Flex PCB Design

Over recent years, there has been the notable advanced development of technology. One of the most appreciated adjustments is in the circuitry field. Flexible Printed Circuit Boards manufacture is one of the major achievements in this sector. Indeed it’s this gadget that has since replaced the need for hand built wire connections, connectors, and harnesses. Now, this guide is solely meant to enlighten you on all the arising issues about this circuit. Stay alert and read till the end. https://youtu.be/IKCHLPv8wCI

What is Flexible PCB?

To kickstart our discussion, consider this. Which kinds of electronic appliances do you use? I suppose they include smartphones laptops, computers, digital Televisions, phone tablets, cameras, and any other gadget.

Iphone 6 Components – Source: Tech Insights

Have you ever wondered about what kind of connections they use? Now, this interestingly brings us to the main course. Flexible PCB is simply a device used in electrical connection simply made by the advanced technology of assembling various electronic circuits, connectors and good electric conductors (copper) into a single unit film base usually a dielectric material.

Flexible PCB

They are distinctively thin, easy to bend due to their flexibility hence can be light and small in size. These qualities make them a popular brand among users. Rapidly, the flex PCB are substituting the conventionally built hand wires connections and connective boards. That’s was the sole purpose for designing these PCB’s initially. One might be thinking if indeed there exist rigid PCB since we got the flexible one. Absolutely, your guess is right. Importantly, flex PCB is also known as flex circuits or flex printed circuits. We shall, therefore, use these synonyms interchangeably. Flex PCB circuits are distinctively designed and fabricated. Now this where most designers go wrong, most tend to design the flex circuits in a manner that is similar to the design guidelines of circuit boards. Note that the Flex circuits are different from circuit boards. Let’s consider this phenomenal – What is the reason for naming Flex circuits as “printed’? It’s simple! A common manufacturing process involves printing the circuits design. However, due to the recent upsurge in technology, most designers photo or laser image as a better method instead of printing.

Laser machine for making PCB – Source: George Fomitchev

So the two types of PCB’s are:

• Flexible PCB
• Rigid PCB

The only design difference between these two types of circuitry lies in their adaptability and usability. Basically, the rigid PCB can’t be twisted or transformed into varied shapes, unlike the flexible one.

 Rigid PCB

Among the lists of electronic gadgets mentioned above at the start of our discussion, the rigid PCB is employed in the laptop, TV’s, desktop computers, audio keyboards among other numerous devices. Elsewhere, Flex circuits boards are usually incorporated in highly sophisticated or technical devices which requires extreme accuracy. Flex circuit boards are incorporated in digital cameras, smartphones, GPS devices, satellites, hearing aids, heart monitors and digital calculators etc.   Interestingly, the rigid and flex circuits could be used together in a single device to provide an outstanding result. Since our discussion is purely based on Flex circuits, we end the comparison of the two and proceed to an important segment of our discussion.

Benefits of Flexible Circuitry

The flex circuits are have got several benefits compared to other conventional electrical connections. Unlike the cables and rigid boards, the Flex PCB boasts of:

• Complete elimination of mechanical connections
• Production of strong and reliable signal
• Decreased wiring faults
• A wide range of operational temperature
• High level of reliability
• Smaller size and light in weight.

Clearly, Flex circuits deserve the lead and praise among the connectors. Don’t you think so? If not satisfied, let’s have a look at a comprehensive benefit of Flexible Printed Circuit Boards.

Flex PCB

1)Reduced chances of Manufacturing Errors

The flex production involves automated and technologically advanced machines. This makes the building process to be efficient and eliminating several production faults which may occur. Conversely, these errors are prone in the manually designed wires and cables.

Flex PCB Connection

This doesn’t mean that Flex circuits are faultless. There may occur some machine-related errors though insignificant. Due to this kind of production technology, the several circuits are only attached to the rightly designed positions.

2)Reduced assembling period and expenses

An outstanding feature of flex assembly is that it doesn’t really require a lot of manual labor for construction just simple and easier processes summarizes the whole technique. Full interconnection systems are installable or replaceable rather than the whole each board.

Flexible PCBs

All these reduce any chances of manufacturing errors from wiring and eventually minimizes the costs that may be incurred in continued connections. Flex circuits also don’t really involve other subsidiary mechanical processes such as soldering, wrapping, and parts routing which are quite costly. In totally, no matter the volume of production of the Flex circuits, you are absolutely sure of savings in costs and time efficiency.

3)Varied Designs and shapes

Have you ever tried to create shapes and designs by twisting flexible material like a rubber band? How many configurations would you achieve? Uncountable right?

 Twisted flex PCB

A major feature of Flex is its’ ability to assume a three dimensional configuration as opposed to the restricted two dimensions of the rigid boards. This a major feature prompting designers to switch their focus to flex circuits. So, this kind of flexibility makes the circuit to possess endless designs and forms just like the numerous options available when working with ribbons. These forms range from highly complex to simple configurations. Significantly, they are tutored to promptly function in up to hostile conditions. Some of the common flex circuit designs are:

• Rigid-flex layered designs
• Pins and connectors
• Fine lines
• Stiffeners
• Shielding
• Assembly pallet
• Coils
• Surface mount and selective bonding

4)Avails adequate Flexibility when installing

This is also another main benefit of working with flex circuits. During execution, they can be used with another element since these circuits can interconnect among planes.

 Sony Z Internal connections – Source: iFixit

Therefore, they only need limited space apart from their increased reduction in weight. In summary, these circuits can be maneuvered severally when installing but they never experience any electrical break down.

5)Avails higher density applications

Flex circuits provide room for the numerous tiny lines. This would, therefore, lead to the production of high-density gadgets.

Flexible PCB

    The indisputable advantage of this mechanism is the fact that this denser device and lightweight conductors may be designed into a commodity. This allows the creation of space for setting up the product’s features.

6)Reliable and durable

Uniquely, flex circuits are capable of moving and twisting for numerous cycles without any damage caused as power and signal is sufficiently delivered without any lapse. This is a feature found in the devices that have got the moving parts. It doesn’t also suffer from thermal effects. Why? Because it’s made of polyimide which has got excellent heat resistivity.

 Flexible PCB – Source: Device Plus

Other advantages due to this are:

• enabling the device to a bare wide range of varying heat effects.
• Provides an adequate base for mounting compared to hard boards.
• Avails a stable solder mask for the circuit parts.

In cases of extreme vibrations or unnecessary acceleration, flex PCB effectively limits impact on itself. Why so? Flex circuits have got ductility and little mass.

7)Adequate thermal regulation

Let’s do some little math. A tiny object does have a large surface area to volume ratio. Flex PCB isn’t an exception. Additionally, it is compactly designed. All these features ensure a small heat path is designated.   Moreover, the flex design can thermally regulate heat from both of its sides ensuring sufficient heat loss.

8)Limited package size and weight reduction

The dielectric in the flex circuit is extremely thin which contributes to the streamlined shape thus no bulky components would be required. Actually, a single flex circuit would essentially substitute several cables and connectors.

Small flex PCB

Latest experimental records report that flex circuits help in sparing about 75 % of the space available thereby creating room for other wiring procedures. The package size is highly limited since Flex PCB is highly flexible and elastic. Importantly, the package weight is another feature that is greatly minimized.

9)Sufficient air circulation

The flex circuits receive adequate air circulation due to their streamlined shape which in turn assists in cooling down the components of the machine.

10)Shortened circuit geometry

The more complex and simplified patterns that were unimaginable difficult to implement in the rigid boards have been made possible thanks to flex circuits.

 Components on flexible PCB

In fact, some recent technologies places surface mount electronics straight onto the circuit. The pros of this technique make the design more reorganized and simpler.

11)Improved system dependability

One major defect of the olden circuits was failure resulting from the interconnection points. This a major defect related to cable wiring. Thanks to Flex circuits, this problem is well catered for since these interconnection sections are greatly minimized which enhances the reliability of the circuit. Now, you may want to reflect on these gains of using flex circuits. I surely trust that you’re now convinced of the needs of implementing flex circuits as opposed to other traditional circuits. Shall we go on?

Primary Materials of Flex PCB

These devices are made from different components from simple locally available resources to other special kinds of substances. The four main materials are:

• The substrate and cover overlay materials
• Conductor materials
• Adhesives
• Insulators

Note that flex circuits may be made from a variety of numerous materials. The main considerations made when selecting the materials to be incorporated in the manufacture are:

• Current conductivity and tolerance
• Capacitance
• chemical and mechanical resistivity
• thermal conductivity
• degree of flexibility

Why don’t we now specifically discuss each item?

· Substrate and Cover Overlay Materials

These include all the other materials that are used to cover the conductor material. The reason behind this is quite straightforward. Although copper is an excellent conductor of electricity, it has one limitation, copper easily oxidizes resulting in the formation of a thin layer on its surface. For this reason, the exposed copper surfaces are covered using substrates and cover overlay materials. The two common materials used to do this are gold or solder.

Flex PCB with overlay material

These two materials are preferred due to their two main physical properties i.e good conductivity and durability when exposed to the environment that is the ability to resist rusting.

·Conductor Materials

Since we’re dealing with electricity here, the main conductor material extensively used in Flex circuits is copper. However, copper is modified into different thicknesses suitable for each customer’s needs. Copper is recommended since it offers value for money, meaning copper is cost effective. The table below summarizes the different types of materials used and their corresponding thicknesses.

Other conductor materials may include:

• Carbon
• Silver Ink
• Inconel
• Constantan
• aluminum

·Adhesives

For flex circuits, adhesives are essential to attach the conductor metal firmly onto the substrate. Selection for adhesives is quite procedural and needs to be done with extreme care since we need the best one to produce the best results.

Flexible PCB with Adhesive

Mostly the choice of the type is done in accordance with customers’ needs and preference as well as the conductor thickness. In some few cases, manufacturers use vapor deposition to bond the conductor onto the substrate. Here are some common adhesives which shall give you the best results.

• Epoxy
• Acrylic
• Pressure Sensitive Adhesives

·Insulators

An insulator is simply a substance which doesn’t easily transmit thermal radiations (Heat). An insulator forms part of the flex circuit in order to prevent the emission of heat during operation. This is helpful to avoid any thermal injury to the user during wiring. Insulators form one of the prime parts of Flex PCB. It is therefore paramount to use an insulator that is as much effective as possible. In fact, the kind of insulator used will directly affect the durability of the Flex circuit. The following are the suitable insulators.

• Polyimide
• Polyethylene Naphthalate and polyethylene terephthalate
• Solder mask
• Flexible solder mask

Now that you know the major parts of flex PCB, next we look at the design structures of PCB.

Types of Flexible PCB Design Structures

A vast range of designs for the circuitry is available in the market depending on the sizes, functionality and even structure configuration. When it comes to Flex PCB’s, only limited types exist. There are very essential differences among the types basically in terms of their manufacturing procedures. The following are the major types.

• Single sides flexible printed circuit boards
• Double sides flexible printed circuit boards
• Sculptured flex PCB
• Multi-layer flexible circuit boards
• Rigid-flex PCB

We shall constructively evaluate each.

·Single Sides Flexible Printed Circuit Boards

Just as the name, this type of PCB consists of only a single layer of conductive material on the dielectric film. In some PCB’s the conductor is placed in between dual insulating materials or on one side. No matter the construction design, it would, therefore, mean that any execution related to the components can only be done from one side.

Single side flexible printed circuit board

Additionally, by means of drilling or laser method, holes may be made on the film base. The holes provide a path for passage of component leads during interconnection. The leads are commonly soldered. Despite the fact that the creation of a protective layer coating is the most common practice for circuitry manufacture, for single sided flex PCB this not a mandatory procedure.

·Double Sides Flexible Printed Circuit Boards

Unlike the single sided one, this type has got double conductor layers. Notably, the conductive materials must have an insulator between them for separation. This flex type is formulated with plated through holes. the major role of these holes is to provide a connection between the two conductor metals.

Double sides flexible printed circuit boards

Though not common practice, the plated through holes may be omitted in some circuits. For such a case, the associated features of the flex will then be accessed via a single side. The protective outer covering layer may be fabricated on both, one or neither sides of the circuit. For most designers, this technique isn’t a consideration and thus the protective layer is usually on both sides. Note that all these customizations shall depend upon preferences and design requirements. Double-sided flex PCB is adored by many engineers since it makes it easier to design interconnections.

·Sculptured Flex PCB

For this one, it’s a combination of single and double-sided flex PCB with plated through holes. for this reason, this kind has got varied thicknesses along its circuitry pattern. The varying thickness results from copper sizes. The copper can be designed to be thicker in other regions compared to others. The difference in thickness is affected by two major factors i.e. current capacity and strength.

Sculptured flex PCB

For current capacity, areas which receive more current is thicker compared to the corresponding thinner regions. Specifically the terminal point of the circuit. Likewise, areas of the flex circuit which needs to maintain a stability should be thicker as opposed to other thinner parts which need to be flexed hence increasing flexibility. The terminal point can thus be used as a connector and plugged directly into a socket. To achieve this thickness difference, the etching process would be perfect for that. The following are advantages of using sculptured Flex Circuits.

• Greatly cuts the budget since it eliminates any need for purchasing connectors.
• The mode of integration utilizes labor costs hence customers may gain savings.
• Savings of Weight and space since connectors are eliminated. This also increases flexibility.
• Highly reliable and superior in terms of functionality.

·Multi-layer Flexible Circuit Boards

This is a type which has got more than three layers of conducting materials. The insulating layer is in between each of the conductor materials. Also, the outer layers may be optionally covered. These types do combine the single and double sided PCB’s into one unit shielded together to form the multi-layer form.

Multi layer Flexible PCB

The layers are additionally attached and connected by use of plated attached through holes. Lamination is a basic procedure that might be considered during the manufacturing process. During the formulation process of the multilayer circuit, continued lamination may be optionally implemented. The only reserved places in this practice are the ones incorporated in the plating through holes. Nevertheless, if you need high flexibility with your circuit, then continued lamination isn’t an option to be considered. For this practice you may consider irregular lamination, the only spared sections are the ones that are bent or flexed. With multi-layer flexible circuit boards, it’s very possible to:

• Control impedance
• Limit crossover
• Eliminate crosstalk
• Regulate shielding

·Rigid-flex PCB

This is a combination of both the rigid or flexible substrates which are laminated and attached together into a whole unit. This type also has two or more conducting material layers with insulating materials in between them. The insulation layers could either be rigid or flexible.

Rigid Flex PCB

Noticeably, Rigid-Flex circuits may seem familiar and challenging to distinguish from multi-layer circuits with stiffeners. The only outstanding difference between the two is that Rigid-flex circuits have got conductor metal on the rigid layers. The plated through holes in this type also electrically interconnect both the rigid and flex zones. However, blind and buried vias are exempted. Optional measures may be incorporated. This include:

• Stiffeners
• Pins
• Connectors
• Components
• Heatsink
• Mounting brackets

Hopefully, you now know how to differentiate different types of Flex PCB’s.

Flexible PCB Design Considerations

The design of flex PCB should be consistent between the electrical and mechanical components of the circuits. All these segments greatly have got an impact on the usability and durability of the circuits and hence need thorough scrutiny. Importantly the mechanical considerations to be made to modify the circuit layout. We shall, therefore, discuss the circuit properties that needs serious attention.

Flex PCB Design

·Trace widths

First, the size range for trace widths of circuits varies accordingly depending on the vendor. A number of factors affect the productions of different trace widths. The most notable ones are:

i.The market demands

Due to the ever-increasing demand for quite small electronic gadgets, flex circuits which got trace widths equal to or less than 50 µm or 0.002″ are increasingly being hugely produced. Whereas flex circuits with trace widths of 250 µm or 0.010″ are easily available in the market. Interestingly those with line widths of 125 µm or 0.005″ have seriously taken up the market.

 ii.Technological aspect used in the manufacturing process

Critically, the tech applied in the manufacturing procedure widely leads to different the minimum trace widths. The technique of using plated up copper sputtered polyimide base circuits results into limited circuit sizes which are otherwise regulated by the photolithographic technique. Hence, only a very few numbers of circuits can be made by the manufacturer. Elsewhere, the etched circuit width traces together with pitch are only dependent on the degree of thickness of the copper foil base. In summary, the trace pitch limit will greatly vary in accordance with the size of the thickness of the copper material used. The following table gives an insight into this. However, note that these values may vary depending on the vendor.

·The conductor on the design

The conductor design is frequently inclined towards its width and thickness. These parameters are significantly determined by the current requirements, the voltage drop and characteristic impedance control measures. A dynamic circuit should be constructed from very thinner copper materials. As a result, any designer needs to opt for wider traces as opposed to thicker ones to be flexible in complying with the basic electrical requirements. In most flex PCB manufacture, 35µm (1-oz) and 70µm (2-oz) copper are used. in addition, 18 µm (0.5- oz) thick and lower copper foils are continually increasing.

·Flexible circuit board sketch tolerances

Now this one of the most crucial aspects of FCB design. The proper and right tolerance application is a matter of concern to both the circuit users and even the manufacturers. It is a recommended manufacturing practice to provide for a larger tolerance allowance for all the features and segments. This is so because the base materials of the components are prone to mechanical breakage resulting in difficulty in providing accurate measurements. To rectify this, then you can use various datums on larger circuits. Moreover, it’s recommended to use one primary or master datum while the others become the secondary or slave ones. This would result in more accurate circuit measurement and gadget placement during assembly. Elsewhere, tight tolerances can as well be provided under special techniques and considerations. Also, tight tolerances are quite expensive due to the high potential risks they possess. Presently most circuit manufacturers produce circuits features sizes of 35 to 50µm while others 25µm and even 10µm.

·Minimum distance to design edges

Actually, the least distance of separation between the conductor and the design edges is usually approximately 375 µm. More advanced finer lines and spaces can as well be produced using polymer thick films inks. The only limitation is that it may interfere with the conductor conductivity.

·Space Between Strands and Pads

This is otherwise called conductor spacing. Remember the pad is the section of the conductor that surrounds a through the hole. The pads serve as connection terminals for other electrical components. They are also referred to as lands or terminals. Essentially, when manufacturing the Flex circuits, a certain minimum quantity space must be accounted for. Why? This is vital in preventing the conductors from coming into contact. This helps to prevent the conductors from short-circuiting between themselves. The CAD conductor spacing needs to be allocated as follows:

• Minimum of 0.001” ( 25 µm) above the specified minimum allowance size for all the unplated copper.
• Minimum of 0.002” ( 50 µm) above the specified minimum allowance size for all the plated copper.

·Conductor Geometrics on the Circuit boards

This includes a combination of the modifications and simplifications done to the conductor to ensure optimum functionality.

Flexible PCB Design

Such issues include Conductor Routing – this involves a number of issues such as maintaining a minimum number of crossover in the layout. This would enable to reduce the layer count and lower the costs. Also, it enables the available space to be maximized. The general recommended design practice for routing flexible circuits is on a perpendicular position to the bend or the fold. The reasons for this practice include:

• To enhance the bending and the folding procedure.
• Limits the amount of stress generated throughout the region.

It’s a common practice to prevent the circuitry designs from assuming a right angle or acute angle during circuit routing. Such discouraged angular positions make the circuit to trap solutions and may lead to etching apart from making it difficult to clean efficiently. A solution to this is to make the corners have a radius. This mechanism helps to reduce the reflections at the turn points hence enhancing signal propagation.

Flex PCB Design Rules You Should Know

Well, some rules are applicable in the design and manufacture of Flex PCB’s. These rules ensure that all the PCB constructions proceed perfectly fine without unnecessary misinterpretation of the aspect ratios. https://youtu.be/AmkD0PPDuDU Let’s have some detailed discussion on these regulations. The aspects of the Flex PCB that need to be regulated include:

1.Bonding Sheet Layout

Bonding sheet is a group of flexible single sheets that are attached together and maybe the peelable one after another to achieve the necessary level of thickness tolerance. Thereafter the bonded segments can be used independently. Remember adhesives are used to attach these sheets. Now to do this effectively consider these two rules associated with bonding sheet.

• INCORRECT: When drawing the bonding sheet layout, the borderline of no adhesive segment is drawn straight. In such a case, open or short circuit hitches will be experienced.
• CORRECT: Design the bonding sheet in such a way that provides a slope angle of 45 degrees in relation to one of the border lines of LCD mounting area or the tail part. An exception for this procedure should be the single side flex PCB.

2.Pattern Design of the Folder Area

Here the pattern design is technically manipulated to achieve one major objective:

• In order to retain or achieve an extreme flexibility by completely staggering the pattern lines.
• This is done by following below procedures.
• Staggering the pattern lines on each layer.
• Staggering of the first and second layers.
• Subsequently staggering the third and fourth layers.
• Consequently, you can now stagger all the layers with each other.

All these measures need to be done for signal pattern line. Importantly, this technique ensures that no layer pattern needs to be situated above the same line. Otherwise, it may limit flexibility.

3.Silkscreen description

The aim of this rule is to avoid any possible default by getting to know the silk screen production condition. To achieve the set objective follow this:

• Text Mark: this usually printed on the circuit board and may entail the customer’s mark, symbol, and date. The mark is usually 2mm in size.
• Component text mark: unlike the text mark this one is smaller in size, at least7mm and maximum 1.5 mm. its position is transferable depending on the customer’s preference.
• Insulation line: – this the mark necessary for preventing any short circuit among the lands. The standard line thickness needs to be 0.15 mm while the distance between the line and land is 0.2 mm.
• Land outline – you need not silk screen the exterior line of the land. Moreover, it can be removed if not an insulation line.
• Alignment line – done according to the customer’s
• Space – the spacing should be placed at least 0.2 mm between the lines.

4. Pattern Line Depth and Tolerance of Raw Material

The table below simplifies the optimum dimensions. (All Units: mm)

5. Through Holes or Pads

Have a look at the following tables for measurements.

6.Stiffener and tape area tolerance (Units mm)

7. Teardrop design

The main aim of this design is to prevent the appearance of open circuits resulting from cracks or disconnection between PAD and pattern. Type:

• 1 amid PAD and through the hole.
• amid lands and pattern

Follow these steps. You will need a teardrop tool cutter to input R size. If you experience difficulty in doing this then you may do it manually.

8.Silkscreen tolerance

9.Cover overlay and solder resist dimensions

10.  Cover lay Open Area Stiffener Specification

The overlap distance for this needs to be a minimum of 0.15 mm.

11.  The gap from stiffener edge to hole

12.  Adhesive Specification

The adhesive need only is applied within the guideline segment.

13.  Goldfingers Design

Note that all the units are measured in millimeters (mm)

14.Stiffener and pattern design

Ensure that the stiffener is 0.5 mm shorter than the pattern line. This prevents the formation of unwanted cracks.

Flexible Printed Circuit Manufacturing Process

Before kicking off the design process, let’s mention some necessary points. You need to understand that flexible circuits need a balancing strategy for both the mechanical and electrical matters. https://youtu.be/WWi7ZQt9Yzg The overall manufacturing process is quite procedural. However, we can break it down into three major segments.

Step 1: Flex PCB Build UP

Laying the Circuit. This is the basic phase of the manufacturing process. Here our focus begins by aiding you in saving the base material to be used. this is mandatory if you would wish to maintain low manufacturing costs. Remember, Flex circuits main material i.e. polyimide is much expensive compared to FR-4 hence the need for proper utilization. The proper way to get this is by ensuring the circuits are as closely spaced as possible. This done by implying the nesting technique. It’s a way of boosting the number of circuits for every panel. You shall realize that the ordinary circuit layout has got four segments per panel, nesting boosts it to eight parts while maximization then takes the number to sixteen. Indeed correct circuit nesting increases the yields of the panel.

·Looping

After that, it’s recommended to add an extra material length to the flex circuit beyond the designers limit. The small material is thus called service loops. Here are the purposes of looping.

• Provides extra length for servicing and assembly of the circuit during usage if the need
• Compensate for any slight variation that may exist between the package and the circuit.

·Staggered lengths

This technique involves the addition of slight material length to each of the subsequent flex layer and importantly diverting from the curve radius. Notably, the added material should be about one-half times the layer thickness. This is vital in preventing the center of bend layers from buckling.

·Sizing conductor

To ensure maximum flexibility of the circuit, it’s worth selecting the thinnest copper especially when the circuit is to be used for dynamic applications. Again, the designer needs to go for the wider as opposed to thicker traces. This is good for the accommodation of any arising electrical needs.

·Trace Breadths

These measurements vary among vendors. However, flex circuits which got traces of 250 µm or above, 125 µm or lower sizes, 50 µm are much common. Actually, due to the ever-increasing demand of smaller electronics, small trace widths now are increasing. Trace widths also are dependent on the technology type applied. Plated up copper sputtered polyimide base circuits are limited in size by the photolithographic mechanism. Therefore, this results in little circuit components. Etched circuit traces, the pitch, and widths are influenced by copper thickness. In addition, the trace pitch limit is almost linear with the copper thickness at some given range. 18 µm copper would subsequently produce 125 µm pitch circuit features. 35 µm copper produces circuit features of 175 µm pitch and above. Whereas 18 µm copper can manufacture 25 µm circuit features pitch.

·Etching

This process is done by etching factor. This a tool used to deal and compensate for any isotropic loses during the manufacturing process. Usually, the line width loss as a result of the etching process is about twice the copper foil thickness. However, many factors have got impacts on the line width such as conductor, type of copper, etch mask and equipment.

·Routing of the conductors

To achieve this on a flex circuit, you need to do right on the perpendicular position to the bend and fold. It would enhance bending or folding at the same time reducing stress in these specific regions. Remember, the circuitry should also be routed to a single layer of copper via the bend or fold regions. During circuit routing, ensure the designs assume an angle of less than right angle. This so because they tend to trap some solutions during the etching process. Also, the corner sections need to be fitted with some radial allowance to increase signal propagation. If you’re dealing with double-sided flex, then the designer needs to allow spaces approximately twice the trace thickness. This is applicable where the conductors have to be routed at the bends and fold areas. To avoid the I beam effects then the designer has to stagger traces from one side to the other. It’s important to avoid vias within the bends to avoid excessive stresses at these sections.

· Ground planes

The ground areas need to be crosshatched if the electrical allocation is sufficient. Afterward, the flexibility of the circuit is enhanced while the weight drastically reduced.

Step 2: Flexible Printed Circuit Board Fabrication Process

In this segment, how procedures shall be based upon the things done on the boards. We shall start with the conductor spacing and width.

Flexible printed board circuit

Common conductor width needs to be spaced at 375 µm. by use of polymer thick films, you can produce smooth lines and spacing. Nominal Polymer Thick Films (PTF) have got capabilities of carrying the current. However, silver-based polymer thick films can carry 25% of the circuit current. Screen printed resistors can as well be incorporated into the PTF circuit designs. Hole sizing The diameters for through holes in flex circuits should be 200 – 250 µm bigger than the component designs to satisfy the practice requirements for automatic placement of components. The most preferred design should be that all the lands and pads needs be about twice the hole diameters.

·Sizing of the holes

It’s possible to design the holes as tiny as possible as long as it’s fine with the manufacturer. These kinds of vias enable the circuit layout to be well inclined. With the advancement of technology, it’s possible to produce via holes as little as 25 -25 µm.

·Filleting

The pads and lands termination points on the flex circuits need to be filleted. This technique multiplies the pad area and distributes the stresses. Plated through holes are good enough if very tiny pads are suitable for making a reliable solder joint.

·Button plating

This a process used to create a substitute plated through hole, the only difference is that the through holes and vias are plated with copper.

Step 3: Consider Possible Chemical and Physical Constraints in Flex PCB Fabrication Process

In this step, we shall basically concentrate on the other components of Flex PCB mainly the cover layer and the cover coatings issues. The cover coating are many and got unique traits, among the common ones that can be applied in the manufacturing process, are:

·Adhesive backed films

This type is highly suitable for dynamic flex circuits applications since it has got its raw materials optimally balanced. Indeed this is the most used by overcoating by many manufacturers.

·Screen-printable liquid overcoats

If you really need to save on your expenses then I would advise you go for this. Apart from being pocket-friendly, it is commonly used together with polymer thick film.

·Photoimagaeable liquid and film polymers

Actually, this is the new advanced method of overcoating. This coats can be imaged and used to view the termination features of the flex circuits. A major advantage of this coating is that it isn’t associated with any irregularities that are experienced by other cover layers. Someone might be surprised and wondering about the functions of the cover layers. Below are the functions of cover layers.

• It acts as a solder mask thereby preventing the solder from circuiting traces together.
• Harbors the circuit from any external electrification.
• Protects the circuit from external and physical damage.

Basically, that’s the information that is entailed in the manufacturing process. The process and segments may vary from one manufacturer to another. Many modifications may be included as suited.

Considerations when Buying Flex PCB

Now, the most challenging task for many is how to pick the best Flex PCB that’s efficient for applications.

Flexible PCB in TVs

Flex circuits have got some unique elements that need to be focused on just to ensure that you acquire a reliable one. Reliability here is interconnected with efficiency, the flex circuit needs to sustain any mechanical stress and hence avails long-term usability without unnecessary breakdown. Whenever you’re ready to make a purchase then you need to deeply bear in mind the following parameters to assist in selecting the most suitable one.

i.Quality and Grade of Flex PCB

These factors depend on the types of flex PCB. As earlier discussed each type of the circuit has got unique and distinct properties that make it suitable for use in certain applications. So, it’s important to consider buying one that would guarantee you service for your application as earlier outlined.

ii.Types of material

Flex circuits are made of polyimide layers. This is as opposed to the rigid boards made which are made of Fiberglass Reinforced Epoxy laminate (FR-4). The major advantages of polyimide are:

• Produces lighter applications. Subsequently making the flex circuits boards much light in weight.
• Results into productions of thin sections since the polyimide layers are always very thin. This is the reason behind the thinner PCB board segments compared to the typical rigid circuits.

Clearly, polyimide is the different material that you really need to select. Apart from this, it’s worth noting that flex PCB materials are expensive compared to FR-4. Consequently, during the designs of the circuits, there is a high need for conservation of the material quantity. Again, this vital since the quantity of material used would have a direct impact on the final costs of the circuits. When the material is sufficiently conserved, this would assist in greatly lowering the costs.

iii.Minimum Tracing and Spacing

Now in order to identify the most efficient circuit, confirm the traces and spacing. Let’s begin with the spacing.

·Spacing

Generally, the circuit needs to be closely spaced. The common way of achieving a suitable close spacing is by optimizing on the number of circuits in a given panel. This technique is known as nesting. Circuit nesting results in the transformation of the circuit layout. An ordinary circuit layout usually got four parts for each panel. After nesting, eight parts per panel circuit is produced. Further testing would result in the production of a maximized circuit layout of sixteen parts per panel. The significance of proper circuit nesting are:

• It impressively improves the circuit panel yields.
• Eventually, it lowers the circuit costs.

·Trace widths

The trace widths of the flex circuits are as well essential in the decision you’re gonna make. However, the minimum tracing widths vary significantly among the vendors as earlier discussed. Therefore, it’s prudent to consider the width trace and spaces good for your applications before buying a circuit.

·Flex PCB Dimensions

Here we shall look at the various dimensional sizes of the Flex PCB parts.

·Number of the layer in Flex PCB

Basically, the prices of the circuits are directly proportional to the number of layers of the circuits. Thus the more the layers, the higher the costs. Therefore since our priority is on how to save on costs and expenses, we thus need to look at some few tricks. You should use two circuits to cover up the job to be done by a single one. Take for instance, in an ordinary vendor shop, a four multilayer circuit is very expensive compared to two double-layer flex circuits. Remember, flex circuits may also be folded to save on the available space and number of layers.

·Surface Finish

It’s time to concentrate on the general finishing of the flex circuits. Here we’re considering the outer appearance of the circuits. This is also another important segment to focus on since it determines the quality of the circuit. Obviously, after manufacturing of the circuits, the external circuit pads would require finishing to protect it from corrosion. As well as ensuring its compatibility matches that one for the application. The most common finishing procedure is electroplating. Various metals can be applied during electroplating procedure such as tin, copper, nickel, silver and hard and soft gold etc. Apparently, electroplating can only be done once all the conductors are together connected to an external bus during fabrication. An alternative electroless procedure is also possible. This is only done when the circuit traces can be connected to a bus in the panel. The alternatives include hot air solder leveling, immersion gold over electroless nickel, and immersion tin, organic solderability preservative (OSP). Above all, the circuits need to be surface finished and that’s your checklist.

·Color

Generally, the PCB color expresses the solder mask color. The solder mask is significant in preventing any occurrence of short circuit of the boards. PCB boards have got several colors, however, the most prominent one is the green solder mask. In the recent, use of different colors have greatly risen, some of these colors used are: white, black, yellow, red etc. No matter the color used, it has no direct impact on the efficiency of the PCB. The only difference will be a unique board resolution.

·Flex PCB Manufacturer

Even though the flex manufacturing technology has existed for very many years now, it’s still a challenge for most. This because it is still a new aspect in the industry and therefore we only got few manufacturers. Unfortunately, there are many who still haven’t perfected their production work. As a result, you need to only contact reliable and renowned manufacturers for quotation. That’s all the information you need to have before making a quotation with any vendor. Shall we move to the next segment of our discussion?

Methods of Connecting Flexible PCB in Electronic Systems

The flex circuits are well constructed to provide an allowance for connection to other electronic devices.

 Connecting flexible PCB

This provision is efficiently scattered for during manufacture and assembly. Again, the connectors can be attached to the flex circuits using numerous methods. Actually, the connector manufacturing companies have effectively designed the gadgets to be readily adaptable for connection to the Flex Circuits. These connectors may seem complex but indeed some are just simple devices. Having known that, let’s dig deep into the connection of the connectors. Now for the socket type connectors, first the male and female pins have to be made. To do this, attach the swaged or brazed pins directly to the flex circuits. To accomplish this method, the following methods may be applied i.e.

I.The sculptured Flex circuit technology

This technique is famous due to the fact that it can wholly integrate the connector straight into the flex circuits. A number of electronic applications apply this method since it doesn’t require any distinct joining of the pins. As a result, there will be edge contacts of the circuits that extend unsupported just beyond the edges of the flex circuit. Now, this makes it possible for creating a suitable pin that will just fit into the socket.

II.Edge card contact constructions

This is a simple and cheap way of connecting the flex circuits. It is made by folding the area of contact of a circuit just next to the stiffener. Now this one is the direct opposite of the edge card contacts in the rigid circuits which got several mating connections. By altering the thickness of the circuit together with stiffeners, it’s possible to create and accommodate various connector designs. A common connector used with the flex circuits are the surface mounted connectors. Therefore, simple connectors are appropriate for use in the flex circuits. These kinds of circuits are most suitable for use with flex circuits which have got tight tolerance.

 III.Lapp connections of the circuits

For this one, a combination of materials is used including conductive polymers, soldiers and adhesives. Doing this results into connections between a flex PCB and mating interconnection structure.

IV.Making the connections directly onto the chips although anisotropic adhesives or even lap soldered connections have to be incorporated

There you go. Those are the simple methods by which Flex circuits are connected to other electronics. We move onto the final section of our discussion.

Applications of Flexible Circuit Boards

The robust properties of the Flex Circuits make it suitable for use in sophisticated applications that need a high degree of flexing or even accuracy. Some of the common applications include:

• Cell phones
• Calculators
• Satellites
• Cameras
• Battery packages
• Automotive engine control systems
• Airbag systems
• Antilock brakes
• Heart monitor devices and pacemakers
• Hearing aid devices
• Fuel pumps
• Printers
• Motion systems
• GPS trackers

Conclusion

At last, this marks the end of this educative guide. It’s important to remark that now you know significant information related to the flex circuits. It’s impressing that you are now enlightened on the benefits of these circuits, their primary materials, the types of these circuits, the design mechanisms that matter most and requires many considerations. Elsewhere, you need to fully understand the rules associated with the circuit designs as well as the manufacturing process. Most importantly, if you are going to place a quotation with a vendor then you shall really need to master aspects that needs meticulous considerations before you make a step. That’s all. Thanks for reading.