Through Hole Assembly

When it comes to printed circuit board assembly, there are two options of mounting components: Through Hole Assembly and Surface Mount Assembly.

Today’s guide will focus on everything you need to know about through hole assembly of printed circuit boards.

It covers basic definition, advantages, disadvantages, techniques, application, practical assembly process, and many other vital subjects on through hole PCB assembly.

Let me take you through:

What is Through Hole Assembly?

Through hole PCB assembly

Until around 1980, through-hole assembly was the primary method of PSB assembly, even though the use of SMT had begun earlier, about 1960.

(You will learn more about SMT in my next series of guides.)

Through-hole PCB Assembly is the process where through-hole components are mounted onto a bare Printed Circuit Board (PCB).

 Bare PCB

Through Hole Technology (THT) involves soldering the components onto the board, with their leads through holes drilled on the board.

The leads are soldered onto pads on the reverse side, either manually (by hand placement), or using automated insertion machines.

PCB with components

Advantages of Through Hole Assembly

Why should you try Through Hole Assembly for your PCB manufacturing processes?

1.Easier Prototyping

Components of a through hole can easily be swapped out; thus they are perfect for PCB prototypes and testing.

2.High heat tolerance

THT bonds have high heat tolerance; hence making it the preferred choice for aerospace and military products

3.Better power handling capability

Soldering in Through Hole assembly creates strong bonds between the board and the components.

It is therefore ideal for larger components which are bound to be subjected to:

• High voltage
• High power
• Mechanical stress.

These include transformers, connectors, and electrolytic capacitors.

4.Stronger physical connections

The leads of the THT component have stronger physical endurance.

They can withstand environmental stress since they are soldered on the opposite of the board (they run through the board).

Stronger physical connection

That is why THT is preferred for led lights for stadiums and billboards. It is also used in industrial equipment and machinery due to their exposure to very harsh conditions

Disadvantages of Through Hole Assembly

Like any other manufacturing or assembling techniques, Through Hole PCB Assembly has its disadvantages too.

These include:

• Longer production time-It takes a lot of time to drill the multiple holes through which the leads are expected to run.
• Expensive-because the drilling process is difficult and time-consuming, it increases the cost of production
• Minimizes space on the boards-drilling the holes on the surface of the board limits the routing area for tracing signals.
• Also, by feeding the leads through the board, only one of its sides is usable, thus making the THT PCB a bit slower in operation speeds.

Through Hole Assembly Components – What are the Best Components for Through Hole Assembly?

There are two types of components in Through Hole Assembly:

• Radial lead components
• Axial lead components.

a) Axial Lead Components

Axial leads go through the component from one end to the other in a straight line, forming two terminals, one on either side of the component.

Axial lead components – Photo courtesy: EBSO

During Through-hole assembly, the two terminals of the lead go through holes on the circuit board.

The component, therefore, ends up being closer and flatter on the board. They are sometimes used to span short distances on the board.

b)Radial Lead Components

As opposed to the axial lead components, radial lead components have leads that protrude out of the component body on the same surface.

Usually, stand perpendicular to the board.

 Radial lead components – Photo courtesy: EBSO

They, therefore, occupy a smaller space and cover a shorter distance on the board than the axial lead components.

Their leads project from the same mounting surface in a parallel manner.

Through Hole PCB Assembly Techniques

There are so many technique you can adopt in Through Hole PCB Assembly process.

Depending on your unique objectives, you can go for:

1)Manual Through Hole Assembly

This process involves individuals and soldering small quantities of circuit boards in order to perfect their PCB designs.

It was and somehow is still used by smaller and start-up companies, which may not be able to afford to automate the PCB Assembly process.

One of the characteristics of the boards assembled manually is that they hardly come in consistent quality.

Humans, unlike machines, are not able to accurately repeat such a process as PCB assembly in one exact way, several times. It is also prone to incorrect placement on the PCB.

Manual assembly process

Today, there are smaller electronics with very small boards on which it is absolutely impossible to hand-solder components.

Manual Assembly is only effective with larger components like through hole chip packages and led resistors.

Another problem with manual THA is speed.

It would be very difficult for an individual to hand-solder, say 500 components within a single sitting.

That means the production capacity for a company using manual THA alone limits its survival chances in the current hi-tech field.

The operator is also exposed to solder fumes and flux fumes, which puts them at risk as these fumes can lead to cancer.

Even the direct threat of getting burnt by the soldering iron is always very real.

Again, the traditional solders which had lower melting points have over time been replaced by lead-free solders. These ones have higher melting points and are therefore difficult to hand solder. Solder wetting with a hand-solder iron takes a long time, and the solder also spreads less.

They also tend to have very small plastic range, meaning the part being soldered has to be kept intact until the solder has cooled and solidified.

That takes a long time and is tiresome if it has to be repeated for several components.

2)Automated Through Hole Assembly of PCB

This is a technique that uses machines throughout the assembly process.

It involves several machines arranged in series, each one performing a specific task in the process.

The machines have a few operators and repair technicians, and that is just like all the human involvement in the process.

You may, however, have PCB stackers to hold the PCB while in progress.

PCB Assembling and Testing Machine

The components of an automated assembly line are:

• A solder paste applier
• A high-speed chip shooter
• A pick-and-place machine
• An infrared oven

When all these are placed on a conveyor, they can apply solder paste and solder even more than 500 parts in one hour.

The combination of these this set up produces a perfectly even solder joint that cannot be matched by hand-soldered ones.

Many companies have fitted their THM systems with Small, vacuum-operated local fans.

They help to reduce the operators’ contact with dangerous metallic compounds like lead, cadmium, and copper.

Through-hole vs. Surface Mount PCB Assembly

Earlier in this guide, I mentioned surface mount technology (SMT).

It will be unfair to read this guide without knowing the differences and similarities of Through hole vs. surface mount PCB assembly.

Let’s quickly look at that before we proceed.

Surface mount technology

Through hole mount technology

Apart from through-hole assembly, the other PCB assembly method commonly is Surface Mount PCB assembly.

Surface mount devices do not need holes drilled on the PCB as their leads do not go through the board to the other end.

Instead, the components have surface mount packages.

The leads are usually somewhere around or beneath these packages.

These leads usually make direct contact with the pads which are located on the surface of the board.

The main differences between Through-Hole and Surface Mount Assembly are:

 Through hole vs surface mount PCB assembly

1.Drilling of holes and the soldering of leads on the opposite side of the board means only one side of the board can be used in THM.

• This minimizes the space available for mounting components. However, In SMT both sides of the board can be used because there is no drilling of holes on the board.
• This makes SMT ideal for making boards with many components. It is therefore used to create smaller and lighter designs, which are more powerful.

2.The components used in SMT are usually much smaller, making it possible to mount several components on a relatively small board.

• This also increases the functionality, thus making SMT the better option for producing more modest and more portable, yet still more efficient circuits.

3.In SMT, the boards are able to accommodate more pin count than in THM; thus the boards have higher component lead count than in THM.

4.Production volumes in SMT can go very high.

• It is because the process is not as involving as in THM-there is no drilling of holes which is usually very difficult and time-consuming.
• This high volume production allows for economies of scale; hence the production cost per unit is significantly lower, making it the more profitable option.

5.SMT requires higher capital investment for setting up the machinery and for production process than THM.

• It is therefore only suitable for large scale production where the advantage of economies of scale helps to surmount these high costs.

6.Using SMT requires higher levels of skill and technology as compared to THMA. The need for more advanced technology also comes at a higher price.

7.Despite the overwhelming advantages that SMT seems to have over it, Through Hole method is still mostly used in testing and prototype applications.

This is because there is always need to manual adjustments at this stage.

Through Hole PCB Assembly Equipment

In Through hole Assembly, a number of equipment are required to complete the process.

Some of the equipment and their functions are as outlined below

I.Soldering Irons

The soldering iron is the central equipment for manual through-hole PCB assembly. It usually has the following parts.

Tips when Considering Soldering Irons

The iron tip is the pointed part of the iron that heats up the solder, making it able to flow around the components being joined.

The function of the tip is to transfer heat and raise the temperature of the metal components to be able to melt the solder.

In most irons, the tip is replaceable.

This will allow you always to choose the shape of the tip you want to use depending on the soldering task you want to perform, and to replace it when it is used up.

PCB Assembly Accessories

Wand

This is the part that holds the tip.

It is usually made of an insulating material such as wood or rubber because it is the part that the user handles.

There are wires and metal contacts for transferring heat from the base to the tip.

II.Wave soldering machine

This is especially necessary for a bulk assembly process that cannot be achieved manually using the soldering iron.

III.Desoldering Braid

Also known as the Solder Wick is a thick copper wire that is braided together that is used for the removal of solder.

IV.Solder Vacuum

It is also known as solder sucker and is used to remove solder that remains behind in through holes during desoldering.

Through Hole PCB Assembly Process

Through hole PCB assembly can either be done manually, or the process can be automated.

The process of Automated Through Hole PCB assembly is quite a complex one.

It involves the use of very sophisticated machines and procedures.

The processes may differ slightly from company to company.

However a typical Automated Through Hole PCB Assembly process takes the following steps (Starting with a plain/clear board).

A.Making the Through Hole PCB

Making holes on PCB

The PCB is designed with pre-printed through holes for the passage of the leads of the components before they are soldered on the opposite side of the board.

But how exactly are the boards made-from an empty board until the point it can now be assembled?

i.Pre-Production Engineering

Data is for the specific PCB is processed (including data for imaging process and drilling programs) based on the data provided by the customer.

The engineers then consider the specifications vis-à-vis the capabilities so as to be able to design the steps and checks.

ii. Preparation of Phototools

The artwork master (photographic image of the PCB pattern that is used in circuit board production) is produced.

This is done using electronic data that is accurately scaled. An artwork master can be:

(1) Conductive Pattern

(2) solder mask

(3) Silkscreen

iii.Print Inner Layers

The third stage is to transfer the image to the board surface.

This is done using photosensitive dry film and a UV light. The purpose of the UV light is to polymerize the dry film.

This is therefore where the electronic data is transferred to the photoplotter.

The photoplotter then uses light to transfer the pattern into the panel or film.

iv.Remove the inner layers

Etch out the unwanted copper on the panel. Now remove the remaining dry film. The copper circuit that remains matches the design.

v.Automatic optical inspection

The circuitry is inspected to verify that it matches the design and is free from defects.

To achieve this, the board is scanned, after which inspectors verify any anomalies highlighted during scanning.

vi.Lamination

An oxide layer is applied on the inner layers, after which they are stacked together to provide insulation between the layers.

A copper foil is then added to the stack on the bottom and top.

During lamination, the internal layers are placed under extreme temperatures of about 375 0F, and pressure of between 275-400psi.

After lamination, the PCB is allowed to cure at a high temperature. The pressure is then slowly reduced before the material is allowed to cool slowly.

vii. Making drills on the PCB

Making drill holes on PCB

The board is now ready for drilling of the holes to enable the creation of an electrical connection within the layers of the PCB.

One of the primary methods is by using a laser drill

viii.Copper Deposition

At this stage, a thin layer of copper is deposited on the walls of the drilled holes.

This process must be controlled to ensure that the copper is adequately plated even on the non-metallic walls.

It creates continuity between the layers and the through holes.

It is then followed by panel plating, which provides a thicker copper deposit on top of the first (About 5-8 um)

PCB

ix.Imaging the outer layers

This process is similar to the method of imaging the inner layer.

Except that here, the dry film is removed where you intend to define the circuitry in order to plate additional copper.

The step must be performed in a clean room

x.Plating

This is where the additional plating is added in the areas without dry film.

Tin is then applied to protect the copper plate.

xi.Remove the outer Layer

This happens in three steps:

• Remove the dry film
• Remove the unwanted copper
• Chemically remove the tin that had been added to protect the needed copper

xii.Outer layer AOI

AOI means Automated Optical Inspection.

Scan the panel that has been imaged and etched. This ensures that circuitry is according to design, and is free from defects.

xiii.Soldermask Application

The next step is to apply Soldermask ink over the entire PCB surface.

Certain areas of the board are exposed to UV light.

The unexposed regions are later removed during the chemical development process. This step is also done in a clean room.

 xiv.Surface Finish

Different finishes are applied to the exposed copper areas to protect the surface and to enable good solderability.

Finishing can be done using HASL, Immersion silver or Electroless Nickel Gold.

xv.Profile

The manufacturing panel is cut into specific shapes and sizes according to the customer’s design in the Gerber data.

This can be done by scoring, punching or routing.

xvi.Electrical Test

This test is to check the integrity of the tracks and through-hole interconnections.

This eliminates chances of short circuits and open circuits on the finished board.

Electrical test

There are two methods involved: for smaller volumes, flying probe is used, while fixture based is used for volumes

xvii.Final Inspection

Each PCB is checked one by one by expert inspectors. The visual checking is done by approved inspectors.

Comparing the PCB with the Gerber is automated, but has to be verified by human eyes. They are also checked for solderability

xviii.Packaging

The boards are packaged and boxed, ready for shipment.

Packaging PCB

B.The Assembly Process

Before you get into ordering the boards (if you do not have them) and the components, you need some technical information.

This is critical when it comes to Through Hole PCB Assembly process.

Customers usually have conditions and preferences that a PCB must meet before the assembly process.

These include:

1.BOM Information

You will need to have information that guides you either order the parts.

Or organize the components if you have them within your company.

BOM – Photo courtesy: PCBCart

You can only know the parts to be used based on the customer’s specifications. This is what we call the BOM.

2.Solder mask

You need to receive Gerber files from the customers that identify the areas of the board which should receive solder during solder-paste deposition.

Solder paste is the solder used in reflow assembly.

3.Placement Data

This includes details on the distribution of spatial coordinates and rotation for every part on the board.

The machine must be configured with specific information about where to place what.

If your company does not make its own boards, then the first step after understanding the customer’s specifications will be to place an order for the boards.

In case you have the boards, the same would still apply when procuring the components to be assembled onto the board.

It is vital to adequately research on identifying which company will supply the best quality boards or parts.

You will, for example, have to know the history of the company in terms of the quality of their products.

Occurrences like the bursting of hoverboards that usually reduce consumer confidence on a company’s product.

You do not want to order for boards and components which once you are done assembling, no company will be willing to buy.

In that regard, the following factors must be put into consideration

4.Certification

The company you want to order PCB from must be certified to sell such products.

It is only through this that you will get high quality printed circuit boards and components.

5. Excellence

What does this imply?

You need a manufacturers that uses high-tech machine and equipment.

Besides, they should adhere to strict quality control and assurance.

6.Experience

Years of experience and expertise in the field of Through Hole Assembly process is also important.

Choosing a good component supplier is very important since the component quality usually determine the quality of the end product.

After making these considerations and selecting your preferred component supplier, the next step is the assembly process itself.

There are some initial steps you need to follow before the real PCB Assembly process commences.

These processes are meant to assess the functionality of the PCB.

You will, for example, have to conduct a DFM check.

Design for manufacturing check is the process that helps the company to prevent, detect, quantify and eliminate waste or inefficiency in manufacturing within product design.

The check looks into all the design specifications of a PCB to identify any missing, redundant or problematic features.

These issues have to be identified because they can harm the functionality of the ultimate project.

You can now embark on the assembly process.

By now you should have assembled all the equipment listed earlier.

Before you get to the Soldering process, there are preparations you have to put into place.

C.Preparation for Soldering

Before you start soldering, you will need to tin the tip of the soldering iron.

This simply implies coating of the tip with a thin solder coat so as to enhance the transfer of heat from the tip to the component.

I.Warm up the Iron

You are just about to start soldering, and the first thing to do is to warm up the iron thoroughly.

Now, you may need to warm even longer if the iron is new because they are usually packed with some coating to prevent corrosion.

II.Get some space

As the soldering iron warms up, get yourself adequate working space.

You need this space to ensure that you position the iron well to avoid any dripping solder from falling on your body.

Also, have some cardboard on which such dripping solder will fall. Also, place a moistened sponge on in the base of the soldering iron on a stand.

III.Coat the iron tip in Solder

If the tip is not fully covered, the uncovered part usually collects flux residue.

This makes it unable to conduct heat to its best potential.

Therefore, you will need to run the solder all-round the tip until it is fully covered.

It is because you will be applying a lot of solder on the component once you start soldering.

Remember, you need the entire tip to be well coated and ready.

IV.Clean the soldering tip

Once the tip is entirely coated with solder, wipe it off using the wet sponge to remove all the flux residue.

Ensure you do this before the flux can dry out.

D.Soldering the PCB

Now, here are some critical steps when it comes to soldering PCBs:

Step 1: Preparing the Surface

If you want to achieve a strong joint, then you will have to ensure all the surfaces to be soldered are clean before you start applying solder.

As you clean the surface, be keen not to abrade the PCB material. You may use 3M Scotch Brite Pads, usually available at the automotive body shops.

Clean PCB surface

You may also use sine grade steel wool if you feel that there are tough deposits on the board which the pads cannot remove.

However, you have to be careful as you do it because the steel wool can lodge between the holes.

Use methyl hydrate or acetone to clean any bits of the pad that may have remained, and also to remove chemical contamination from the surface of the board.

Ensure you test the solvents first if the board is silk screened because they can remove ink. Also, use hot air to remove any junk that may have remained in the holes.

Finally, also wipe the component leads to remove any tarnish or glue that may have accumulated there over time.

Step 2: Component Placement

When placing the components, start with the smaller and flatter components like the resistors and the signal diodes.

Then, move to larger and taller ones like the transformers and the power transistors.

Fixing the smaller elements between the larger ones may prove to be difficult if you began with the larger.

Therefore, starting with the smaller is to ensure that the board remains relatively flat and the iron can easily reach the board to solder the leads onto the holes.

Also, the more significant components, once placed, may make it difficult to turn the board when putting more components.

To make it worse, they may even damage the board if not handled with precision.

Some components are also more delicate and sensitive than others.

Placing components – Photo courtesy: PCB Way

These should be kept until all the hardier components have all been soldered.

It is because they risk getting damaged in the course of placing and soldering other components if they are placed earlier.

When placing the components, insert the leads through the right holes on the board.

Then, bend the leads appropriately to hold the components in place.

Let the bend rise slightly above the soldered area to minimize the amount of heat and solder required.

If the leads are too short to bend, you may hold the component in place using masking tape.

Step 3: Heating the Lead and the Pad

Heating lead pad

Apply some solder to the tip of the soldering iron to help in conducting heat to the board and the component, and to create contact between the board and the lead.

Rest the tip of the iron at the meeting point of the component lead and the board.

Ensure both the board and the lead catch sufficient heat.

The joint should be ready to solder after one or two minutes.

Do not heat the pad until it starts to bubble beneath.

Step 4: Apply Solder To The Joint

After heating the lead and the pad, bring the tip of the solder to the component lead and the solder pad.

Then touch with the tip of the iron. The solder should flow around the pad, and the led, so long as the two have been adequately heated.

Soldering

Add more solder until it coats the pad completely and forms a small mound.

Once this is achieved, stop adding solder, and then remove the soldering iron.

Do not move the joint to allow the solder to cool and solidify.

Moving the joint before the solder fully resolidifies leads to cold joints, which usually have a dull, grainy appearance.

If this happens, apply some little solder again and allow it to cool well this time.

Step 5: Joint inspection and Cleanup

When you are done with soldering the joint, inspect for any cold joints or poor flow.

Trim the lead above the solder joint.

Check resistance using a meter and examine the joint using a magnifying glass to ascertain if the process was perfect.

Testing PCB

You will also need to clean the entire flux residue from the board using methyl hydrate and a rag, though some may require stronger solvents.

If not well cleaned, the flux may absorb water and become a conductor since some fluxes are hygroscopic.

After you have cleaned all the flux, use hot air to blow dry the board.

How to Hire Through Hole PCB Assembly Service Provider

Finding reliable through-hole PCB assembly services is never a difficult task as long as you know the specifics of how you want the job done.

Several through-hole PCB assembly companies are easy to find online, but there are also small-holder PCB assembly workshops in almost every town.

Below are some of the main aspects you need to consider before you hire a through hole PCB assembly service provider:

• Have well-trained staff that are able to provide consistently high-quality assembly services.
• Have attention to detail: the ability to accurately interpret the customer’s requirements is very important in ensuring that customer expectations are fully met by the final product.
• Cost-effectiveness- A good PCB assembly provider is one that charges reasonably for their services.
• Time-The best Assembly company is one which has the capability to clear the work within the specified time.
• Reliable-Should be one that you can always trust to deliver the best quality Assembly services.

Capability of the Through Hole Assembly Service Provider

Some of the main capabilities to consider include:

·Wave Soldering

Using a wave soldering machine enables bulk soldering.

This allows the assembly service provider to assemble several boards within a short time that cannot be achieved by manual/hand soldering.

·Hand insertion of components

There should be enough skilled workers with perfect knowledge on the best component insertion procedures.

It will ensure the components are always inserted accurately

·Hand component soldering

Hand soldering is usually error-prone.

Only the most experienced and skilled soldering specialists should be trusted with hand-soldering.

·Conformal coating

Should be able to add a conformal coating material to protect the components and the board.

This is usually done using thin polymeric films along the PCB contours.

The films are waterproof, moisture proof and provide corrosion protection to the PCB

·Potting

Also called PCB encapsulation; it helps to harden and permanently protect the assembly.

It offers complete protection because it gives both electrical and mechanical stability to the PCB.

To achieve this, the assembled board is dipped into a chemical.

It can be left there for some time so that it hardens enough to withstand environmental pressures and “threats”.

·ROHS soldering

With governments pushing towards reducing the amount of lead getting into the environment, companies are rushing to adopt ROHS (reduction of hazards substance) soldering.

However, due to the challenges that come with lead-free soldering-lead-free solders require very high temperatures to melt-some still use lead solders.

In sourcing for through-hole assembly services, therefore, ensure you confirm that the service providers have embraced ROHS soldering.

·Ability to provide PCB prototypes

To be sure that the assembly services will meet your demands, the service provider should be able to offer prototypes for your confirmation before full assembly commences.

The accuracy of the prototype building will also let you know the other capabilities of the service provider.

They should be ready to do this for all volumes of assembly.

·IC Programing

Some boards require to be programmed with instructions.

You should therefore confirm that the service provider has the capability to work with programmable ICs.

Besides, they should correctly feed them with your defined instructions.

· Functional tests and automated inspection

Using automated inspection is very critical in minimizing turnaround time to ensure timely delivery of the boards.

Conclusion

As you can see, Through Hole Assembly of PCB can be time consuming.

However, in some applications, it is worth it more so when mounting heavier and bulkier components.

You can easily achieve the strength by soldering the components on the reverse side as you have seen.

In fact, it is virtually impossible to pull components assembled using Through Hole Assembly.

The good news, today’s guide has simplified the Through Hole PCB Assembly technique.

Now, it’s you turn…

Do you have questions or inquiries on Through Hole Assembly process?

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