Lead-free PCB Assembly: The Ultimate FAQ Guide

In this guide, you will find all information you’re looking for about lead-free PCB assembly.

Whether you want to learn about features, specifications, quality compliance, or other information – you will find everything you need right here.

So, keep reading to learn more.

What Are The Benefits Of Lead-Free PCB Assembly?

The Benefits include:

  • Reduction in size of PCB: it has simplified the PCB designs hence boards can be made smaller leading to mass production.
  • Reduction of environmental impact by PCB: the use of RoHS guidelines has made it possible to recycle and prevent lead poisoning.
  • Maintained Reliability of PCB: by introducing RoHS, a significant reduction in PCB problems has been achieved. This has majorly to do with the use of lead-free soldiers.
  • Fine pitch component support: the use of lead-free soldering enables the production of fine pitch and extremely tight PCBs. Small designs can now be used to assemble semiconductors of high density.

Lead Free Assembly Reduces Environmental Degradation Done By PCBs

Lead-Free Assembly Reduces Environmental Degradation Done By PCBs

What Are The Steps Involved In The Lead-Free PCB Assembly?

Assembly of the Lead-Free PCB is divided into two main parts namely the process of pre-assembly and the process of active assembly.

The steps are as follows:

Pre-assembly Steps

There are three main steps in pre-assembly Lead-Free PCB.

The steps have proven to be precise and free of error during the assembly.

  • Profiling

This process resembles the prototype process.

A completed lead-free PCB is taken as a prototype by the manufacturer.

This is to ensure their compatibility for assembly.

The PCB can either be functioning, dummy parts are taken, or a completely non-functioning one.

Their stencils are traced by profiling them for assembly.

  • Solder Paste Inspection

It is very vital to carry out a meticulous inspection on the lead-free solder joint because it has a metallic appearance.

The IPC-610D standards are used to inspect the solder paste and the board’s profile.

Due to the PCB’s exposure to extreme moisture levels, the moisture content is also tested.

  • Bill of Material (BOM) and Analysis of the Components

The client here has to confirm the Bill of Material to ensure all components are lead-free.

Due to their affinity to moisture, the manufacturer has to bake lead-free components in an oven.

Active Assembly Steps

This is where the assembly of the Lead-Free PCB takes place. The detailed steps are as below:

  • Placement of Stencil and Application of Solder Paste

Here, the profiling stage provides the lead-free stencil that is placed on the PCB.

After this, the application of lead-free solder paste takes place using SAC305 as the paste material.

  • Component Mounting

The components are mounted on the PCB immediately after the application of the solder paste.

The placement of the components can either be automated or carried out manually.

In as much as it is a pick and place procedure, every component needs to be on the BOM verification stage.

The labeled components are picked and placed by the operator or machine to their assigned location.

  • Soldering

This is the stage where manual soldering or lead-free through-hole soldering takes place.

Whether it’s Surface Mount Technology or Through Hole Technology, all soldering needs to be lead-free.

It Is At This Step That The Assembly Of Lead Free PCB takes Place

It Is At This Step That The Assembly Of Lead-Free PCB Takes Place

  • Placing the PCB inside Reflow Oven

The solder paste on the PCB has to be heated at extreme temperatures in order to evenly melt it and meet RoHS standards.

They are placed inside a reflow oven to melt the solder paste.

Cooling the melted solder paste at room temperature enables it to solidify.

This ensures the components are firmly fixed in their position.

  • Testing and Packaging

The IPC-600D are the international standards used to test the lead-free PCBs after Assembly.

This step also involves the testing of solder joints.

Testing begins with visual inspection then AOI and finally X-ray inspection.

Before final packaging, a physical and functional test is carried out.

How Does Lead-Free PCB Assembly Compare To Lead PCB Assembly?

During PCB assembly, a soldering process is used to attach components either by through-hole or surface mount technology.

Previously, ten-lead (Sn-Pb) alloy has been the solder of choice for a very long time.

Tin-Lead alloys have a melting point ranging between 183°C to 188°C.

Temperatures may rise up to 235°C during Reflow when attaching surface mount devices (SMDs).

This is because the solder is changing to liquid before cooling down.

Lead-free PCB assembly usually involves temperatures higher than the Sn-Pb solder.

The reflow temperature may go as high as 250°C affecting the PCB construction and the selection of components.

Board components containing very low decomposition temperatures may be permanently damaged when exposed to such heat.

Reflow should be carefully done to prevent component damage since exposure to heat is prolonged.

The main concern for the Lead-Free PCB is the high temperature damaging the PCB.

Rework on the PCB is also a major challenge

Another challenge is the materials because the temperature characteristics of laminates and substrates have to be considered.

What Are The Surface Finishes Used In Lead-Free PCB Assembly?

Some of the available Lead-Free surface finishes used in Assembly are:

i) Electroless Nickel Immersion Gold (ENIG) – it is 2-5µ inches and provides the PCB with a surface that is electrically conductive. It inhibits oxidation and gives aluminum a wire bonding surface when it us used.

ii) Organic Solderability Preservatives (OSP) – when the flatness of the PCB is very extremely vital, then OSP is used as the surface finish.

iii) Hard Gold – they are used to inhibit oxidation and prolong the PCB shelf life when used for gold fingers (30µʺ minimum). The solder should not exceed 17µʺ.

iv) Lead-Free Solder- preferred for solderability in instances where the PCBs flatness is not vital. The thickness ranges between 2-10µʺ.

v) Immersion Silver- it is preferred for solderability when the PCBs flatness is important in mounting components. Its thickness varies between 2-10µʺ.

vi) Soft Gold- used to inhibit oxidation and for wire bonding. Thermionic bonds have a minimum of 30µʺ while Ultrasonic has a minimum thickness of 2 µʺ.

An ENIG Surface Finish

An ENIG Surface Finish

What Is The Importance Of Lead-Free PCB Assembly Regulations?

A company’s failure to adhere to the RoHS guidelines can affect the company in the long run.

Such regulations should be put into consideration when planning to Assemble any Lead-Free PCB.

The regulations are important because:

i) Protect Workers Health

Exposure to lead both in the short term and long run can lead to negative health complications.

They range from loss of fertility to headaches.

Regulations compliant with RoHS protect workers and vulnerable individuals who share their homes with the workers.

ii) Protect the Environment

Pollution to the water, ground, and air supplies can be eradicated by reducing or getting lead in PCBs.

This serves to safeguard public health and protect wildlife while creating recyclable PCBs.

iii) Sale in Critical Locations

Very many countries are currently advocating for the sale of strictly RoHS-compliant electronic products.

Assembling lead-free PCBs broadens a lucrative target market and increases sales.

When Should Lead-Free PCB Assembly Be Used?

There are some important instances where benefits of Lead-Free PCB Assembly reap big:

a) If regularity standards are a prerequisite of the PCB Assembly by the Original Equipment Manufacturer.

b) If the target market is a country or region strictly adhering to the regularity standards.

c) If the human health risk is a major concern and the environment needs protection too.

Lead Free PCB Should Be Used Where The Environment And Human Health Is At Risk

Lead-Free PCB Should Be Used Where The Environment And Human Health Is At Risk

What Are The Reflow Temperature Concerns In Lead-Free PCB Assembly?

The concerns brought about by Reflow during Lead-Free PCB Assembly are as follows:

  • The shelf life of Lead-Free pastes is greatly reduced compared to Tin-Lead
  • Cleaning is very important but with Lead-Free PCB assembly, it is mandatory after reflow
  • During reflow, temperatures reach up to 260°C which increases unnecessary stress on the PCB and oven.
  • In some circumstances, a Nitrogen blanket may be required for good solderability
  • Compared to Tin-Lead solder, the Lead-Free paste does not wet out as fast.
  • The Lead-Free paste is thicker with a bigger tack force. This requires a modification of the speeds.
  • There is a major concern of voiding during reflow]

What Are The Process Issues Arising From Using Lead-Free PCB Assembly?

The mainstream application of various laminate materials and PCB finishes are the major changes for many board manufacturers.

Such kinds of laminates that can withstand extreme process temperatures were not common before.

In the process of moving the PCBs through the process of manufacturing, existing process parameters have to be altered.

Multi-layer PCBs have higher temperatures during their press process with varied process times and pressures.

The speeds and feeds of the drill have to be adjusted as well as the drill hit constraints.

Boards with a high thermal capacity will require longer dwell times and other process changes to be efficiently processed.

The legend ink and solder mask must be free of hazardous material so as to meet the requirements of RoHS.

They should also be able to tolerate extreme process temperatures without experiencing any conspicuous discoloration.

What Are The Manufacturing Areas Affected By Lead-Free PCB Assembly?

Lead-Free PCB Assembly has affected three major areas of manufacturing.

They are:

a) Cleaning the PCB Assembly

The composition of the Lead-Free Solder flux greatly differs from the usual leaded solder flux.

Cleaning materials specific to the lead-free processes are required to efficiently clean them.

This entails the use of a varied range of temperatures outside the regular cleaning process.

b) Temperature

The melting temperature of lead-free solders ranges between 220-240°C compared to lead-based at between 180-190°C.

The difference in temperature exerts extreme pressure on the PCB and the corresponding assembled components.

The PCB can experience multiple failures because the coefficient of thermal expansion (CTE) is heightened by high temperatures.

c) Moisture

During assembly, moisture tends to get absorbed and trapped inside the Lead-Free PCB due to the high temperatures.

This is a major challenge because of the damage to the board and components sensitive to moisture.

The board has to be baked before assembly to prevent delamination and get rid of excess moisture within them.

What Are The Challenges Of OEMs In Making Lead-Free PCB Assembly?

There are numerous reasons for Lead-Free PCB failure after assembly due to the circuit design complexity of each board.

Some common challenges OEMs face during assembly include:

a) RoHS Compliant Components

The surface finish of the Lead-Free PCB needs to have a very high melting temperature.

This means that the required components need to be verified for full compliance with RoHS.

No prohibited materials are allowed in a circuit.

Also, components should be scrutinized well during assembly as some like switches may not work in high temperatures.

b) Ball Grid Array (BGA) Solder

Solder joints are a part of ball grid arrays.

It is a challenge sometimes to match the lead-free assembly type with the Ball Grid Array metallic solder.

Many components are not compliant but still work well.

As for BGA, exposing the solder balls, which are non-lead-free, to reflow and lead-free temperatures leads to their failure.

Ball Grid Array Solder

A Ball Grid Array Solder

c) The PCB Base

The substrate impacts the PCB in terms of its resistance to insulation, the voltage it can withstand, and it’s dielectric constant.

This means lead-based board assembly has to be separated from the lead-free finishing and substrate.

If there is a need for rework, then substrates like FR4 may bring about temperature issues due to repeated heating.

Materials with high-temperature tolerance should be used since they can be processed severally with RoHS compliance.

d) Moisture Sensitive Components

All surface finishes applied to Lead-Free PCBs are sensitive to moisture.

The components coated with such contain an expiry date printed on the moisture-protective package.

Continuous use of expired components by manufacturers may lead to permanent damage caused by moisture.

Open packaged components or expired ones can be utilized in RoHS prototyping after being heated.

 What Are The Factors To Consider When Choosing A Contract Manufacturer For Lead-Free PCB Assembly?

Enough research has to be done on a Contract Manufacturer for Lead-Free PCB Assembly before beginning the process.

Some factors to consider on their ability include:

  • Lead-free Materials

The PCB should be exclusively assembled with materials that are lead-free in order to be compliant with RoHS.

This includes things like solder paste, solder, and every other process requirement including cleaning agents.

The procedure should be fully documented by the CM for RoHS material verification.

  • Assembly Processes that are RoHS-specific

Compliance with RoHS means the PCB can never have lead-based solder materials.

The CM has to have a clear mechanism of separating led from lead-free process to ensure purity.

  • Documentation

The CM has to prove that the Lead-Free PCB has been appropriately built following the RoHS guidelines.

The CM has to have a system that documents all materials, components, and their chemistry as compliant to RoHS.

  How Does Temperature Impact Lead-Free Joints In Lead-Free PCB Assembly?

Lead-Free solder has a significantly low melting point.

The variations in temperature as the solder are cooling affect the solder joints on the PCB.

Thermal protection is presently used to sustain the PCB’s ambient temperature for efficient operations under extreme heat.

The temperature alters with the solder joint’s internal stress and its microstructure leading to solder joint failure.

Electronics majorly fail due to this component failure.

The varied effects of temperature on solder joints include thermal shock, aging, and thermal cycling.

What Are The Lead-Free PCB Assembly Material Characteristics?

After complying with directives from the RoHS, normal solder made of Tin or Lead paste cannot be used such PCB materials.

Solder-free solder paste is used to take them through a rigorous process of soldering.

Soldering at very high temperatures is a prerequisite of this process hence the PCB material should be stable at such temperatures.

This means that the PCB material must exhibit a very high glass transition temperature.

The PCB material should also have a very low coefficient of thermal expansion with a large decomposition temperature.

A low dielectric loss coupled with a high insulation resistance is the electrical properties required.

How Does The Reflow Profile Affect The Process Compatibility Of Lead-Free PCB Assembly?

The peak temperature can be termed as the major parameter in the reflow profile in the assembly of the Lead-Free PCB.

Sufficient reflow temperature is required for melting of the solder, wetting, and copper interaction on the pad.

It also assists in component termination and for the PCB to form a rigid bond after cooling and solidifying.

Basically, a superheat temperature of about 30°C (over the melting temperature) is required.

Due to the component’s thermal stability concerns, ways have to be found to reduce the soldering temperature in Lead-Fee PCB Assembly.

A peak temperature of 235°C is recommended as the peak reflow temperature for large-scale PCB manufacture.

This factors in the robustness of the process, the various PCB finishes, the thermal stability of the oven, and the tolerance.

The profile of the reflow can either be a straight ramp or pre-heat plateau for standardizing the board’s temperature distribution.

How Are Lead-Free PCBs Packaged After Assembly?

After the IPC-600D standards have been tested and visual inspection is done, functional testing is carried out before packaging.

Packaging of the Lead-Free PCBs strictly has to be done using anti-electrostatic discharge bags.

This is an assurance that the PCB will not undergo any static charge interference during transportation due to motion.

This procedure is carried out by experts to ensure perfection and strict professionalism.

How Does Lead-Free PCB Assembly Impact Design Compatibility Of The PCB?

A few major changes may be effected on the design rules when switching t Lead-Free PCB Assembly.

Volume manufacturing is widely ongoing to gather enough data to have a comprehensive answer on the changes.

Changes applicable in wave soldering of Lead-Free PCB Assembly are essential.

This is to incorporate the physical property differences between the Lead-Free and Tin-Lead solder alloys.

The orientation of the board in relation to the soldering direction and other general guidelines are still applicable in Lead-Free Soldering.

Temperature variation across the board is minimized by optimizing the copper distribution and the board layout.

Round corners or overprints in reflow soldering are a requirement to elude corners that are exposed after the reflow.

What Are Lead And Lead-Free Solders In PCB Assembly Made Of?

Lead solder is basically composed of a metal alloy with Lead and Tin acting as the base components.

The fumes and dust emanating from lead soldering is extremely toxic when inhaled.

This is the reason for the lead solder phase out due to its negative health and environmental impact.

It has recently been replaced by the lead-free solder which is recommended by RoHS and given tax breaks.

Modern lead-free solder is composed of Zinc, Copper, Nickel, Silver, and Tin each having a varied percentage of the composition.

The ultimate goal is to achieve a superior solder alloy that produces outstanding mechanical properties in combination with other metal alloys.

Lead Free Solders Are Made Up Of Non Lead Materials

Lead-Free Solders Are Made Of Other Nonlead Materials

What Are The Major Considerations Of The Flux Used In Lead-Free PCB Assembly?

There are three major flux types that are attuned to Lead-Free PCB Assembly.

They are No clean, Organic Acid Flux (OA) and Rosin Mildly Activated Solder Flux (RMA).

The considerations to be taken while using the Fluxes include:

  • The issue of cleanliness is very vital because lead-free assembly is leaning more to no-clean flux.
  • Choice of flux to use should be compatible with PCB
  • Flux used must be easy to clean
  • Activation temperature should be balanced with the alloy melt temperature of the Lead-Free PCB
  • Application of the Flux can either be through brushing on, spraying, dipping, or foaming

How Does Thermal Stability Affect Lead-Free PCB Assembly?

An increase in temperature leads to a mismatch between the coefficient of thermal expansion and the laminate material.

The mismatch also includes the copper and glass fiber.

The effect will be an increase in the copper stresses leading to cracks in the plated copper vias.

This scenario is impacted by factors like the layer count, the material used in the laminate, and the PCB thickness.

Other issues like blistering and delamination brought about by the high soldering temperatures affect the PCB.

Alternate laminate materials with Coefficient of Thermal Expansion such as FR2 can also be used.

For all your lead-free PCB assembly, contact us now.

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