The effects of lead-free on PCB fabrication: assemblers may bear most of the brunt of the movement away from lead, but board shops won't be getting off unscathed. Materials, finishes and processes are all likely to be affected.

The push for lead-free solder assembly has led to many investigations into PCB materials and finishes. These are primarily based on discerning the capability of the materials to withstand the higher temperatures of the lead-free soldering process.

This new lead-free arena brings different PCB materials and surface finishes to the table as well as specific changes required in the PCB manufacturing process to support lead-free compatibility. It's important to understand the effects these different materials will have on PCB fabrication, as well as the need to respond to these changes.

PCB materials must be able to withstand lead-free soldering temperatures of up to 260[degrees]C during the assembly operation. Some materials, particularly FR-4, are not capable of withstanding the elevated temperatures required to melt the lead-free solder. And the upcoming Restriction of Hazardous Substances in Electrical and Electronic Equipment (RoHS) Directive deadline of July 2006 is raising concerns about PCB laminates.

Certain materials that are relatively new to the market and are considered to be high-reliability type FR-4, sometimes called phenolic cure materials. These phenolic cure materials have higher temperature resistance to be able to support the 26[degrees]C peak soldering temperatures.

As these materials enter the market, fabricators need to start characterizing how well the materials are processed in their facilities and how they compare in cost and electrical performance. Fabricators also need to characterize factors such as conductive anodic filament (CAF) growth in order to deem the material a direct replacement for the standard (not lead-free) FR-4 resin systems.

There are some phenolic cure lead-free compatible materials on the market that can support the lead-free soldering operations, and a few are more beneficial for the assembly process than others. TABLE 1 lists various types of materials that are currently being considered.

In some instances, the solution lies with materials from the high-performance category. Because of lead-free soldering compatibility issues, a standard FR-4 material could be replaced with a higher-performing material with a resin that's more resilient to high temperature processing. However, these materials can, and probably will, add cost and affect the performance of the finished product. Design modifications, because of the higher-performance material, may be required.

Taking this avenue is beneficial when manufacturing high-performance daughter cards as well as high-performance backplanes that are high in layer count and thickness, and that take the soldered components.

A Few Good Finishes

Right now, a small selection of lead-free PCB surface finishes is available: Electrolytic nickel/gold, electroless nickel/immersion gold (ENIG), immersion tin, immersion silver and organic solderability preservative (OSP). Currently, these are the only finishes commercially available for lead-free applications.

These surface finishes must be able to withstand the elevated soldering temperatures required for lead-free processing. Silvers are lead-free soldering-compatible (if processed properly) and high-temperature OSP coatings are capable of supporting these temperatures.

Immersion tin is also compatible, although it is primarily used for back-planes. Typically, an immersion tin surface finish is not used for a non-back-plane-related product because of concerns about metallic growth between traces (commonly known as whiskering). See FIGURE 1. That could exclude immersion tin from the leadfree soldering arena for daughtercard-related circuit boards.

[FIGURE 1 OMITTED]

Will these finishes change the cost of the final product? Probably not. However, each finish does have its advantages and disadvantages. For example, ENIG offers better planarity and a longer product shelf life, but the solderability of components is less desirable. A finish that contains silver provides an easier surface for soldering, but has decreased planarity and a lower shelf life.

What about operational changes? Depending on the materials selected, the fabricator may need to add additional surface finish processes or outsource processing to support the changing customer requirements. The PCB fabrication industry is shrinking due to Asian expansion, and fabricators may not have every surface finish process at their disposal.

The other option would be to out source, which could increase cost, cycle time and quality management. Also it's important to note that even though the legislative mandate for lead-free solder is still moving ahead, only 17% of global assembly manufacturing has converted, in large part because full conversion will run into the tens of billions of dollars.

An important consideration moving forward is material compatibility with the PCB manufacturing processes. When changing the additive to the resin system, the user must first investigate whether the material is interchangeable or backward compatible with legacy programs. Will it be a plug-and-play system? In other words, how will these newer types of materials, such as the high-reliability FR-4s, work in systems already in place in the field? Users of these new lead-free materials will need to run simulations or do testing to ensure compatibility.

Materials Testing 

As the RoHS compliance date of July 1, 2006 approaches, many new material systems are being tested and developed to satisfy lead-free soldering parameters. Tin, silver and copper (SnAgCu or SAC) alloys are the predominant choice to replace tin/lead (SnPb), with a melting point of 217[degrees]C vs. 183[degrees]C for SnPb. The qualification of material deemed lead-free must be able to survive multiple 260[degrees]C reflow cycles without impacting reliability (per iNEMI criteria for lead-free reflow).

The following are some factors to consider when choosing materials:

* Elevated reflow temperatures threaten through-hole reliability and increase the risk of delamination.

* Higher aspect ratios increase reliability concerns.

* Larger, high-layer-count boards have significant thermal mass, which taxes the limits of reflow equipment to safely reach peak temperatures.

When exposing laminates to higher temperatures, glass transition temperature (Tg), coefficient of thermal expansion (CTE) and decomposition temperature are data points to consider.

The glass transition temperature of the resin system is the temperature at which the material transitions from a rigid state to a softened state. Tg is the material property typically used by industry to compare thermal robustness of laminates; however, it is not a good indicator of lead free compatibility.

CTE is a measure of the material expansion both below and above the Tg expressed in parts per million (PPM). Lower CTE values increase PTH reliability. With the elevated temperatures associated with lead-free soldering, it is now more important than ever to focus on the CTE rate above the material Tg.

Decomposition temperature (Td) is a value derived by measuring the weight loss of the sample vs. temperature. Td is an important material property used to assess thermal survivability. Material decomposition temperature is reported at a different percentage depending on the laminate manufacturer. Material decomposition temperature is the temperature at which material weight changes by 5%. And, as test data from Polyclad shows in FIGURE 2, decomposition can start at a much lower temperature. Even a 2-3% loss, especially when exposed to multiple thermal cycles, can degrade thermal reliability. This decomposition point is critical when determining reliability.

[FIGURE 2 OMITTED]

Most design issues are tied to material choice and finish. The primary alloy of choice looks to be a SAC alloy (Sn96Ag3.5CuO.5). However, the process window for lead-free is smaller and fragility is a big concern.

Most likely, every market will need to go lead-free because device manufacturers are not likely to support two component finishes--one with lead and one without. For the military and defense markets, for example, this presents a dual problem. Much of the defense market is typically excluded from the Rolls lead-free soldering requirements. However, as component manufacturers begin making lead-free products, the defense applications will ultimately be affected because no other type of soldered products will be available. Although lead-free soldering itself is compatible with a board that contains lead, there may be some cases when defense contractors will have to change their whole thought process for PCB assembly, depending on types of solder used in wave soldering equipment.

With the RoHS initiative in play, we will continue to witness a domino effect throughout the industry that will affect everybody, including fabricators that primarily supply the defense industry, because they have to supply product with lead-free compatible surface finishes.

When changing materials that have different thermal and electrical properties, how will you know that your design will function correctly? The answer is, you don't. A preparedness checklist (see sidebar) is essential to determining the costs and technology required to convert to lead-free materials and processes and ensure that the product will work.

In the move toward lead-free, it all boils down to the customer and assembler (contract manufacturer) being aware that some materials aren't able to support the higher temperatures required for assembly. While understanding the reflow process and profile --peak temperatures, ramp and effects on reliability--and running multiple pass assembly simulation tests are essential, a collaborative effort by the OEM, contract manufacturer, fabricator and laminate supplier is also beneficial for achieving a smooth transition toward compliance in 2006.

TABLE 1. Suppliers of lead-free soldering-compatible materials

            MATERIAL                                TG
SUPPLIER      TYPE         RESIN SYSTEM      ([degrees]C DSC)

Polyclad     FR 226            FR-4                140
             FR 370            FR-4                170
            FR370HR     FR-4 with Phenolic         180
             Getek             PPO                 180
             LD 621            APPE                190

Isola        FR 406            FR-4                170
             FR 408       Modified FR-4            180
             IS 410     FR-4 with Phenolic         180
             IS 415       Modified FR-4            220
             IS 620       Modified FR-4            205

Nelco      N4006-6FC           FR-4                175
            N4000-11    FR-4 with Phenolic         175
            N4000-13      Modified FR-4            210
            N4000-12      Modified FR-4            190
           N4000-13SI     Modified FR-4            200

            MATERIAL            TD              LEAD-FREE       HALOGEN
SUPPLIER      TYPE       ([degrees]C TGA)       COMPATIBLE       FREE

Polyclad     FR 226            320                 No             No
             FR 370            310                 No             No
            FR370HR            350                 Yes            No
             Getek             340                 Yes            No
             LD 621            400                 Yes            No

Isola        FR 406            290                 No             No
             FR 408            360                 yes            No
             IS 410            350                 Yes            No
             IS 415            360                 Yes            No
             IS 620            330                 Yes            No

Nelco      N4006-6FC           300                 No             No
            N4000-11           360                 Yes            No
            N4000-13           350                 No             No
            N4000-12           370                 Yes            No
           N4000-13SI          350                 No             No

Note: Data obtained from product data sheets

Material Checklist

* Number of reflow cycles at the higher temperatures that the material will be able to withstand.

* Additive effect of multiple cycles on the material.

* Glass transition (Tg) temperature.

* Higher temperature resins demanded by higher temperature reflow processes.

* Z-axis expansion rate of the material.

* Impact of PTH reliability.

* Decomposition temperature of the laminate.

* Understand materials are rated to withstand high temperatures for limited times. Extreme temperatures can break chemical bonds, and weaken material.

Design Checklist Laminate

* Is the glass transition temperature rating more than 170[degrees]C?

Reason: CTE: Z-axis expansion, via and PTH reliability, through-hole copper plating thickness and potential delamination.

Final finishes

* Is the finish capable of multiple reflow cycles? Reason: Some finishes degrade more rapidly at lead-free assembly temperatures and solderability and solder joint reliability may be reduced.

Circuit panelization

* Will the board layout on the panel be affected by the higher temperatures?

Reason: The number of support points may need to be increased (e.g., for route and retain assemblies). New or alternative board retention strategies may be needed.

Board marking

* Will the board have a lead-free designation mark or marking?

Reason: Some certification markings will likely be required for ID purposes, and they must be legible following processing.

Component Checklist

* Identify materials used in component construction.

* Need temperature rating of 220[degrees]C for 90 sec with 260[degrees]C max.

* Choose finish: (e.g., ENIG, Sn, Ag, Pd, etc.).

* Evaluate and choose suppliers with required capabilities.

* Check JEDEC moisture sensitivity level.

* Verify component performance.

* Verify finish solderability.

* Establish ID protocol for marking lead-free components.

Logistics

* Identify and segregate or purge temperature-sensitive materials, and inventory control/segregate lead and lead-free finishes.

* Dispose of components that can't be converted.

* Assure document changes are correct.

* Develop timeline for implementation of required changes.

REFERENCES

Information on the Rolls Directive can be found at: http://europa.eu.int/eurlex/pri/en/oj/dat/2003/ 1_037/1_03720030213en00190023.pdf.

The Impact of Lead Free Design--PCB East, Joseph Fjelstad SiliconPipe, Inc., San Jose, CA.

BOB MCGRATH is a high-performance circuits design and applications engineer with Teradyne Connection Systems. He has more than 26 years of experience in printed circuit board manufacturing, with expertise in both backplane and daughtercard technologies. McGrath has a BS in industrial technology from Northeastern University. He can be reached at [e-mail].