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.