Engineering & Design

The switch to lead-free assembly affects virtually all aspects of the Engineering function. Engineering personnel will have to pay close attention to design, components, PWBs, solder alloys, fluxes, and the printing, reflow, wave soldering, rework and cleaning processes and equipment.

Established PCB-design rules may need to change during the transition to lead-free soldering. Currently, industry guidelines govern component lead-pad and land size, track width and spacing, via and through hole dimensions, and similar factors to ensure manufacturability and reliability.  However, the physical characteristics of any solder include subtle factors, such as its ductility and elasticity. In addition, the local heating of component leads and their pads causes some thermal expansion during operation, which tin-lead solder accommodates and matches.

In determining design solutions, Design should try to remain with as many standard parts as possible. This will reduce the unpredictability encountered with atypical parts. In addition, if the assembly is designed to have a long life, factor in the reduced moisture resistance of parts. Furthermore, Design must factor in the higher temperatures required for connectors.


Material Considerations
The first critical duty is to ensure that the parts to be used will be compatible and reliable for their particular application. Compatibility relates to components , PWBs, solder alloy and flux. Reliability relates to component concerns, which includes such factors as Moisture Sensitivity Level (MSL) Rating, wetting and tin whiskering.


Component Reliability Concerns
The higher melting temperatures of the lead-free solders that are coming into use mandate components that can withstand the increased temperature stresses of the soldering process. Life -test data for many components at these higher temperatures is less comprehensive than it is for tin/lead processes. To maximize reliability, Engineering should start looking now at all critical components, design rules, fabrication processes, component engineering, and reliability records.

A critical factor in the transition to lead-free assembly is the MSL rating of components. To date, industry testing has demonstrated that there is no generic solution for maintaining an IC's MSL with a higher reflow profile. However, it has been demonstrated that degradation of MSL may increase with increasing profile dwell above 200 C and that MSL typically degrades by one level for every 5 to 10 C increase of peak reflow temperature. Therefore, all ICs must be reclassified for lead-free applications and the impact to MSL. This could result in an increased need to pre-bake parts and more stringent storage methods.

It should be noted that these different materials have different wetting characteristics and that Engineering should consider wetting when specifying components. Engineering also needs to balance the fact that increased reflow temperatures can improve wetting, but worsen reliability. In addition, Design should be aware of reduced solderability on second-side reflow and through-hole processes.


PWBs / PCBs
Several PWB lead-free surface finish options exist. Many of these, such as OSPs and Au/Ni immersion, have been available for years. Engineering should determine the finish of choice based upon wetting, storage, planarity and cost issues. In addition, it must be ensured that board materials can withstand reflow temperatures without warpage or other damage. For many cases, FR-4 will remain acceptable, but other applications may require a modification.