Task force reports on lead-free reliability

iNEMI's High-Reliability RoHS Task Force has published recommendations for Pb-free manufacturing of complex, thermally challenging electronic assemblies.

iNEMI's High-Reliability RoHS Task Force has published recommendations for Pb-free manufacturing of complex, thermally challenging electronic assemblies.

These recommendations, which focus primarily on thermal requirements for components, laminate and PWB materials, and equipment, are intended to help ensure product reliability.

Most high-reliability products have a Pb solder exemption or are out of scope under the RoHS Directive, and the companies that manufacture these products are evaluating the impact of Pb-free SAC solder assembly on the reliability of these more demanding applications.

The complex assemblies found in high-reliability products often feature broad component mixes and pose many thermal challenges as a result of board thermal properties and the resulting thermal gradients across the assembly.

Pb-free assembly is already challenging for manufacturers due to the higher temperatures required and the resulting tighter processing windows.

When assembling complex assemblies, these challenges are exacerbated by the variability of component mass, large size and high layer count of PWBs, the need to rework, and the high likelihood of mixed through-hole, surface mount and back side assembly.

"New soldering materials, maximum qualified component temperatures and primary attach and/or rework equipment all need to be addressed relative to reliability in the face of Pb-free assembly requirements", said Joe Smetana, Principal Engineer, Advanced Technology, for Alcatel and cochair of the iNEMI High-Reliability RoHS Task Force.

"These recommendations were developed to communicate the needs of the high reliability segment to the supply chain and the relevant standards groups that must address these needs".

The following are some of the key recommendations.

Industry standards for components have not yet fully encompassed Pb-free processing requirements, particularly for thermally complex assemblies.

Recommendations address temperature requirements for nonhermetic devices, glued and wave soldered SMD components, wave soldering, and BGA coplanarity requirements over temperature.

The task force recommends specific test methods, including CAF (conductive anodic filament) and thermal/temperature cycling, to validate acceptable performance of materials in the appropriate use conditions.

Manufacturers need to work with equipment providers to integrate and resolve the issues of process speed, peak temperatures, flux chemistry, solder pot contamination and soldering gas atmosphere.

Key issues (from a materials perspective) that need to be addressed include the ability of flux to handle higher temperatures at longer preheat times (135-200C for 3-4min) and total profile times of 8-9min.

In the meantime, there are a few steps that can be taken to improve processing: slow reflow oven speed to guarantee a good solder joint; preheat the whole board to reduce heatsinking effects of PWB power and ground planes; and improve adjacent component "shielding" from hot gas rework temperatures to prevent secondary reflow.

The full set of recommendations are available on the iNEMI website.

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