Pb-free Assembly

An ideal lead-free transition team has trained personnel in purchasing, inventory management, fabrication and process control. Purchasing and procurement specialists are versatile enough to fully understand and track a variety of changing conventions association with Pb-free assembly. Inventory personnel understand differences between eutectic and lead-free components. Particularly, they are attentive to the adverse consequences of mistaking one for the other.

They also maintain stringent control over incoming vendor components from a variety of supplier bases; and once in house, segregate those components and maintain proper inventory of them.

There are differences between eutectic and lead-free assemblies, not only in terms of different thermal profiles, inspection and rework criteria, but also because lead-free assemblies tend to have latent defects that are absent in eutectic assemblies. One of those defects is “whisker formation.” This phenomenon is created when tin whiskers or electrically conductive, hair-like structures grow outward from tin, zinc, gold, cadmium, indium or silver.

Solder joint embrittlement is created over time as a result of a latent defect. This is due to the absence of lead in the joint, which makes the remaining alloy hard. Over time, this hardening alloy can show signs of cracking and micro-cracking. Signs of fatigue are more prevalent in PCBs subjected to motion- or vibration-based applications. This latent defect places greater pressures on process controls and require tighter assembly re-work and inspection tolerances.

Lead-free solder and assembly reliability is achieved if trained specialists carefully implement several process requirements. There are a number of variables that must be taken into consideration to achieve lead-free solder joint reliability. These include alloy melting temperature; alloy wetting and surface tension properties; solder balling and bridging potential increases; and cosmetic effects of flux at higher re-flow temperatures.

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