Reliability of electronic substrates after processing at lead-free soldering temperatures; Part 2.

The requirement to comply with European RoHS regulations which ban the use of lead, is driving the adoption of new materials both in PCB assembly and bare board manufacture. The alloy traditionally used for soldering in electronics manufacture, SnPb, can no longer be used. But the preferred replacement (a tin/silver/copper alloy) has a higher melting point of ~217°C, and concern has been expressed that the higher stresses involved during leadfree processing may affect the reliability of the PC substrates. In consequence this collaborative study aimed to (i) provide the electronics manufacturing industry with a test method for measuring the effect of the higher temperatures associated with lead-free soldering on the reliability of electronic substrates (ii) improve industry confidence in the reliability of PCB substrates during and after the change to lead-free processes. This study investigated the reliability of daisy-chained vias in four commonly used types of laminate materials - both standard and high glass transition temperature (Tg) epoxy systems, with both dicy and phenolic curing agents. The assemblies were conditioned prior to thermal cycling using reflow soldering profiles with peak soldering temperatures of 230, 245 or 260 oC, for up to 8 reflows. The fatigue failures were dominated by barrel cracking, generally in the mid-part of the barrel. Micro-sectioning of failed vias highlighted heavy concertina-like collapsing of barrels, and fatigue cracks through the plated copper walls. These failures were mostly in vias where the stress during reflow was highest (i.e. materials with a high z-axis, high aspect ratio vias, and/or high number of reflow profiles). Electroless nickel and immersion gold (ENIG) finishes were used, and the results show that the electroless nickel was significantly stronger than the copper. In many cases in which copper failures occurred the nickel remained intact. Clear differences in performance were noted between the four laminates; phenolic materials were better than the dicy-cured laminates. All were sensitive to the peak reflow temperature and the number of passes. The z-axis CTE was a better indicator of susceptibility than Tg, for example, it was observed that high Tg and high z-axis CTE samples failed before low Tg and low CTE laminates. The higher the via aspect ratio, the greater was the susceptibility to failure. Failure was monitored as a loss in electrical continuity of the daisy via chains. The electrical resistance was monitored either periodically (every 500 cycles) or continuously with an event detector having a 200nsec response time. The continuous monitoring detected failures at an earlier stage, typically at 50% to 75% of the fatigue life for the first failures for periodic monitoring. This is thought to be related to contraction effects closing fatigue cracks during continuous monitoring - periodic monitoring is carried out only at room temperature.