Does Copper Dissolution Impact Reliability?

Craig Hamilton, Polina Snugovsky, Mario Moreno
Teng Hoon Ng, Juthathip Fangkangwanwong
Celestica

Matthew Kelly, Marie Cole
IBM Corporation

The process issues relating to high copper dissolution rates have been well documented within the industry. The copper dissolution rates of SAC305/405 plus many of the leading alternative lead-free wave alloys have been characterized and pin-through-hole rework process windows assessed. One remaining gap within the industry is the understanding of how copper dissolution or thinning of the through-hole barrel wall/knee locations impacts the thermal and mechanical reliability of a pin-through-hole joint.

This paper provides a summary of previous work performed to study and characterize the copper dissolution rates of various Pb-free wave alloys, as well as attempts to assess the impact of high levels of copper dissolution on the thermal reliability of a 14 I/O PDIP connector. A total of five PDIP connectors were designed onto a test vehicle, each individually daisy chained using bottomside traces, for in-situ monitoring. A design of experiment was defined as an attempt to vary the level of plated copper remaining after simulating the rework of the five PDIP connectors. Each board was subjected to 0-100C ATC for 6,000 cycles with in-situ monitoring to detect failures throughout testing using data loggers.

The results from this work indicate reliability concerns when a severe degree of copper dissolution occurs and shows signs of reduced reliability due to thinning of the copper plating thickness at the knee location. In addition, mechanical and thermo-mechanical loading are key elements necessary to confirm the reliability of a pin-through-hole joint, this paper however, focuses on thermal fatigue reliability assessment. Lastly, this paper will highlight limitations of the copper plating measurement methodology used and suggests alternative non-destructive options to inspect for copper dissolution within manufacturing environments.

Based on this and other studies on this topic, copper dissolution is indeed a natural phenomenon which occurs during formation of a solder joint to a copper plated surface and there are a number of factors which jointly interact to influence the rate at which the copper surface is dissolved. It is clear from this study, that severe degrees of copper dissolution can have a quality and/or reliability impact.

Even though the inspection methodology had limitations in accurately portraying the exact copper dissolution of the PTH barrels, there were samples which: (1) electrically failed immediately, (2) had early fatigue failure or (3) survived up to 6,000 cycles - each with varying degrees of copper dissolution. These experiments, and the results of other industry work, have shown that having a remaining copper plating thickness of at least 0.5mils provides a reasonable level of plating to resist barrel cracking, under thermal fatigue loads.

This result, however, would need to be verified under mechanical and thermo-mechanical reliability testing to further validate this specification under all conditions. In addition, this work has shown that there are currently non-destructive 3D x-ray and horizontal cross section techniques which can be used to measure copper dissolution on a product, however, additional work is required here.