Authored By:
Paul Reid M. Sc.
PWB Interconnect Solutions Inc.
Nepean, Ontario, Canada
Summary
It should be noted that this is an overview paper that represents the early stages of an ongoing investigation into the causes and effects between conductive anodic filament (CAF) formation and printed wiring board (PWB) material damage. Our belief is that certain or specific types of material damage can increase the propensity for CAF formation.
The preliminary data collected suggests is that there is no statistical correlation between the general definition of material damage (cohesive failure) and CAF. The resulting dichotomy is that we find no CAF failures in some coupons that have obvious material damage and we find CAF failures in coupons that don't exhibit material damage.
Since the advent of the European Union's legislation for Restriction on Hazardous Substances (RoHS) lead (Pb) was removed from solder in surface finishes and pastes used in the component assembly process. The alternative metals and alloys to traditional tin/lead (Sn/Pb) solder required that the assembly temperatures be increased to achieve the higher melting point of the lead free solders.
The traditional assembly temperature reached a level of 230 degrees C, lead-free can require up to a maximum of 260 degrees C, although most assembly houses are using a more modest 245 degrees C. Multiple exposures to the additional 15 degrees C to 30 degrees C has demonstrated a negatively impact to the integrity of the FR4 and halogen free dielectric material used in PWB substrates.
Quantification of material damage is now possible through new techniques that utilize capacitance measurements to identify specific levels of bulk capacitance change that signify degradation within the resin system. This technique was employed to non-destructively identify both the locations within the construction and the magnitude of the change, traditional microsectioning was completed to confirm the results of the capacitance testing. This new technique, including equipment used is described.
Many of the commercially available materials have not demonstrated sufficient robustness when exposed to multiple lead-free assembly and rework thermal excursions. The reality is that these higher assembly and rework temperatures are increasing the risk of material damage. One would naturally expect that the increasing levels of material damage would produce an opportunistic path that would provide an increased possibility for CAF growth. In order to understand this very complex environment it is necessary to lay the ground work for how and effective quantification can be determined.
This paper reviews the results of some initial work, our strategy for improved test vehicles design, including features for measuring material damage and CAF formation, the assembly and rework environments, the material and CAF testing methodology and the protocols that will be used. Our ultimate objective is to establish whether correlation can be found between the various types of material damage and the propensity to CAF failure.
Conclusions
Based on these results there is no correlation between the presence of material damage and CAF failures during testing. This is a counter intuitive finding. What one would expect is that once there is an opportunity (path) that CAF should form relatively easily. Since we don't find this to be the case it becomes problematic in understanding what is happening.
Various hypotheses are possible that explain the atypical results. It may be that the size of the material damage, or the type of material damage has a greater affect on CAF growth. It may be that crazing produces a more effective path for the formation of CAF, while adhesive delamination and cohesive failures do not provide an environment for CAF formation. It may be that adhesive delamination and cohesive failure produce cracks that are too large for conductive materials to "fill" the crack, while crazing produces a capillary action that readily moves water into the dielectric.
The uncertainty of root cause is sufficient grounds for further investigation; we are committed to developing the tools and techniques necessary to ultimately answer this question.
Initially Published in the SMTA Proceedings
|
