Authored By:
Udo Welzel, Norbert Holle
Robert Bosch GmbH
Schwieberdingen, Germany
Summary
Voiding in SMT (surface-mount technology) solder joints has been a topic of intensive discussions particularly since the introduction of lead-free technology. This is also reflected by various IPC guidance documents, as IPC-7093 (Design and Assembly Process Implementation for Bottom Termination Components) and IPC-7095 (Design and Assembly Process Implementation for BGAs), which contain some information on typical voiding levels and the impact of voiding on assembly reliability. In terms of quantitative requirements and limits for acceptability of voiding, values are provided only for ball-grid arrays in current versions of J-STD001, IPC-A610 and IEC 61191-2.
However, recent discussions during standardization-related meetings as well as the vast amount of papers dedicated to voiding reveal a need for a more extensive coverage of voiding in standards. This is particularly true in automotive electronics, where customer specifications already often contain requirements for voiding in SMT solder joints. This fact obviously calls for a more extended coverage of voiding in the automotive addenda for J-STD-001 and IPC-A610 in the future. To accomplish this, two challenges have to be addressed: (i) a common understanding of the impact of voiding on assembly reliability between different stakeholders in the supply chain has to be established. The design for reliability of electronic assemblies has to cover all aspects, ranging from solder-joint lifetime to other aspects as electrochemical reliability.
Any material and process optimization should thus be oriented towards the goal of an overall assembly-reliability improvement rather than focusing exclusively on the reduction of voiding. Adopting too tight limits on voiding may even impede the implementation of solutions for overall reliability improvements, as has been shown for some cases. Moreover, the notion of voiding does not generally make sense for all types/geometries of solder joints: For flat, laterally extended solder joints, e.g. at exposed pad soldering areas, solder coverage is a more appropriate measure for thermal transfer than void percentage. (ii) typical X-ray inspection systems used nowadays in automotive mass production do not generally satisfy tight requirements on gage repeatability and reproducibility (gage R&R), i.e. such systems do not qualify as measurement systems for voiding.
This can be understood considering that their primary purpose is the detection of soldering defects like bridging, wetting failures etc. For introduction of void level requirements, a certain level of X-ray reproducibility would be required. This paper will provide an overview of current activities in standardization related to the above-discussed challenges.
Conclusions
Voiding in solder joints is not generally a reliability concern unless untypical, excessive voiding levels are taken into consideration. Mass production, as it is required in automotive electronics, however, may result in a few assemblies with elevated voiding levels, as voiding generally exhibits considerable scatter, i.e. the full width of the underlying distribution function for voiding, including its tails, will actually occur in assemblies due to the large part count. This scatter, together with high reliability requirements for automotive electronics, may explain why the discussion on the acceptability of voiding is particularly lively for automotive electronic assemblies.
Consensus concerning voiding has been achieved among various stakeholders in the supply chain of automotive electronics for a set of solder joints geometries considered as most important. This consensus emerged from discussions among various stakeholders in the automotive electronics supply chain within a German standardization working group of the DKE (DKE – Deutsche Kommission Elektrotechnik), DKE AK682.0.7 ‘Assembly and Interconnect Technology in Automotive Electronic Assemblies’, considering typical voiding occurrence, typical environmental loads, required functional performance characteristics and processability conditions of solder joints. The such agreed requirements and criteria may serve as a starting point for discussions among additional stakeholders in the supply chain of automotive electronics.
As of today, X-ray inspection systems have very limited gage repeatability and reproducibility (G R&R) performance. This has also been addressed by working group DKE AK 682.0.7 by initiating a round-robin study on X-ray imaging G R&R performance. This study, in addition to documenting the current state-of-the-art, will also enable conclusions on improvement potentials for Gage R&R performance, to be implemented by equipment manufacturers.
Initially Published in the IPC Proceedings
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