Udo Welzel, Marco Braun, Stefan Scheller, Sven Issing, Harald Feufel, Robert Bosch
mbH Schwieberdingen, Germany
Driven by miniaturization, cost reduction and tighter requirements for electrical and thermal performance, the use of lead-frame based bottom-termination components (LF-BTCs) as small-outline no-leads (SON), quad-flat no leads (QFN) packages etc. is increasing. However, a major distractor for the use of such packages in high-reliability applications has been the lack of a visible solder (toe) fillet on the edge surface of the pins: Because the post-package assembly singulation process typically leaves bare copper leadframe at the singulation edge, which is not protected against oxidation, and thus does not easily solder-wet, a solder fillet (toe fillet) does not generally develop.
Solder-joint robustness is also increased by the presence of a robustly wettable singulation edge, but this is not the primary benefit (the number of cycles to failure under thermal cycling is typically decreased by up to about 20 %, in the absence of an outer, visible fillet). Users, primarily those involved in the mass production of high-reliability, mission-critical, e.g. automotive, applications, have insisted that a solder fillet be visible at the outer edge of each contact to enable a robust inspection for wetting failures by automatic optical inspection (AOI). The possibility to inspect the integrity of the solder joints by AOI avoids the need to employ X-ray inspection methods, which involve additional costs and layout restrictions, as certain keep-out zones for traces and components are necessary for avoiding disturbing effects in the solder joints X-ray images.
Package suppliers have responded to these needs with various pin modifications that enable a portion of the terminal-edge surface to remain plated after singulation, as two-step sawing or dedicated etching processes. However, for such pin modifications to be useful in the context of AOI under series production conditions, the pin modifications must meet certain geometrical requirements, in order to robustly distinguish a wetted pin ('good solder joint) from a non-wetted pin ('defective solder joint') in AOI. These geometrical requirements will be investigated in this work considering also typical assembly-related process variations. The geometrical requirements enabling robust AOI of LF-BTCs in mass production will be derived.
The influence of the wetting height of lead-frame based bottom-termination components (LF-BTCs) with wettable flanks on the performance of automatic optical solder joint inspection (AOI) has been investigated. Stable wetting up to a certain height can be achieved by using certain pin modification, as step-cut or hollow-groove type modifications, which enable parts of the pin edges to remain plated after singulation. Whereas an experimental assessment of the effect of wetting height on AOI performance would be cumbersome, if not impossible, a simulation-based approach enables a thorough and efficient tool to assess the AOI performance. Various analyses involving also AOI systems from different brands, which make use of different detection principles and image analysis algorithms, demonstrate that wettable flanks must have a minimum wetting height of 100 micrometers in order to enable robust AOI in the mass production of high-reliability electronic assemblies, where low escape rates and low false-call rates are both required.
Initially Published in the IPC Proceedings