Does the Presence of Components Make a Difference? A New SIR Test Protocol to Characterise a Lead-Free, Electronic Production Process

摘要

Surface Insulation Resistance (SIR) Testing, has been used traditionally tc characterise process materials, particularly solder fluxes. Existing Surface Insulation resistance (SIR) test methods are outdated and ur representative of modern circuit technology. A recent European research programme found that, using the existing international standards, the SIR value of a typical no-clean flux, could be over estimated by a factor of 10 when compared to a test using parameters representative of today's technology. This new test method has, in addition, proved easier, cheaper and faster to perform; the equipment required is now readily available and a new IEC (Draft IEC 6-1189) process characterization specification is soon to be available. Here, in light of these European findings, an updated SIR test method is used to characterise a lead-free and VOC free electronic production process, including board surface finish, solder resist, paste, flux, wire and conformal coating - and using dummy components to more accurately determine their influence on the test protocol. The SIR test method is very simple in concept, and involves measuring the resistance across two inter-digitated comb patterns, whilst the sample is exposed to artificial ageing conditions of heat and high humidity. If a low SIR is seen on the test sample it is likely that the residues, if left on a PCA, will have a regative effect on the reliability of the circuit in the field. Whilst the principle is simple, the successful implementation of a test is not trivial. Historically the test was implemented simply with a single current metering nstrument capable of measuring fractions of a micro-amp. Modern test equipment allows frequent monitoring of a large number of samples at sensitivities of nano-amps or better. This increased sensitivity has resulted in the European group's ability to study the SIR values over a range of track and pitch widths, using test patterns that are located underneath components. It was found that the coupons and voltage gradients defined by present standards, again lead to higher SIR values and fewer failure incidents, as compared to the results obtained for a coupon co nprising a track width, pitch and voltage gradient combination representative of today's technology. The Swedish research institute IVF and Delphi-Delco have shown that synergist c interactions with other process chemistries used in the manufacture of PCAs can affect the resulting SIR and hence the resultant reliability of the product. IPC J-STD-001B Appendix D does tackle the subject of process validation using SIR, but references the same SIR test methods for isolated flux qualification, that need updating.