The Effect of FR-4 Laminate Materials on the Surface Insulation Resistance of Wave Soldering Fluxes
Authors: Karen Tellefsen, Anna Lifton, Mike Previti, and Mike Murphy Company: Alpha Assembly Solutions Date Published: 6/6/2017
ICSR (Soldering and Reliability)
Abstract: It has been observed that, since the implementation of Pb-free processing, different flux chemistries produce different surface insulation resistance (SIR) results depending on the test coupon laminate used. The most likely reason for these results are differences in the FR-4 laminate chemistry used in the test coupons because of the higher processing temperatures used in Pb-free assembly. SIR has been measured per IPC SIR IPC-TM-650 methods 126.96.36.199 (J-STD-004B) and 188.8.131.52 (J-STD-004A) for several fluxes and types of laminates. In general, the less expensive FR-4 materials with lower glass transition temperatures (Tg) gave higher SIR values when wave soldered comb down with most of the fluxes. The more modern FR-4 materials with higher Tg gave the lowest SIR for comb down wave soldering. These high Tg laminates are sometimes filled with talc and similar hydrophilic materials. For some fluxes the difference in comb down SIR values between the best and worst laminates was 2 orders of magnitude. The effect laminate type had on comb up wave soldering was much smaller, and sometimes the higher Tg materials were a little better than the lower Tg materials. Reflowed solder paste SIR was also higher on the low Tg materials than the high Tg materials but to a much lesser extent than comb down wave soldering fluxes.
Many wave soldering fluxes with rosin content, even as little as 1%, pass SIR testing per IPC-TM-650 when soldered comb-down on the high Tg FR-4 laminates. In general, the fluxes with higher rosin content have the highest SIR comb-down on these more modern laminates.