Issue link: https://iconnect007.uberflip.com/i/745476
64 SMT Magazine • November 2016 new lead-free capable materials. What this tells me, though, is that the boards that were passed through lead-free assembly initially would have passed electrical and functional testing, and would then fail later on in the field (latent failure). It seems like this portion of the seminar has been swept into the dustbin of history. Most folks today are only focusing on whether or not the PCB delaminates after assembly. If there is no delamination then the board is deemed reli- able. But is that the whole truth? Study for Via Life Impact I designed an experiment testing this no- tion that an unbaked PCB that did not delami- nate during initial testing is as reliable as a PCB that has been properly pre-baked prior to as- sembly. Here are the details: Goal: The goal of this study is to determine the effect of properly pre-baking PCBs made from IPC 4101/126 or /129 laminate destined for lead-free reflow assembly. Test Vehicle: For this study, we will use the Interconnect Stress Test (IST). This test uses spe- cifically designed coupons that represent spe- cific technologies. Electrical current is used to generate a thermal cycling environment that stresses the vias. Resistance is measured to de- termine via failure. Our test panels represent a standard 8-lay- er PCB with 0.012" and 0.010" drilled hole siz- es, representing a "heart of the range" of tech- nologies. Sample Preparation: We will pre-con- dition the test vehicles using three passes at a peak 245°C lead-free reflow profile. Prior to this reflow pre-conditioning, how- ever, we will add a moisture conditioning step as follows for three groups: Group A: This sample set will be pre-con- ditioned by going through 8-hour live steam aging with five gallons of water. This will rep- licate one-year of shelf life under worst condi- tions. Group B: This sample set will be pre-baked for four hours at 150°C, per Isola guidelines for lead-free laminates. Group C: As is. These samples will be tak- en from a manufacturing environment, pack- aged, and dwell on a shelf for 30 days to repli- cate a standard dock-to-stock scenario. IST Testing—Methodology [1] The principles behind IST are actually quite simple. A current is passed through specific traces on a specially designed coupon. This cur- rent generates heat in those traces, which then heat the surrounding area to include vias, pads, laminate, etc. While the coupon is being ther- mal cycled in this manner, resistance is contin- ually being monitored through the other traces and vias. Once the change in resistance mea- surement exceeds 10%, the coupon is deemed a failure and the number of cycles to that point are counted as passed cycles. Typically, the test is concluded after passing 500 cycles. IST Testing—Results Test 1: 8-hour steam + 3X Pb-free reflow: TO BAKE OR NOT TO BAKE