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108 SMT007 MAGAZINE I SEPTEMBER 2020 convection reflow assembly simulation (IPC- TM-650 2.6.27B) and current-induced ther- mal cycling (CITC, IPC-TM-650 2.6.26A) are explored in this study. Dr. Cauwe began by updating the results of Phase I testing in the ESA program. The results indicated that for the three build-up layers, the semi-stacked inside (L1-L2/l3 stacked) was superior (no failures) compared to the semi- stacked outside (L1/L2-L3) construction for both the 0.8-mm pitch D-coupons and for the 0.5-mm D-coupons using polyimide materi- als. The modified high-Td epoxy SI materials passed at both pitches, but he noted that the single-prepreg used was 25 microns thinner. This was verified in failure analysis that the high Td SI materials strain was 9960 ppm at 190°C compared to the polyimide's 11340 ppm at 210°C. Theoretically, it is 22% lower than the polyimide due to CTE-Z and thickness, ver- ified by modeling. Also, IMEC used finite ele- ment analysis to help determine the effects of microvia interfacial stress. Jerry Magera What was the source of the weak microvia inter- face? According to Jerry Magera, senior staff prin- cipal engineer at Motor- ola Solutions, it all began with the microvia target pad in his presentation on "The Complete Path to Least Resistance." He focused on the often maligned electroless copper process and proclaims that the IPC- 6012E performance specifications for metalli- zation for PCBs of "sufficient for subsequent plating" is too vague, sets low expectations for deposit quality, and is the reason optical microscope views of well-formed electrolyti- cally copper filled microvias fracture during solder reflow thermal excursions. Continuous resistance measurements dur- ing component reflow assembly revealed ther- mally induced microvia failures that were sub- sequently located by cross-section analysis at and in the vicinity of that electroless copper deposit. The fascinating elegance of electro- less copper is not appreciated, relegated today as a transient step in the copper metallization process, with function reduced to a conductive liner within the laser-ablated microvia cav- ity that bridges the target pad to the adjacent copper layer to support electrolytic copper fill plating. However, it must form a metallurgical bond between the target pad and electrolytic copper plating to survive reflow assembly. Four-wire resistance measurements of micro- via daisy-chains confirmed substantial chain to chain variation attributed to the electroless copper deposit in the microvia. The results of four-wire resistance measure- ments completed on simple L1L2 and L3L4 microvia daisy-chains at ambient temperature are presented for samples prepared with pro- duction-ready processes by PWB manufactur- ers. The measurements objectively revealed the actual variation in the quality of the electroless copper deposit lining the microvias that was missed by weight-gain and backlight assess- ments. Published electroless copper deposit thickness ranged from 0.3–3.0 µm for immer- sion times of 4–30 minutes, depending on the process implemented. Structural variations exist in microvia field- failure interfaces on products that had passed existing IPC and OEM validation tests and micro-section inspections but had retroac- tively failed IPC 2.6.27A testing and been investigated using micro-sections produced by focused-ion-beam (FIB) trench-machining techniques. These revealed defects that had been missed by conventional optical micros- copy. Three modes of interface failure had been observed: between electroless copper and target pad, between electrolytic copper fill and electroless copper, and within the electro- less copper. There was often a mix of all three failure modes. Scanning electron microscope (SEM) exami- nation of the surfaces of target pads after laser drilling and electroless copper showed some interesting variations in grain structure, which Magera explained in terms of the different rates of growth of (100) and (111) planes in the face- centered cubic crystal structure of copper. The evidence suggested that substrate morphology,