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FEBRUARY 2020 I SMT007 MAGAZINE 105 assembly entered the reflow zone. Provided the movement was not excessive, surface ten- sion would pull them back into place as the solder joints were formed, but in cases where the substrate was thin or flexible, supporting pallets were necessary to maintain yield. Willis demonstrated a simple test involving printing dots of solder paste on ceramic tiles with an etched stencil and using a microscope and video camera to observe and record their behaviour when reflowed at 250°C. His video examples showed two different pastes com- pared for solder-balling, one considerably bet- ter than the other, although he commented that present-day paste formulations displayed less tendency to balling effects. A somewhat more elegant test was to print paste onto the copper sheet through a stencil based on J-STD- 005 apertures chosen to represent the most dif- ficult features of the design. The IPC-TM-650 Test Methods Manual listed a series of tests for solder paste, including powder size, metal con- tent, viscosity, slump, solder-balling, tack, and wetting. Further, a series of joint industry reference standards (J-Standards) had become estab- lished as the predominant authority for elec- tronics assembly manufacturing worldwide. IPC J-STD-001 covered requirements for sol- dered electrical and electronic assem- blies, describing materials, methods, and verification criteria. IPC-J-STD-004 covered requirements for soldering fluxes with IPC-J-STD-005 addressing soldering pastes and IPC-J-STD-006 covering solder alloys. Having discussed phenomena such as spitting and solder spotting and a technique for observing them using a watch glass, Willis considered ways in which solder paste residues could be assessed, initially by using glass slides to simulate component stand-off height and enable the performance of clean- ing systems to be monitored. Large components presented particular chal- lenges. For example, after surface insu- lation resistance (SIR) testing, it was difficult to optically inspect without removing components and possibly destroying evidence of dendrites. X-ray or back-grinding were possible alternatives. He showed video illustrations of dendrite formation and exam- ples of good and bad SIR traces. Willis examined some of the details of solder paste printing with reference to the require- ments of the IPC-7527 standard for positional accuracy and shape of deposit and demon- strated simple test procedures using plastic film overlays for trial prints to minimise wast- age of test coupons or production PCBs. Turn- ing his attention to the assessment of solder- able finishes on PCBs, he described the solder spot test pattern he had developed in cooper- ation with the National Physical Laboratory, which could be used either to compare the sol- derability of different finishes under different conditions, or, alternatively, to assess the wet- ting characteristics of solder pastes or to com- pare reflow cycles. Willis also discussed "over-printing" in the context of pin-in-paste through-hole-reflow technology, where it was necessary to print a large amount of paste on to the pads in order to fill the complete plated-through-hole with sol- der, but the pad size was not large enough for the total volume required. The technique was to print over-size, beyond the immediate pad Poor paste release from stencil apertures.

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