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104 SMT007 MAGAZINE I FEBRUARY 2020 of defect levels in surface mount assembly, and he set out to explore and explain basic facts and to suggest practical test methods. "Understand the materials and the differences in the mate- rials that are offered to you. Let's understand what's actually happening with solder paste," he said. Willis made it clear that all major sup- pliers to the industry provided good and consis- tent products, and that the variations or specific features of one product over another might be difficult to detect or just unique to a company's specific requirement. What classes of solder paste were avail- able? Willis listed solder pastes for printing, dispensing, jetting, dipping, and rework and repair. He commented that most paste types were designed for a no-clean process, but because an increasing number of customers preferred to clean, then techniques were being developed for cleaning "no-cleans" and sev- eral pastes had been specifically designed to be cleaned. Regarding storage, provided stocks were carefully controlled first-in, first-out, and used within a few days. Refrigeration was gen- erally not necessary, and containers of paste could more easily normalise in the production environment. Stencil printing had been established as the standard application technique, but as fea- tures became finer, dispensing and jetting had become increasingly used as alternatives: they're slower but independent of hard tool- ing and adaptable to very small batch sizes. Dipping paste was very fluid and exceptionally good for rework and repair on area-array pack- ages. The component was dipped in the paste and placed on the substrate. Willis showed illustrated examples of jetting and dipping for package-on-package assem- bly and rework, using video clips to demon- strate joint formation with traditional j-lead and gull-wing packages. He described a video reflow simulation technique capable of repli- cating the profile of a convection reflow oven, permitting direct monitoring of actual assem- blies with LGA and QFN packages. The addi- tion of X-ray enabled the technique to be used for process optimisation and fault-finding on area-array packages. X-ray simulation was also effective in characterising the formation and movement of voids, particularly in inac- cessible areas, such as via-in-pad or underfill, where there was no direct escape route. It was also useful for studying the characteristics of pin-in-hole intrusive reflow. Next, Willis described simple but effective methods for a quick, initial assessment to char- acterise the property or effect and understand the nature of the problem and its probable causes before spending time and money car- rying out an in-depth investigation. In many cases, these simple, shop-floor test methods had been used in the industry for solder paste evaluation since the pioneering days of SMT. Semi-automatic equipment was now avail- able for testing solder paste in line with IPC standards, which allowed solder paste tack and slump, wetting, and solder-balling to be assessed and measured under a microscope and recorded on video. In the absence of specialised equipment, an effective shop-floor comparison test to give an indication of initial tack and usable tack-time was to print a series of paste samples, place representative components after various time intervals, tilt the substrate to a steep angle, and observe any slippage or loss of adhesion. Wil- lis commented that some pastes could main- tain their tack for several days after printing. It was interesting to observe the extent to which components could move out of position as the Solder balls after reflow.

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