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Page 72


www.us-tech.com


Using Continuous Vacuum Reflow to Reduce Voiding... Continued from page 65


splatter. If it is too slow, time is wast- ed and the time above liquidus (TAL) is extended. The vacuum level is the amount


of vacuum the board is exposed to. Levels as low as 1 Torr and as high as


250 Torr are possible. But, even with very large vacuum pumps, it takes longer to go to low vacuum levels. Vacuum hold time is the time


the board is exposed to the vacuum level selected. Hold times can be as short as a second or as long as a few


minutes. At 1 second there is little time for the voids to exit and greater than 40 seconds have been proven not to be necessary. The equalization rate is the


speed at which the vacuum is released and the chamber brought back to atmospheric pressure. Cur - rently, there is no known ad verse effect with high equalization rates. However, it does take more time to reach atmospheric pressure with low vacuum levels. The total time for the vacuum


portion of the reflow profile includes the four listed items plus the time to transport the board in and out of the chamber.


To Be Skillful


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 control and assurance


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Vacuum Trial Results Mike Meilunas from Universal


Instruments’ Advanced Process Lab (APL), with assistance from BTU, developed a test board for void stud- ies. The board included QFNs/MLFs with various-sized thermal pads, resistors and D2PAKs. A study was designed to evaluate the effect of vacuum levels and hold times on voids with SAC305 solder paste from numerous suppliers, two board fin- ishes and a variety of screen configu- rations. A ramp to peak thermal profile


with a linear heating rate of 3.6°F/s (2°C/sec) and a peak temperature of 464°F (240°C) was developed. Various levels of vacuum hold times, and pump down rates were evaluat- ed. X-ray analysis showed that pad size, vacuum level and hold times affect the void level in thermal pads. Trials run with 20 Torr displayed the least amount of voiding. The researchers found an issue


with fine-pitched CSPs, when they are subjected to hard vacuum levels. Voids in the balls combine, and instead of escaping to the surface, expand the ball. When the balls make contact with each other, they can stay in contact (stick) and cause bridging or shorts between circuits. Dipping the CSPs in flux made the issue worse. Although the industry is focused


schleuniger.com


on the overall percentage of voiding in thermal pads, the real issue may be the location and size of the voids. With this in mind, Meilunas analyzed


 


   


  


  


     


March, 2020


the voids’ size distribution on QFN - 100s at various vacuum levels and found that once below 120 Torr, the odds of having a void greater than 15


Test board for void studies with QFNs/MLFs, various-sized thermal pads, resistors, and D2PAKs.


percent in size of the total distribution were very slim. The results of this trial show


that vacuum reflow is an excellent way to decrease the void level in thermal pads of MFLs. Levels below 5 percent were obtained with hold times of 20s and vacuum levels of 20 Torr. It was also found that relative- ly soft vacuum levels of 120 Torr and short hold times can still significant- ly decrease voiding. Contact: BTU International,


Inc., 23 Esquire Road, North Billerica, MA 01862 % 978-667-4111 Web: www.btu.com r


The Evolution of Automotive Leak Testing


Continued from page 66


lation, ionized oxygen molecules will be able to flow through the hole and the ions will connect to the inside of the battery cell, without igniting it. This will show a 0 percent insu-


lation percentage reading on the instrument and the operator or machine can then reject the part. The B28 can be automated into a pro- duction line to support fast pass/fail testing of the cell. The faulty part could then be taken to a rework bench for manual leak localization testing by using the Ioniq’s testing wand to find where on the cell the testing needle reads a low percentage on the instrument. ATEQ’s involvement in the bat-


 


 


tery industry does not just stop at leak testing. ATEQ also has several battery testers ranging from charg- er/dischargers, voltage balancers, cell monitoring and diagnostics. In fact, ATEQ is the proud provider of electric vehicle battery module bal- ancers to Renault-Nissan. But, before any battery can be charged or analyzed, it has to be leak tested. Contact: ATEQ Corp., 35980


Industrial Road, Livonia, MI 48150 % 734-838-3115 E-mail: hfranklin@atequsa.com Web: www.atequsa.com r


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