I used an off-line paste height measurement device to control my paste process. I did not inspect every board, just a random sample once an hour. I have used both manual and automatic devices. I like the automatic becuase: 1 - I have no "operator" variation. 2 - I get more average sample readings than just 5 pads per board. 3 - I get the increased readings in less time.
I let the automatic 2D systems on the screen printers monitor the process in between sample readings.
The paste process is THE most important part of your SMT process. The paste process can be subject to many different factors. Temperature, humidity, time on screen, and many other factors interact to constantly affect the paste. What works this morning may not be working this afternoon. Plain visual inspection is not enough to control the process. The paste process can be at the root of so many types of defects. Control chart it, monitor, control it, and your SMT process will go MUCH better. If you don't control it, you will always have variations in your process. How good do you want the process to be. At one company, we had it under 50ppm, and that includes doing 20 mil or less fine pitch. Much of that is the result of monitoring the paste process, and carefully planning all the steps in the process and reducing their variation.
Brian W.
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