I am often asked the question about how to handle components that are close to the outer edge of a PCB. Today a question came in on Circuitnet to highlight this problem:
Title: Issues with BGA Components Near PCB Edges
What issues are we likely to see when we place BGA components very close to PCB edges?
What impact might it have on reliability?
Will equipment (screening, placement, reflow, etc.) require modification?
I leave it to the screen printer, pick and place and reflow oven guys to answer the equipment part of the equation, but I can answer how one can determine with a profile if your BGA is getting what it needs as well as how other aspects of your PCB are impacted.
Across the Belt Uniformity:
There can be anywhere from a 2 – 5+C variation in temperature across the belt. For example, BTU uses this homemade fixture to test for uniformity. The idea is fairly simple. With a set of type K calibrated thermocouples, you can easily monitor 6 TCs across the belt. You want obviously to see the least amount of variation (if you were wondering the front TC is for measuring air temperature which is also used for automatic mapping of the profile to the oven zones with KIC2000 software).
Profiling for Reflow:
BGAs typically require more heat to reach their peak temperatures than smaller massed components like electrolytic capacitors. For example, your BGA might have a peak temperature of 245C.
While your electrolytic capacitors cannot tolerate as high as a peak temperature, therefore you want to set their maximum peak temperature lower, for example to 235C (this is just a relative example).
With KIC2000 software, you can define each component in isolation. If the BGA is off on the edge, I might need to bump up even further my peak temperature spec since in many reflow ovens, the outer edge near the rail is the coolest. This is why you see some ovens with heat tape running along the rails! Keep in mind of course as you crank up your oven to reach higher temps to reflow your outer edge BGAs, everything else on your board is also going to be impacted. More the reason you better be hooking up thermocouples to temperature sensitive components to ensure they do not get fried while you are focusing your attention on your BGAs. Profiling software that can “balance” the board is a must. If there ever was a case where software can help solve complex problems in profiling, here you go!
I had a webinar back in July talking about BGA profiling. There is also a video that illustrates what I explained above. You can find this in an earlier posting: http://profilingguru.com/reflow/profiling-bga-webinar/