|Ask the Experts|
October 17, 2017
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?
|Expert Panel Responses|
Thermal Profiling is the key to determine if you are going to have a reflow problem with a BGA, or any other component, near the board edge.
A careful thermal profile of the board and BGA near the edge as compared to the middle of the board will need to be performed to characterize the nature of the oven near its rails. Many reflow machines run several degrees cooler near the rails, and as you know, this can make the difference between a good and bad BGA reflow process.
Place T/Cs at all four corners of the BGA and one or two others at a BGA closer to the middle of the board and compare the resulting profile temperatures and times above liquidus. If you do not have the exact assembly for this characterization profile, then use most any scrap assembly with a similar component density, even cutting it to size in order to position a BGA near the edge of the board.
This will let you know if the reflow process is going to give you problems. There are several ways to solve this problem in roughly descending cost order:
Senior Project Engineer
Electronic Controls Design Inc
At least on the reflow front, you can use profiling as a tool to ensure you are reaching the correct peak temperatures for larger mass BGAs, while not frying more sensitive components on your board. Also remember across the belt uniformity can range anywhere from 2 to 5C+. Often the edges of your PCB is the coolest in the oven, therefore requiring even more heat for your BGAs to reflow correctly. This makes even more critical that you use profiling as a method to "balance" your board. For example if your BGAs are normally set to a max peak temp of 245C you might need to bump this up slightly, but you better keep a close eye on components like electrolytic capacitors. With some software like KIC2000, you can redefine these sensitive components, for example, lowering their max peak to 235C. I just posted today some great pictures to show you what is being used to measure uniformity across the belt from one oven manufacturer as well as examples of balancing your board with profiling.
Global Account Manager
In regards to the stencil printer, you will have to make sure that depending on how close to the edge of the board you are talking about that any PCB clamping system that is present on the top side of the board during printing is designed to not affect stencil contact at the print point or you will need to ensure that your printer has the capability to pull the board flat and then retract any top side clamping system before printing.
If your printer has board edge clamps you may need to put a relief pocket on bottom side of the stencil to assure good stencil to board gasketing in the area close to the edge of the board.
Vice President Technology
From the PCB design side, with BGAs close to the edges, you may have problems routing the traces away from the component. From the reflow side, your higher mass BGA may require additional heat for reflow, as a result you may overheat the edges of your board during soldering. From the rework side, your BGA on the edge of the board may cause edge warping during rework of the BGA.
Regional Sales Manager
OK International Inc.
When the PCB is post-assembly separated frompallet/panel form to individual form, the various methods of"depanelizing" will load different levels of stress along the PCBedges (e.g., V-Score vs. tab-rout, both individually and in combination). As SMD distance to board edge decreases, component solder jointsusceptibility to the mechanical stresses of depanelization increases. Can'tpredict joint force loading through modeling? Have your PCB house provide a"print & etch" sample for practical evaluation.
Circuit Connect, Inc.
There are a number of other factors to consider whenplacing BGAs closer to the edge of the PWB other than those already mentioned.Here are some other considerations:
I would not stressthat much about this. However- few things to consider:
Engineering and Operations Management
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