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The Knowledge and Know-how Connection
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June 17, 2013
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Views: 3360
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In my experience we've tried both round and square apertures on our BGA's with mixed results. We switched to a 1 to 1 aperture using a rounded rectangle with perfect results (corners rounded to .003")
Doug Christen, Ascentron Inc., USA
I have my stencil house convert the round BGA to square by keeping the same area, this results in a smaller square but the same volume of solder paste is deposited and that also reduces the amount of overprint. I had to do this to get better release of the paste from the stencil when there is not very much solder paste to work with on the micro BGAs.
Scott McKee, Goldenwest Technologies
I have my stencil house convert the round BGA to square by keeping the same area, this results in a smaller square but the same volume of solder paste is deposited and that also reduces the amount of overprint. I had to do this to get better release of the paste from the stencil when there is not very much solder paste to work with on the micro BGAs.
Scott McKee, Goldenwest Technologies
Using square stencil apertures and printing square paste bricks for round BGA or CSP (including flip-chip) pads may lead to disaster. Here's why:
1. Whenever printing paste, it is seldom a good idea to print paste off the pad. Solder fines have a tougher time agglomerating into a single solder joint (during reflow) when overprinted off of the pad and onto the solder mask, even more so if the pad is not solder-mask-defined because then they need to overcome the gap between the solder mask and the edge of the pads.
As a result, you can end up with many renegade fines trapped under the BGA that are not part of any solder joint. These may well go undetected, even when using an automatic X-ray scan because quite often they are too small or are outside of the X-ray's "target zone."
Typically, for automatic X-ray equipment, the software is quite similar to an AOI, and of course some X-ray inspection machines are actually a combination of both, called an AOX, AXI, or OAX, depending on which configuration and inspection methodology is used. The machine first finds a localized fiducial, and from there it goes into the programmed inspection routine. This routine is typically telling the machine to perform a series of inspections based (first) on the offset distance from the fiducial, then into the step-and-repeat sub-routine where it measures every ball based on the pre-programmed pitch, target ball size, roundness algorithm, and voiding gray-scale measurements (remember I said typically, not all operate the same way).
What this means is that NO inspection is performed in the areas between each target location. If there are individual fines or, worse yet, semi-agglomerated renegade solder balls in these "nether regions", they will go undetected by the automatic X-ray. Yet they can be large enough to violate the minimum electrical clearance required between the vias and the BGA solder joints, or between the vias and adjacent conductors or the finished solder joints and adjacent conductors.
The result is can be poor high-RF performance (crosstalk), and either permanent or intermittent solder shorts caused by all of these renegade conductors floating around the finished CCA. If water soluble chemistry is used, the wash process usually will get rid of some of these renegade solder balls, but never all of them.
If no-clean or RMA flux chemistry is used, there will be hundreds of these entrapped renegades, and while they may be held in the no-clean or RMA flux residues they will eventually work themselves loose and cause all kinds of havoc.
2. Some of those nice folks reading your column may interpret what you are saying as "square pads and square stencil apertures may provide better paste printing results for BGAs." Again, this can also lead to disaster. In this scenario, the round BGA, CSP, or flip-chip solder balls are placed onto square pads with square paste deposits. Instead of a finished solder joint similar to a round vase on a round table, you now have a solder joint that is round on the upper half connected to the round pad on the bottom of the BGA, but where it connects to the PWB the solder joint has four corners.
During the cool-down stage of the reflow profile, and during its service life where the temperatures are cycling, differing coefficients of thermal expansion between the BGA and the circuit board causes severe stresses on the solder joints between them.
When four stress risers are present on every square pad under the BGA, and if the stresses are sudden and severe enough, the corners of each pad will be pulled up. This upward leverage causes the bond between the PWB pad and the PWB itself to fracture, often severing the traces connected to these pads.
This is called "pad cratering", and with the higher temperatures required to solder lead-free solder plus the more brittle nature of lead-free solder joints this phenomena is being seen more and more every day. It is a bad enough issue with standard round pads, adding the four stress risers has been shown to make it orders of magnitude worse.
In both of these examples, the issue can go undetected until after all CCAs have completed assembly, inspection, and test and are deployed into the field. More than one company has found out the hard way they have just generated a whole bunch of scrap, with the full cost of materials and manufacturing lost with no possibility of recovery.
As always, you must warn your audience not to make these changes without going through some type of qualification steps to ensure these issues are not present before jumping in and implementing them on production CCAs. With all due respect to your audience, some may not understand the ramifications of not performing the due diligence aforehand. I do not want that to happen to anyone, hence this note.
Richard Stadem, Advanced Scientist/Engineer, General Dynamics Nuclear and Space Products Division
If converting to square pads from round on the stencil, would the correct conversion be to make the square sides the size the diameter of old round pad?
James Puritch, VP of Research and Development, Reliable Controls Corporation
For a moment I thought that you were going to say round ... but as you said, in my experience we found that squares for BGA worked better than circles.
It is good to be in-sync. Keep it up! Regards from Mexico amigos.
Guillermo Velazquez, Manufacturing Engineering Manager, Rainbird