Selective Solder Paste Printing for BGA Components



Selective Solder Paste Printing for BGA Components
Could we print paste only on the pads that have functional requirements? Could we skip printing on 50 or more of the interior pads since they serve no function? Phil Zarrow and Jim Hall, The Assembly Brothers, answer these questions.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil

And welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting. Up here high above Mount Rialto at ITM intergalactic headquarters, where we are here to answer your process, material, component questions.

Jim, today we have a question from A. J . We use common BGA packages on many of our low-end products. Could we print paste only on the pads that have functional requirements? Could we print all the perimeter pads at 120 pin BGA, but skip printing on 50 or more of the interior pads since they serve no function?

The cost of solder paste is just so extreme. You just have to pinch pennies wherever you can.

Jim

I agree. When you think about all that paste being used in printing and cleaning the stencil and the losses and everything, that is a small amount of paste. It is hard to imagine it could be a significant cost.

But if you are printing a high volume, consumer product where you are printing a lot of these, maybe it is.

One technical thing that I want to raise to cite the example, we could print the perimeter pads on the 120 pin BGA and skip the 50 interior pads, are you sure they are non-functional? They may be non-functional electrically but they may be very functionally thermally.

It is very common to use pads and balls right under the center of the dye to conduct heat out of an active dye within the BGA package down into the board to keep the IC cool and happy and running with no flaws and at a temperature and speed you want and everything.

Before you write off those center balls, make sure they are not required for thermal cooling of the package, in which case you definitely want to solder them down to the pads of the board.

You may even want to put through-holes in those pads to dissipate that heat down into the board. Your component manufacturer should give you those instructions.

Phil

So, the answer to your question A.I. is yeah, you can. But assuming that those balls don't indeed have a function, be sure of that. Beyond that, is it really worth it? That is for you to decide, the economics of it.

Jim

One other concern, which I don't think is applicable here, but in a high rel situation you would be concerned about, by not soldering all of the balls, the long-term reliability of that part, the thermal cycling, vibration and so forth.

But with these cost considerations, I am assuming that this is a low-end product or that would not be a concern.

Phil

That is what he basically states, a low-end product so I have to wonder about the economics as well.

You have been listening to Board Talk with Phil Zarrow and Jim Hall. I would just like to add that however you are soldering your balls, please don't solder like my brother.

Jim

Don't solder like my brother.



Comments

In addition to the expert's reasons not to skimp on paste, please consider: All of the solder joints on a BGA or other array package act together as a stress distribution network. Skipping paste on any balls, whether at the center or otherwise, will increase the stress on the remaining solder balls during operation, thus shortening the number of temperature cycles before failure. Also consider; the component vendor will not warranty the components if you start getting cute with the selection process of the balls to be soldered. If they all end up falling off the board after 6 months, the BGA manufacturer has no liability. It's YOUR fault! Still want to save a penny per BGA?
Odin Stadheim, Analog Technologies Corp
And the bga might not seat flat when reflowed causing opens on pins that are intended to be soldered.
Joel Ronning, Get Control, Inc
The cost of the solder paste used on almost every assembly is less than the lowest cost passive component. Skipping a few BGA IO's by starving them from paste is not a measureable cost reduction, at the risk of thermal and mechanical resistance.
Mitch Holtzer, Alpha Assembly Solutions
You may already know my take on this the best way to do this is to use dip paste. No extra cost of mini stencils and no problems in access to tight spaces. Dipping paste is commonly available in the industry for PoP assembly and a simple solution to both standard balled area array packages. Alternative not that common these days is to print on to the termination's, can provide more paste for high temperature non collapse balls.
Bob Willis

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