Transcript
Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, who by day work for ITM Consulting, here to answer your SMT and assembly process questions and address the issues. Jim, what is today's question?
Jim
What issues are we likely to see when we place BGA components very close to the PCB edges? What impact might it have on reliability? Will, equipment, screening, placement, or re-flow require modification?
The first thing is you have to be quantitative about this. What do you mean by very close?
Phil
Yeah. We've seen some real goofy things in the last 30 years. It was that one cell phone manufacturer, remember that? They wanted components literally right up to the edge.
Jim
They wanted to build a batch of single cell phone boards without a panel through all their processing and they wanted to put components literally right on the edges.
Phil
Why don't they put them on the edge itself? You know, the side of the board.
Jim
The general answer is any given process is going to have some limitation on how close you can get to the board edge.
Phil
Oh, let me say that process never happened, by the way. I don't think that company is making cell phones anymore.
Jim
Screening depends upon how much your board hold down is. If your board hold down clamps the board from the top, there's some definite keep out area. Even if you're doing edge clamping, you can't count on the the pressure and the squeegee to be absolutely identical, right out to the very edge. Now, is that, 3 mils, 5 mils, 10 mils, You should talk to your screen printing manufacturer.
Likewise, placement equipment will not typically place right out to the exact edges of the board. Think about this, think about trying to support that edge. I guess if you have a single sided board, you can use flat plate supports, but still, how are you gripping your board?
You might find equipment that would be more tolerant. With re-flow, most of us are using edge conveyors. They're designed to have a minimal thermal impact, but, boy, you try putting a large component like a BGA very close to the edge, so that the balls end up right over the pin of a chain that's carrying the board. You're taking risks.
Phil
I remember, I think it was on a custom basis, or 30 thousandths and even then you held your breath because of factors like board warpage and rail parallelness and it compounds itself.
Jim
And what about the impact on reliability? You're always concerned about edge singulation.
Phil
I think when the board is out in the field, a good solder joint is going to be a good solder joint, but again, it depends on the environment it's living with and whether that card is going to be flexed when it's put into an enclosure or whatever. Probably one of the most volatile issues for components in general near an edge is singulation.
So let's assume this board is being built in an array and there's going to be some sort of singulation. How close to the edge can you get? I know that one of the pizza cutter manufacturers recommends that the component be no closer than 5 millimeters to the V score of the board because of the stresses. So that's not only BGA's, but think about ceramic capacitors, those little hot patooties.
Jim
So I guess my overriding comment is really think about whether this is a good idea. You are going to increase the risk of problems with your process, or perhaps reliability, depending upon final handling. You could probably find equipment that is more tolerant than others, but I think you're going to find that most pieces of equipment; printers, placement machines and soldering equipment, are going to have some level of keep out area, at least desirable.
I think you should really ask is this necessary? This should really give us an advantage, that we're willing to take these other risks and additional costs for special equipment and so forth.
Phil
Right. Definitely a DMF issue. All right, well, on that note, that's it for today's installment. This is Jim Hall and Phil Zarrow of ITM Consulting, otherwise known as the Assembly Brothers, wishing you a good holiday, or good hollandaise, if you happen to be, you know, making asparagus, and whatever you do.
Jim
Don't solder like my brother.
Phil
Don't solder like my brother either, and keep the hollandaise out of the solder pot.
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