Hello all, I read about Steve's headache a few articles back and I'm really interested by his tip. However, I'm still wondering why square apertures release solder paste better than round ones. For the same volume of paste, the surface area of the aperture walls would be larger for square apertures, right? Wouldn't that provide more of a drag on the solder paste? Anyway, I'm still gonna try it out. My hat off to Steve for his tip.
Joe ####################################
See IPC Technical Paper S9-4 (page 7) for discussion of merits of square uBGA stencil apertures. Seems like some people use this on BGA too.
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Hi Joe!
If you click on the link I slipped in at the bottom, it'll take ya' to Tessera's WEB site (they're the company that started this whole MicroBGA business...hold the patent on em' and license it to everybody else) Then click on "Assembly Process Development for Chip-Scale and Chip-Size MicroBGA" and there you'l find a bunch of good stuff about design and assembly for MicroBGA)... there's a buch of good stuff on the rest of the site, so cruise around there. But here's part of the article that was authored by Vern Solberg that talks about using squares instead of circles:
Solder Stencil Development for CSP
Unlike the narrow rectangular land pattern geometry typical for attaching the leaded QFP while the contact sites for BGA devices are circular or square. Land pattern geometry recommended for chip-scale array device attachment is 0.25 to 0.30 mm (.010" to .012"). The stencil opening can be equal in size to the diameter of the land pattern or square and slightly larger as detailed in Figure 13, to meet specific requirements. To achieve a more robust solder connection, process engineers may specify an expanded stencil opening as shown in Figure 14 to furnish a higher solder paste volume at each attachment site as compared in Figure 15.
Defining Stencil Aperture and Geometry-
Changing the shape of the openings from round to square has proved beneficial for CSP applications.
Although the fine-pitch CSP land pattern is generally a circular shape matching the ball contact size, stencil openings that have a square geometry furnish better solder printing quality. The square stencil pattern can serve two purposes. It can increase the solder volume slightly and when adapting the smaller pitch CSP and, the square pattern promotes a more uniform release of the solder paste from the stencil surface.
Another technique that has improved paste transfer on the smaller land geometry�s is the tapered land pattern opening. The tapered wall is formed by chemical etching or with a laser. During stencil fabrication, the opening that will be closest to the board surface is made one or two mils wider than the opening at the top surface as illustrated in Figure 16.
The square stencil pattern serves two purposes:
1. It can increase the solder volume slightly when adapting the smaller pitch CSP
2. The square trapezoidal pattern releases paste from the stencil surface more uniformly.
As to why that is, I think that because of the corners of the square is providing a focused point in which the solder paste will start releasing itself from the aperture, unlike what a circle would do.
To put things kinda crudely, it's like when you open a bag of potato chips, a lotta times if you take your teeth and nip at the top providing a focused point while you're pulling on the bag, and it rips open easily, but if you try and just tear it with your fingers it stretchs and stretches and then all of a sudden the bag flies open and you got chips all over the place. I don't know if that's a good analogy, or if potato chips and solder paste have anything in common at all, but you kinda' get my drift, huh?...maybe somebody can explain it better than I can.
Now an update on my superbly designed 20-piece kit...'scuse me for a second here...
(HA!HA!HA!HA!HA!HA! SUPERBLY DESIGNED!!! HA!HA!HA!HA!HA! I slay myself sometimes!!!HA!HA!HA!HA!...WHEW!...as I wipe the tears of laughter from my eyes)
Okay...I'm back...just had to compose myself for a second there.. HA!HA!HA!...sorry guys, I made a funny!...Anyways, someone finally listened to me here for a change..They're giving the whole thing back! (and the crowd goes wild!)
I started looking a little closer at the fabs, and Justin brought-up something that I noticed on these things. You know I had told ya that the MicroBGA pads were actually filled via's, Justin had mentioned something about being careful just in case the FAB shop had "boned" the process filling them...well they did. Upon closer inspection (under a scope) there were some that looked like they had little "divots" in them (not filled all the way) some had pinholes in them (not good at all) and I spotted a couple of blowholes so big that I could see copper on the inside of the barrel! Cripes!
I showed this to my boss, made him come out and see this junk with his own two eyeballs, and finally convinced him that the only reason we got brought this crap was because nobody else would touch it...if we tried to build it, we would fall flat on our face and the customer would try to blame us because the board wouldn't work. He said; "You know Steve, I think you're right, I'm gonna turn this job down" Hallaluya! I was beginning to think that my opinion didn't mean anything around here...
Even still, there is that little part of me that was looking at this as a challenge...I'm a little weird that way sometimes (aren't we all?) But I'm happier that I didn't have to mess with it...I got better things to do.
C-ya L8'er Alligators!
-Steve Gregory-
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Hello all,
I read about Steve's headache a few articles...
- Apr 28, 1999
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