Category Archives: Screaming Circuits Blog

EMS provider Screaming Circuits offers tips and thoughts on design and manufacturing

Extra-Fine Pitch BGA Pads

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One of the annoyances of the world of trade secrets and proprietariness is that we can’t all learn from each others’ experiences. That is important, and even generally necessary, in a competitive world. If you put in some hard work, you should get the first right to profit from it. Otherwise what incentive would you have to put in that hard work?

There are times, however, when it would be helpful for the industry or the economy in general if we can all learn from someone else’s challenges. Times when, for example, the entire auto industry and therefor the safety of the general public would benefit if all companies shared what they have learned about the reliability of electronic throttle systems.

Bb Good SMD pads Here’s another chance for open source hardware to shine. Take the Beagleboard. The TI folks who designed it pushed technology in a number of areas and by presenting what they have created as open source, we can all benefit from it. Even stepping outside of the great work in the schematic, they have done great service in the areas of manufacturing complex devices as well.

A while back, I wrote about soldermask defined (SMD pads) vs non-soldermask defined (NSMD) pads on 0.4 mm pitch BGAs. The basic idea is that while with most Bb Bad NSMD pads BGAs, you want NSMD pads for better mechanical strength, with the really small BGAs, like the 0.4 mm pitch OMAP processor, you want SMD pads to prevent shorts.

The messages that the Beagleboard team learned here are, first, it’s true that you want SMD pads and second, make sure that your PCB fab house follows your instructions in that regard.

Many fab houses have their own rules and will set the soldermask up based on what they feel is best. They may have never used your part though. Make sure the board house does what you need. By insisting on closing up the soldermask, the Beagleboard team went from 90% failure with the NSMD pads to 96% good and no BGA shorts with the SMD pads. (This info and the photos come from the Beagleboard ESC presentation by Gerald Coley.)

Duane Benson
The worms do.

http://blog.screamingcircuits.com/

Another Via-in-Pad Reason

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Just the other day — no not that one; the other one — I was reading through some of the open source Beagleboard information again and I came across an interesting tid bit. In one of the early revisions, they had some issues with SMT connectors ripping off the board. The pads detached from the board. I know that’s not  a common issue, but it does happen.

BB Empty pcb via in connector pads

Their solution was to put vias in the pads to strengthen the connection between the pads and the PCB. I hadn’t thought of that, but it makes perfect sense. Note the four dimples in each of the pads on the audio connector footprint in the image above. Also note that they are small and closed off.

If you are concerned about losing your SMT connectors, you might want to consider the via-in-pad solution. Please cap or plug them, though.

Duane Benson
Who plays pinochle on your heart?

http://blog.screamingcircuits.com/

How to Fill a Via

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We here at Screaming Circuits keep telling people to fill and plate over vias in BGA pads. The same goes for small passives too, like 0402s and 0201s. As I closed in this post, best option is to fill and plate over. Bigger parts can use some of the solder mask techniques, but BGAs — especially 0.5 mm and small pitch — and tiny passives really do require filled and plated vias.

The question of the day, however, is: “Just how do I get those vias filled?” Here again is another place where our old Gerber file format falls a little short.

There are quite a number of different methods for filling vias: Copper plugs, copper plate, surface material (i.e.  HASL), conductive epoxy, non-conductive epoxy. Maybe others. But, the Gerbers don’t have a method for specifying this.

So here’s what you do.

1) You need to contact your fab house to determine what techniques they have available and what they recommend for your fab project.

2) You have to create a set of fab drawings or special instructions that point out which vias need to be filled and with what material.

Duane Benson
Maybe raspberry jelly filling?

http://blog.screamingcircuits.com/

Large Via In Pad

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I haven’t written about via in pad in a while, but the issue hasn’t quite gone away yet. This particular QFN, below, has the vias tented, which is good. However, it could be better. If you look close, you’ll see that they’re tented on the bottom of the board with solder mask.

Tenting on the bottom will usually prevent solder spillage out on the back side of the PCB, but with vias this large, the solder will probably flow down into the space, leaving quite a bit of voiding under the part. Sometimes outgassing will pop open the little tents too causing the spillage. And with immersion silver boards, outgassing can cause corrosion in the vias if you have the bottom tented and the top also sealed — like by the part.

If it’s a low speed, low temperature QFN that just needs a little ground connection to the center pad, that voiding might not matter. But, in most cases with QFNs, you need minimal voiding for thermal or noise reasons.

The best option for manufacturability is always to have the vias filled and plated over at the board house, but that can be expensive. If you are going to tent with solder mask, this next image illustrates the three ways to do it.

A is the best: a cap on the component side about 100 to 125 microns bigger than the via. B, a larger cap on the component side, or C, a cap on the bottom, will also work but both come with a greater risk of excessive voiding.

Duane Benson
Do solder mask tents need a rain fly?
In Oregon – probably yes.

http://blog.screamingcircuits.com/

Underness as a Common Theme Again

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I didn’t mean to, but here I seem to be with a common theme again. Like yesterday, and again not too long before that — parts under parts seems to be the thing of the hour.


Maybe the caps are shy and are just trying to hide. Or maybe we’ve been writing too much about package-on-package lately. Dunno.

I do wonder though, how many of these issues are caused by parts substitution (the cap originally chosen was narrower), the wrong CAD library footprint, or just a rushed layout. Who wants to vote on that one?

Duane Benson
Just push on it harder. We’ll make it fit.
Maybe…

Underlapping

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Sometimes, it’s the simple things in life that we must stop and appreciate. Like the fact that just because there aren’t any pads in a spot, doesn’t mean that you can use the space.

Take this PCB, for example. U3 is placed so that one and a half of its legs will be underneath a big RF module. The module isn’t shown in the picture, but the pads and outline are shown. U3 is right into the lower left corner of the module as you can see by the rows of pads and the silk outline. The module that U3 steps under is one of those module on PCB using castellated mounting holes.

If U3 were your hot little sports car and the big module were a dump truck, you most certainly wouldn’t do this. You’d look first. That’s what mirrors are for.

C6 up on top is pretty darn too-close to the module too. Move it a few mils to the right please.

Duane Benson
Mirror mirror on the wall
Who is the overlappingest of them all

‘PC Load Letter’

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I remember back in the olden days when we didn’t have any space and everything needed to be abbreviated or put into obscure symbols. We had eight characters for file names, plus a reserved extension. So, for example, on disk, this photo might have to be called “APTC6010.BMP.”

And, years had to be saved in databases as “82” instead of “1982” because space was expensive. I think what happened, is that we all got in the habit of being obscure and obtuse because of that. We’ve since passed that habit to newer generations too, so that today, even when there is room available, we still do the “PC LOAD LETTER” thing instead of just being nice and clear.

This cap footprint, pictured to the left, does have a polarity indication in the silk screen. Either that, or the silkscreen had some printability issues and was cut off on one side. It’s probably positive up, but just to be sure, why not put a little + sign on the positive side to remove that bit of ambiguity? There’s plenty of room on this board. If space was tight, I could see leaving it off, but what’s wrong with a bit of extra safety when you can get it?

Duane Benson
Baseeball been very very good to me


http://blog.screamingcircuits.com/

Don’t Do It

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Friends don’t let friends wire-wrap.

Do people still do this? Do you still do this? When’s the last time you did some wire-wrap?

Digi-Key still sells an assortment of wire-wrap wire and wrapping tools, including the simple little hand tool that I own priced at — holy mackerel! — $34.35! I should sell mine on eBay. Somebody must still be doing this if they’ve got such a nice selection. Amazing.

Duane Benson
Who wants spaghetti?

http://blog.screamingcircuits.com/

Panelization

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A lot of small quantity PCBs come individually routed these days, but when they get too small or come in larger quantities, panels can be very nice to have. When you have your PCBs panelized, what’s your preferred method? And why?

We get questions about this reasonably often: “What’s the best way to panelize my boards?” For our shop, we have some guidelines on how to go about it (make sure to follow the specific guidelines from whatever manufacturer is assembling your boards), but the guidelines don’t specify whether you should use V-score or tab routed. That’s a decision left to you.

What if you don’t know? Well, it depends then, but you can easily eliminate a few options. For example, if you have curves in your board outline, you can’t V-score. V-scoring only works in straight lines. With curves or odd shapes, you have to use tab-routed. If your outline is a pure rectangle, V-scoring tends to require less board-edge, so you can get a bit more out of your PCB real estate. But it’s more difficult to deal with on very thin boards and V-scoring leaves a rougher edge after snapping the boards apart.

Two of the key disadvantages of tab-routing are the greater waste area and the nubs that stick out after separating the boards. You can leave the nubs, sand them down or use a clean-up router.

Here’s my take on it: A) If it truly doesn’t matter, use whichever method is less expensive or that you think looks prettier. B) If you have curves or other odd shapes, you’ll probably need to go with tab-routed. C) If your boards are rectangles and you can deal with the less-smooth board edge, go for the V-score.

Duane Benson
Tab. Not Diet Coke.

http://blog.screamingcircuits.com/

1.0, 0.8, 0.5, 0.4, 0.3 …

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As pitch gets smaller, things get more difficult. Did that blinding flash of the obvious hurt your eyes? Mine too. It’s not always just more difficult though. Sometimes it’s different as well.

Like BGAs, for example. With 0.5 mm and larger pitch, we and pretty much everyone else recommend NSMD (non solder mask defined) pads. However, once the pitch drops to 0.4 mm, some manufacturers are recommending solder mask defined pads to prevent bridging between solder balls. Make sure your fab house can nail the mask registration.

With LGAs and QFNs, IPC recommends NSMD pads for 0.5 mm pitch and larger. Once the LGA or QFN (DFNs too) pitch drops below 0.5, does everyone suggest solder mask defined pads like with BGAs? No. Actually, IPC tells you to go ahead and remove the solder mask web between your land pads with 0.4 mm pitch parts. With spacing that close, the solder mask can’t reliably stick to the PCB. That would be messy. As to why a solder mask defined pad would not be a good thing, I can only speculate at this point. My assumption is that without the solder ball like the BGA has, the whole land area is needed to ensure a good solder joint.

Duane Benson
Fight Uni!

http://blog.screamingcircuits.com/