Happy St. Patrick’s Day!

SC4And some trivia. You may have noticed that the soldermask used on most PC boards is green, as is the paint used on most steel truss bridges. Why is that? And what do the two things have in common? Why green PCBs and why green bridges?

To answer, I brought in color expert expert Patty O’Patrick O’Dell, who stated: “Many bridges and PCBs are green because they absorb red and blue light, only reflecting the green.”

That wasn’t quite what I was getting at, but close enough. The important thing, is that, generally, in commercial products, the PC boards are hidden, so the color doesn’t matter that much. With prototypes and a lot of the hobby or development boards, that is not the case, so many companies have chosen to use a different color as a part of their identity.

Arduino products are blue, as are most boards from Adafruit. SparkFun makes theirs red. Ti Launchpads are red as well. The Beaglebone uses color, essentially, as a model number; Beaglebone black, Beaglebone green. This is possible because major PC board fab houses have made different colors more economical than they used to be.

I’ve been asked if the color makes any difference electrically. In general, no. If you’re dealing with super high speeds, RF, or other exotic conditions, it’s always best to ask your board house. In those fringe areas, a lot of things have the potential to make a difference. Other than that, if you can afford it, and want to make a statement, go for it. You can often get different color silk screen legend too. Just make sure there’s contrast between the two. White silkscreen on white soldermask would not be the best choice.

Duane Benson
Beware the monsters from Id

blog.screamingcircuits.com

A Sticky Situation

Figure 1. Two SMDs have misaligned during reflow due to uneven pad sizes and disproportionate solder liquidus surface tension.

Figure 2.

Figure 2. Temporary solder mask glue applied to two corners of each component prior to reflow keeps them in place.

Figure 3.

Figure 3. Post-reflow, the adhesive is easily removed, and the SMDs are perfectly positioned as they should.

The surface tension of liquidus solder exerts a considerable pulling force on a component during reflow. This is why, once upon a time, small components could be relied on to self-align on SMT pads during reflow. They still can, of course, providing that all things are equal, such as pad dimensions all around. But if they’re not, you can expect problems.

In this case, two components had shifted away from their center location on the PCB footprint during reflow (Figure 1). This was due to the fact that a large SMT pad on one side of the components, see photo, was exerting a stronger pulling force on the component than the ordinary-size pads on the opposite side. More surface area means more pulling force, and consequently component misalignment. It doesn’t matter that the placement machine put the part in the right location beforehand.

Mechanical fixturing simply wasn’t a practical solution. Instead, two dots of temporary solder mask glue were applied to the corners on one side of each component, prior to reflow, to hold it in place. The glue acts as a temporary adhesive and prevents the parts from moving during reflow because it is stronger than the pull of the liquidus solder. After reflow, the glue is easily removed, and the SMDs are perfectly centered. Problem solved!

Roy

rushpcb.com/rushblog/

OMG and MacDermid? OMG!

Not nearly enough attention is being paid to the pending acquisition of OMG’s electronic chemicals business by MacDermid’s parent company.

This deal will throw even more market share to MacDermid, and the big question becomes how will smaller fabs (i.e., the vast majority of the North American and European markets) handle it? Many of them already use one or the other, and will doubtlessly be affected by the merger. I can’t imagine they are looking forward to this.

 

Super Small Via-in-Pad

Via-in-pad is an old issue that still pops up now and then. Our standard answer hasn’t changed: No open vias in pads. But one of the questions we get related to the subject is: “What if we make the vias really small?”

Logically, that makes sense. In fact, in some cases, the via is so small that it’s essentially closed. If it’s so small that it really is closed, then it’s not an open via. But look close — if it’s closed with solder, that solder may melt during reflow leading to an open via.

The images here show some pretty small vias. I believe they’re 0.3mm in diameter.

In the first picture, on the left, it appears that the vias are open. They aren’t though. This board (an unstuffed Beagleboard) uses solder mask on the back side of the PCB to close off the vias, as shown in the image on the right.

Our recommended method (see more detail here and here) is to plug the via with copper or epoxy and have it plated over at the board fabricator. Next, we’d recommend via caps on the component side. Finally, capping the back side with solder mask, like this example can work, but it comes with the risk of voids. The via caps and also pop open, leading to an open via.

Duane Benson
No more open vias-in-pad, I mean it!
Anybody want a peanit?

http://blog.screamingcircuits.com/

US Commitments

When I heard early this month that Foxconn (Hon Hai) chief Terry Gou offered to train Americans in electronic manufacturing I recalled one of Apple’s excuses for putting its production in Asia, much of which went to Foxconn which now has over a million workers. Apple stated that America just did not have a sufficient number of qualified and trained technicians and engineers (tens of thousands Apple said) available to build its products here.

Then I thought, why would Gou make this offer? He certainly has not shown himself to be a good Samaritan in the past. The only conclusion I could reach was that he was planning to establish assembly operations in the US and would need a qualified work force to achieve this. Note that production of iPad minis are behind schedule and market demand. Labor costs have risen rapidly and continually in the PRC over the past five years. Hon Hai has been plagued with labor problems and a high factory worker suicide rate in China during the past few years. Gou reportedly is reported to be conducting evaluations in cities such as Detroit and Los Angeles where there is a large available labor pool. It should be noted that Foxconn has debunked the stories associated with the possible establishment of a US manufacturing base. But then, is it possible that Mr. Gou has become a good Samaritan when it comes to helping the US’s manufacturing capabilities?

Meanwhile, Gou, at a recent public event, noted that the company is planning a training program for US-based engineers, bringing them to Taiwan or China to gain first hand experience in the processes of learning product design and manufacturing. He has already been in touch with MIT regarding the program. They will also be in an environment to learn Chinese.

All this begs the question: Where are the American companies, government agencies, and elected officials that claim that they want to bring manufacturing jobs back to the US? Where is the commitment? Where is the investment? What steps are being taken to entice American manufacturers to the table? What motivation is being offered? If a foreign company can find it attractive to do so, why can’t an American company find it so, too? Even more interesting is the question, “What is the U.S. government doing to keep its current manufacturing base viable and growing?”

Is Japan’s interconnect future on shaky ground? Third-quarter results from Taiwan’s leading board makers (suppliers to Apple, automotive companies, and tablet makers) indicate that the center of HDI manufacturing has already undergone a major shift from Japan toward Taiwan and China. Taiwan’s government has been extremely supportive of this and other high-tech activities and investment by its “native” electronic (and other) companies.

“Rumors” persist that Taiyo is attempting to buy Goo Chemical in Japan. Goo owns 51% of OTC, Taiyo’s leading solder mask competitor in greater China.

Via in Pad X 8

Here’s an interesting via in pad case. On the one hand, the footprint is very symmetrical and clean looking. On the other hand, it has open vias in the pads.

At first glance, I thought this was a DIP footprint with extra long pads, but it’s not. It’s for an SMT part. Personally, I would have put mask between the pads. Looking at the rest of the board (not shown), the spacing between pads and mask is pretty wide, so there may be a good reason. I’m not sure though.

Definitely, though, I would not put the vias in the pads like that. Those open vias will cause solder to flow down to the other side of the board, make a mess there and leave the chips without sufficient solder.

Duane Benson
Sucking solder through a straw – or via

http://blog.screamingcircuits.com/

Passively Annoying

Passive components can be kind of offensive sometimes. I can understand them in analog circuits or charge pumps. But the fact that we need to put them all over our digital logic is just rude. Technically, I understand why they have to be there, but philosophically, they violate my basic principles of life.

Back in the early days of personal computers, there allegedly was a company that had its engineers remove bypass caps one by one until the motherboard stopped working. Then they’d add the last one back in and smile about the short-term cost savings. Well, that was a bad idea. The reality is that we need them.

I’ve written about some of the problems that can show up because of passives (or other small two lead parts like LEDs and other diodes). Like here, here and here. That last example has popped up recently and I have some more thoughts on it. Essentially, I’m talking about multiple two-lead components that have one lead tied together. That’s a pretty common scenario with bypass caps or LEDs (or the LED current limit resistor).

There are a couple of ways to do this. Some error prone and some not. First, the general rule of thumb for two lead passives is, if at all possible, to have the same amount of copper going into both sides. That means that if you have one 8 mil trace going to one pad, have one 8 mil trace going to the other. Also make sure that you have solder mask stopping the solder from going off pad.

Passively annoying bad way here is bad. It might just barely meet IPC standards, but it still is really not manufacturable. First, there are no thermals. That makes the solder melt much slower on the right side which can lead to unreliable solder joints or tombstoning.

Second, even though the theoretical solder mask openings don’t touch and the keep-out (it’s not shown but is just a hair narrower than the mask area) areas don’t touch, they are close enough that you might not have any mask between the parts on the thermal pad. That can lead to components shorting.

is also bad. You have your thermals in there so that’s good, but the parts are still so close together that you might not get any mask between them, leaving a path of bare copper between the parts that can cause them to drift around and mess things up.

Method B1, on the right here has the same issue. Likely no solder mask between the parts and a bare copper path between the parts.

Method C here is fine. The parts are still at risk of not having mask between them, but there isn’t bare copper running straight between them. There will be mask between the parts and the pad so there isn’t any way for solder to bridge or the parts to drift.

Method D here is also okay. You do need more room to spread the parts apart. That’s a bummer, but sometimes “bummer” is the cost of reliability. Here, there will be solder mask between the parts and there are thermals. Everything is happy.

Use method C if you have a little side to side room to play with or method D if you have a little top to bottom spare room.

Duane Benson
Prevent flanking maneuvers.
Don’t be like the Solders at Thermopylae

Small Open Vias

Pad parts change and so do vias. Our standard policy here is that open vias in pads are bad. We from time to time recommend ways to plug them. Generally, you have several options. Like this post shows. However, with vias in the pads of really small parts, those solder mask options will probably not work. Solder mask generally isn’t put down with enough precision to cover holes on tiny pads, and further, the solder mask would probably mess with the clearance. On the left is an example of a small QFP with open vias in the pads. Those are some small vias.

So, if solder mask isn’t going to work, what QFN center void open vias will? Filling and plating over them. That’s what will work. You really only have two choices: fill and plate, or live with a bunch of voids under the part and solder slopped on the bottom side of the PCB.

Here on the right are two illustrations representing the issue. In the top half of the image on the right, I’m representing the vias with copper plugs and plated over at the board fab house. As with all parts of this sort, there may still be tiny voids. IPC and the manufacturer will have guidelines on the maximum allowable voiding. On the bottom, you see what happens with the vias left open. You get two problems: big voids and solder on the underside of the PCB.

Certainly there are some applications where this doesn’t matter. That’s why there is a second choice: “Live with a bunch of voids and slopped solder.” If you can’t live with voids and solder slop, you have to bite the bullet and pay the extra for a PCB with filled vias. Board houses that do this have a variety of materials to use including copper, electrically conductive epoxy and thermal conductive epoxy.

Duane Benson
Please sir, may I have some more voids?
No! No voids for you!

http://blog.screamingcircuits.com/

Random Via-In-Pad Myth #5

Myth #5: When you need thermal vias, more is better, bigger is better

Hmmm. Logically, this would seem to be the case. There are limits though; especially if you want a reliably assembled product. Older parts with heat slugs easily accessible for bolting on heat sinks didn’t have this issue. Just bolt on a piece of metal and maybe blow a fan across it. It’s different with a lot of the new, Padinvia smaller surface mount packages. Many have a heat slug on the bottom, which requires carefully placed thermal vias to a copper pad on the underside of the board.

An extreme case of flooding the land with vias can be seen in this illustration here on Padinvia_alt the left. In terms of assembly, you can hack this together for a prototype, but it’ll never fly in a production environment.It would be much better to use fewer smaller vias and have the center land covered with solder mask except where the metal on the chip is exposed, as in the illustration on the right.

Duane Benson
Place one carrot seed in each via and cover it with planting soil

http://blog.screamingcircuits.com/

0.4mm Pitch BGA Redux

I’ve written about it before, and again here.

When dealing with new technology parts, it’s really important to look up all of the manufacturer’s component information that is available. I’m going to quote from the Texas Instruments document “PCB Design Guidelines for 0.4mm Package-On-Package (PoP) Packages,” Section 10 (PDF page 8):

Industry reliability studies have revealed that NSMD-type pads are highly recommended for most 0.5mm pitch BGA applications. However, there is a problem with this approach at 0.4 mm pitch.

Real-world assembly experiments with the BeagleBoard and the OMAP35x EVM revealed a tendency for solder bridging between pads when NSMD were used. There was insufficient solder mask webbing between the pads to ward off bridging. Therefore, a SMD design was used which resulted in much better assembly yields with no solder bridging.

If you are using a 0.4 mm pitch BGA with the balls aligned in a grid (as opposed to staggered), read the design guidlines from the manufacturer before laying out the board.

In a presentation about the development of the Beagleboard, Gerald Coley, Beagleboard designer, notes that their first two runs had non soldermask defined pads, resulting in a 10% yield. After another run of PCBs where the pads on the PCB were the same size as the pads on the device and the PCB pads were soldermask defined, yields rose to 96%. And verify that your PCB house does in fact follow your instructions. Some will think they know better and will change the mask layout.

If you are still unsure or think your design will have different requirements, call an applications engineer at the component manufacturer and discuss your project and the layout.

Duane Benson
Trust but verify

http://blog.screamingcircuits.com/