PCB Assembly Services - Screaming Circuits: Tombstoning



February has been very lean for blog posts. In the first few months, I was averaging about one every other day. I've put down just four in February, though - five, if you count this one.

While down at the Apex show in Los Angeles last week, I heard mention of tombstoning. Tombstoning small SMT parts isn't something we see a lot of, but it does happen. If you're not familiar with the problem, essentially, it means that the part has stood up on one end, like a tombstone. One end of the part will be soldered and the other is up in the air, not soldered.

I understand that tombstoning also refers to the practice of jumping or diving off of high, shear cliffs into not necessarily well known waters. That's not what I'm talking about and I certainly wouldn't recommend it. It's probably a lot worse than the tombstoning I'm discussing here.

It is much more likely to occur on smaller parts, such as 0402s and 0201s than it is with larger parts. There are a couple of causes, some process related and some design related. The process related causes are mostly up to us, so I'll just cover a couple of the design related causes.

Essentially, what happens is that the solder on one of the two pads melts and captures the part end before the other side melts. Surface tension from the melted side can then pull on the part and stand it up.

The keys to reducing the chances of tombstoning then are to make sure the board surface is level and make sure that both pads on each side of the part heat evenly. HASL (Hot Air Solder Leveling) boards often have uneven surfaces. If the pad on one side is higher because of HASL solder deposits, tombstoning is more likely to occur. This is one of many reasons that immersion gold and immersion silver surfaces are becoming more popular.

An even bigger factor is the amount of copper. If one pad is larger than the other or if one pad is connected to larger traces than the other, the extra copper will act like a heat sink, slow the melting on that side and may cause tombstoning.

Tombstoning_2_1 In the simulated case of tombstoning image on the left, the adjacent via sinked heat away so the solder on the other pad melted first, popping the part up on that side.

If you are using 0402 or 0201 size parts or if your designs have had tombstoning problems, first we would recommend that you avoid HASL finish boards. Second, look carefully at the layout of your small parts. Make sure that the traces connected to each side are equal in width and connect the pad at a similar spot; ideally centered on the X-axis.

Sometimes the problem can be inside the board as well. If an inner layer has a large area of copper the extends under one pad of the part and not the other, that area may heat a little slower than the other side without inner layer copper.Tombstoning_1_1

In the second simulated case on the right, here, one of the  solder pads had too much solder paste. The extra thermal mass, again, caused that side to melt a little slower.

Following these guidelines won't guarantee to eliminate tombstoning, but it will reduce the chances of it happening. We'll do our part with the process related causes and hopefully, you'll never see a tombstoned part.

Duane Benson
No, don't jump!


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