Comment by onetimeuse92304 | Hacker News Reader

Comment by onetimeuse92304 | Hacker News Reader

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onetimeuse92304

I am electronics / PCB hobbyist and I can definitely see how their explanation can be true. I can't say it is, but I can see how it could be.

If you design a PCB for a given size of the resistor but then decide to use larger resistors without redesigning the pads, you may have reflow problems and weak joints. This is simply due to the fact, that the components are positioned due to surface tension during reflow process (they are pulled into place as the solder melts). If the pads are for smaller components, there will be too little solder for larger surface and weight of the component and working at a wrong angle to pull it into place causing potentially higher rate of failure.

> What's more is that the component pictured is a capacitor.

And that means what? From the picture I can tell that there is very little solder between component and the pad. Potentially too little to hold the component well in place.

> The only conclusion I can draw here is that the guy has no clue what he's talking about

Maybe he does, maybe he doesn't. Have you considered a possibility you are not an expert either?

As someone who designs circuit boards professionally, the explanation is clearly lacking. There might be a thermal issue or there might not be. There is nothing conclusive in the pictures either way. What I do see is the following:

1. Underfill (the brownish-tan smooth material surrounding the components towards the bottom of the picture) around the IC, which is typically done to make parts more mechanically robust.

2. No evidence of overheating on any of the thermal interface material that is left stuck to most of the components and no evidence of overheating on the PCB or the components themselves.

3. Completely insufficient evidence to declare a soldering issue. The way to prove this one way or another is x-ray inspection to look for voids in the solder or a mechanical cross-section of the suspect solder joints.

While this certainly could actually be the problem, I see insufficient evidence to conclude one way or another. Manufacturers don’t put underfill under a part unless it’s required through testing or experience with similar package types in prior designs since it adds cost, additional process steps and makes it a PITA or impossible to rework any bad components in the area.

As to the pad size/shape, there are three general classes of design defined by the IPC (standards body that deals with PCBs and PCB assemblies). Depending on how space constrained your design is, there are different recommended pad designs for passive components like these. They might be using one of the tighter spacing guidelines, but if their process is well controlled, it can be perfectly fine for the design life of the product.

If you want to see small pad layouts done well, look at an iPhone logic board.

If you want to know more about pad design for SMT parts, search for IPC-7352

My totally unsubstantiated guess from the description alone was 'I wonder if they switched to a larger package component and forgot to update the pads.' That could be described as the 'component being too large for the device' and while it might just fit, it may be borderline mechanically and electrically stable. That could also explain the added underfill. Is that possible?

It’s certainly possible someone did a BOM substitution and didn’t due diligence on it, but I doubt it. PCB assembly houses tend to notice components that are suddenly too big for their pads because they’ll have fallout in AOI or later testing.

The underfill was likely added before full production as the result of reliability tests that showed some mechanical susceptibility of that IC.

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