3 ms·

The scenario you describe is very unlikely (the conductive part of chip resistor is enclosed in protective coating). What is a lot more likely is one of the fo

by fest 6y ago

The scenario you describe is very unlikely (the conductive part of chip resistor is enclosed in protective coating). What is a lot more likely is one of the following: * Engineer sent incorrect assembly drawing (sometimes when components are shuffled around with silkscreen/assy drawing layer hidden, the label for component can end up in unexpected place). * Someone manually updated the part numbers from one revision to another and missed the one resistor. * A new engineer joined a project, and after minor unrelated change to project, regenerated the fabrication outputs as one should. But he was not aware of changes somebody else did in BOM file by hand. * PCB assembly was performed by hand for prototype and contained an incorrectly placed part, but the assembled PCB ended up being shipped because of complete lack of production planning, somebody in sales selling stuff before it's ready and this being the only option to avoid contract cancellation. I have personally witnessed every one of these and then some. Yes, product engineering sometimes really is that shoddy and people being people stupid things.

jjoonathan 6y ago
I spoke from experience, not from speculation. No, resistors are not always fully encapsulated, especially if they are small, cheap, and/or designated for high speed applications, and no, the ones that are encapsulated are not protected well enough to make high or intermittent resistance failure modes particularly unlikely after being poked. I've seen both cracking and scraping in the wild. Not as many as I've seen shorted MLCCs, but that would be a tall order. Sure, it could also have been a design failure, but every time I've found one bad resistor in a bank of identical resistors, it has always been a physical problem with that resistor. Shrug.
- fest 6y ago
  That is indeed interesting, I haven't seen it in the wild. Lot's of bad solder joints, shorted MLCCs- sure. I'm now tempted to scratch the ones used in designs I have access to.
- carlsonmark 6y ago
  Disclaimer: I'm a software guy Another source of cracking could be related to moisture in the SMD components. We do primarily small volume PCB runs where I work. In house, with parts stored here, pick-and-placed here, etc. We have had components crack as well, and I recall one of the suspicions being the source of the components. They were a supposedly reliable supplier, but components from them failed more often than those from another supplier so we stopped getting parts from them. I am not sure if resistors are as prone to humidity effects, but if plastic parts are not humidity controlled before going into a reflow oven, they are prone to cracking. Even as a software guy, I have seen several brand new boards with cracked components on them over the last 15 years. Often the cracked components are difficult to find, since they are so small and do not fail completely (they are just off by a wide margin.) https://www.ti.com/lit/an/snoa300a/snoa300a.pdf https://www.ti.com/lit/an/snoa300a/snoa300a.pdf

by fest 6y ago

jjoonathan 6y ago

I spoke from experience, not from speculation. No, resistors are not always fully encapsulated, especially if they are small, cheap, and/or designated for high speed applications, and no, the ones that are encapsulated are not protected well enough to make high or intermittent resistance failure modes particularly unlikely after being poked. I've seen both cracking and scraping in the wild. Not as many as I've seen shorted MLCCs, but that would be a tall order. Sure, it could also have been a design failure, but every time I've found one bad resistor in a bank of identical resistors, it has always been a physical problem with that resistor. Shrug.

fest 6y ago
That is indeed interesting, I haven't seen it in the wild. Lot's of bad solder joints, shorted MLCCs- sure. I'm now tempted to scratch the ones used in designs I have access to.
carlsonmark 6y ago
Disclaimer: I'm a software guy Another source of cracking could be related to moisture in the SMD components. We do primarily small volume PCB runs where I work. In house, with parts stored here, pick-and-placed here, etc. We have had components crack as well, and I recall one of the suspicions being the source of the components. They were a supposedly reliable supplier, but components from them failed more often than those from another supplier so we stopped getting parts from them. I am not sure if resistors are as prone to humidity effects, but if plastic parts are not humidity controlled before going into a reflow oven, they are prone to cracking. Even as a software guy, I have seen several brand new boards with cracked components on them over the last 15 years. Often the cracked components are difficult to find, since they are so small and do not fail completely (they are just off by a wide margin.) https://www.ti.com/lit/an/snoa300a/snoa300a.pdf https://www.ti.com/lit/an/snoa300a/snoa300a.pdf