I was troubleshooting some boards manufactured by a contract assembler recently and ran into a strange problem. Some of the 1206 SMT ceramic caps conduct slightly at supply voltage but show open with an ohmmeter.
In one case, a 0.1uF cap pulled to +5 via a 4.7K resistor was sufficiently leaky that the processor on the board wouldn't come out of reset. Upon removal, the cap was tested and showed the correct capacitance value. When tested with an ohmmeter it showed infinite resistance but with 5V applied across it, the cap it would conduct >2ma of current. Replacing the cap solved the problem. I've also seen the problem in another part of the circuit where a 0.1uF cap is pulled to +5 by pulses from the processor or pulled down by an external load. Same symptoms and fix.
I suspect the contract assembler used the cheapest parts he could find but was a bit surprised by the high leakage at just 5V. Has anyone else seen anything like this?
"James T. White" wrote in news:45863820$0$97588$ email@example.com:
If the contractor used a 'no-clean' active flux it can leave residue that might be slightly conductive if it gets humid. If you replaced it, using a rosin based flux, you'd probably dry and seal any residue while it was hot, so it's possibly that. If you can rule that out, great, but check it out carefully if you can't.
Do you issue component specs ? I don't mean in ultra detail but I tend to use the term 'major vendor commercial quality generic part' to indicate to the subbie that I have some expectation that they don't stuff the board with junk.
I will sometimes talk to them specificially about certain parts that concern me most.
Apart from what others have said, the cap appears to be defective after removal. I've had that problem and it was simply that the reflow thermal profile was creating hotspots on the PCB, and beyond the limit of the devices.
If it was a hand assembler, check on the equipment he used to put the parts down quite apart from what parts were used.
I've seen it happen to SMD caps that were placed under mechanical stress, as when a PCB is flexed too far. Apparently, tiny fractures can occur in the substrate, causing the failure mode you are seeing.
As we didn't get them built, I don't really know what component specs the manufacturer were given but I suspect that they were likely inadequate. The product and board design were done by a "design service", the OEM then has the boards fabricated and assembled and now that they are having problems, the came to us to try and figure out what is going wrong. I know that the "design service" provided a data book of component specs for the board but how much of that made it to the manufacturer is something I'll have to investigate.
Leakage can be the result of either damage to the cap, or contamination underneath / around it. Typically a cracked cap will read some nominal ohmic value on a meter - these parts don't repair themselves once cracked. Cracked caps can be caused by stressful dapanelization, mechanical assembly at standoffs w/ screws, pressfit connector installation, attachment or separation of daughterboards, overclamp ATE fixtures, touch-up rework with a hot soldering iron, etc. There's lots of things that can fracture mlcc's. Sometimes mapping out the failure sites on a grid can lead you to the source of the problem. Do you used these parts at multiple locations, and do the failures show up randomly at all of them, or mostly at the same locatins? I think you can find a nice tutorial on cracked caps on the AVX or Kemet website.
The reflow smt process, lead-free or not, is not likely to crack a ceramic part. Those assembly profiles just don't have the ramp rate (typically 2 degrees C / sec) or excursion (250C-260C) to fracture these parts. Now, a bottomside wave part can easily be cracked. If the preheat temperature is more than 100C below the wave temperature, then the thermal shock they experience going over the wave can certainly crack the larger parts. Once again though, those cracks, which often are invisible underneath the end terminations, cause ohmic changes in the parts which are usually detectable with a meter - several hundred kohm vs. open.
If these are bottomside glue parts - more and more of a rarity in these days of selective pallets, then you might be seeing something going on with voiding in the adhesive, which entraps flux at wavesolder. This has the potential to be a very serious problem, especially underneath caps which are usually across Vcc-gnd. Certain adhesives are very prone to voiding and subsequent leakage after wavesolder. The flat cross-sections look like a slice of tomato with flux in the cavities between the terminations.
What do you know about the board's assembly chemistry - "leave on" or OA washable? In general the leave-ons are OK just as long as they're properly reacted in the process. Left on the board as a liquid, say for instance when used in a "post assembly" touch-up process where the operators like to use a flux bottle, they can be trouble. If the board is an OA process build, then cleanliness is a huge issue because the chlorides and bromides in those fluxes must be washed off completely. Look around for any signs of etching around the leads of other smt parts (QFPs, underneath BGAs) that can be the result of improper cleaning. Any "fern-like" things (dendrites) growing across these caps, or smt resistors?
Do these boards undergo any ESS / humidity testing or use in a harsh, polluted environment (near power plants, dense automobile traffic, sulfur mines)? All can cause interesting problems with passive components.
Something else to consider is the layout of the board. Do you have vias underneath these caps? A tight layout that does this can get into difficulty with solderballs between the end termination and the vias. These could look OK on an unpowered board, but show up as you mention, when it's powered up.
As for the parts themselves, you could have some bad material, especially if it's coming from a broker. It's worth finding out where they came from. As for the CM buying the cheapest part they can find ... that's most likely true; however, CM's typically buy to the customer's AVL, which calls out the generic component description and the allowable vendors w/complete part number. I'd be very surprised if your cap is just listed by value, voltage, tolerance and size, and the CM is free to buy any vendor's part that fits that generic description. Your AVL probably calls out a Kemet, AVX, or some other part number(s) that the CM must purchase. True, they will hunt down the least expensive part they can within your allowable selections, but anything they buy will be a part the OEM has specified. No CM that I can think of would want the liability for a non-working assembly that was the result of having independently selected a component inappropriate for the application.