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Check Your Passives First

Mislabeled passive components (reels of resistors and capacitors) are surprisingly common in the supply chain. Testing SMT passives first can be the best option.

Once placed on boards, the wrong part is very hard to detect and expensive to correct. We buy reels of SMT passives by the thousands for our contract manufacturing business, usually through major distributors. Several times a year we get a mislabeled reel, so that the true value of the component is different from what the label says. If we don't discover this error before placing the parts, at 5,000 parts per reel, that's a lot of bad boards.

Worse, normal production tests are unlikely to catch this type of error once it occurs. Most surface-mount technology (SMT) passives we use today are 0603 (0.06″ x 0.03″) and smaller, and not individually marked due to their small size. One hundred percent automated optical inspection could confirm that a passive with the right package size is placed in the right location, but its value cannot be determined: a 10-ohm resistor looks exactly like a 10K-ohm resistor to a camera.

In-circuit testing could catch this type of error in perhaps half of the cases. However, most boards for commercial and industrial products aren't subject to the extensive level of testing that would be needed. Consider a few examples:

  • A board with hundreds of ICs, each with a decoupling capacitor. All the capacitors are connected in parallel to the power and ground. In-circuit testing would show the sum of all the capacitors but not the individual values. With values ranging from nano-farads to tens of micro-farads, it would be impossible to know whether a 1nF capacitor was used instead of a 10 nF. The board would also work fine in functional testing. But it would probably fail for EMI, which is not a production test.
  • A coupling capacitor of the wrong value on a microprocessor. This could cause a slight delay before the microprocessor turns on, but it would turn on and the board would pass a flying probe test. It may then exhibit erratic behavior in the field, such as race conditions in the timing or failure to turn at certain temperatures.
  • A board with the wrong pull-up resistor in a circuit. The device may function properly, but draw too much current. This would most likely show up as end user complaints about short battery life.

How does the wrong part get onto a board?
Occasionally, the manufacturer labels reels with the wrong part number. More often a distributor makes an error when creating a barcode label for inventory control and tracking through the supply chain. Sometimes, we make an error when we enter the part number in receiving. Occasionally an SMT pick-and-place operator loads the wrong reel into the machine. All are human errors, either making typos or misreading a long number (often from a long list of numbers). All are very hard to avoid.

Ask any manufacturer when they last built a board with the wrong part, and the answer will probably be within the past than six months. Ask what procedures they have put in place to prevent it from happening again, and the answer will probably involve more human intervention: checking and double-checking the data entry and barcodes and machine setup.

This is not inherently a bad thing, but human intervention is usually the most expensive and least reliable solution to this sort of problem.

A better solution is to set up automated testing to verify the component value before placing them. At Z-AXIS we use pick-and-place machines from MyData that have the option to test passives for correct value — and for diodes, also the correct polarity. By enabling this option and checking several components every time a new reel is loaded, we avoid placing wrong parts without adding a lot of labor cost or significantly effecting throughput.

18 comments on “Check Your Passives First

  1. t.alex
    August 12, 2013

    Indeed passives are the most ignored components, even during development stage, not to mention production stage. How dangerious it is to have a mislabelled reel ! A zero-Ohm resistor that's accidentally slipped into the board could kill every thing. 

  2. Wale Bakare
    August 12, 2013

    I think it's a worthwhile nice piece for hardware engineers/designers. It's very good to have an alternative or perhaps better testing for passive components.

  3. prabhakar_deosthali
    August 13, 2013

    Could a  ” bed of nails”   kind of test point out such wrong passive parts?

    May be yes . But building such bed of nail test rigs for each board to be tested itself is a task which takes a lot of human effort.

    So a system where each reel is checked manually for correct value , is the safest way to avoid wrong passive components getting inserted .

  4. Mr. Roques
    August 14, 2013

    They test a few from the reel, not every single passive component, right? If the problem is mislabelling, then its not that bad. 

    If the problem is organizing them and that you can get a mix of values, that's harder and more expensive to solve.

  5. _hm
    August 14, 2013

    What a true life story! I have spent days to find these little guys in most urgent schedule. It takes a lots of effort and rework is very painful. Sometime you have 25 or more parts with same value and all wrong by one decimal point.

    My advise for CM is to perform incoming inspection for these little fellows. Just measure value for two to five samples in 5000 pc reel.

     

  6. elctrnx_lyf
    August 15, 2013

    If it is not really expensive all the pick and place machines would come with the feature to measure the value of the resistors and polarity of the parts. I believe the reason for not having the feature is both the economical and technical constraints.

  7. _hm
    August 16, 2013

    It may be that this source of error in passive first may be attributed to select few manufacturers. Also, if detail analysis is done, it may turn out that only some kind of machines or operators are involved for this source of errors.

    Is it possible to investigate and analyze how the this error occurs? It may not be that difficult to prevent. And it is very essential to prevent it. We do not want to change 0201 and 0402 resistors in 10s or sometime 100 per PCB assembly.

  8. FLYINGSCOT
    August 16, 2013

    It is not the major issue but I was wondering if the supplier has any liability for a mislabeling error or is the onus on the consumer of the goods to check suitability?

  9. t.alex
    August 16, 2013

    FLYINGSCOT, this is a good point. Pehaps supplier should have some sort of guarantee and compensation should their reels are mislabelled. 

  10. Hailey Lynne McKeefry
    August 18, 2013

    @Michael, do you have any sort of stats about how common mislabing is? Is it more common in certain types of parts? (I am assuming, smaller parts more mislabeling)

  11. Hailey Lynne McKeefry
    August 18, 2013

    The human element is always the most common way that mistakes get introduced into systems. Ironically, it's probably a human element, in this case, that is also teh best panacea.

  12. Hailey Lynne McKeefry
    August 18, 2013

    @_HM, sorry for your pain! Do you think these kinds of problems would make you look to source products elsewhere? Or is it just something that happens and you have to remain alert to it?

  13. ahdand
    August 19, 2013

    @Hailey: Good question. My advice would be to be on your toes and try to figure out at the beginning itself so you can mitigate the issue then and there itself before it gets out of hand.       

  14. Michael Allen
    August 19, 2013

    Hailey,

    We mostly see it in the passives like resistors and capacitors. Seldom are IC's miss marked. We have seen them missed marked from the manufactures as well as by the distrubutors. It sort of reminds me of the old days when non ROHS parts came in marked ROHS but our XRF equipment clearly indicated lead on the end caps.  So it all comes down to trust but verify.

  15. Hailey Lynne McKeefry
    August 19, 2013

    @Michael, “trust but verity” is a great motto in many instances. And passives are something like flour in the baking world–no one thinks about it but you can't bake without it.

  16. SunitaT
    August 25, 2013

    The supply chain has been facing these problems for a while. The parts that come on board need to be tested and retested. Human errors prove to be costly and often a company has to provide support that costs even more money.

  17. SunitaT
    August 25, 2013

    The supply chain needs better testing equipment. As the article explains that a ten ohm resistor looks the same as a high output variant. However this is easily said than done. The process of testing when done through human hands is tedious, and when done through high tech testing equipment proves costly.

  18. Eldredge
    September 3, 2013

    Michael,

    Thanks for a good overview of the potential problems of incorrect passive components. Sometimes it can even come down to a device with the correct value, but incorrect tolernce in a particular circuit application, producing borderline circuit behavior.

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