Can I assume counterfeit components?

Question

I recently received a run of 10 PCBs from a manufacturer in China, and am worried they have begun to cut corners and source counterfeit parts. Here is why:

I had them do full turnkey production (PCB Fab, component procurement, assembly). I've used them in the past and they've been pretty good, albeit with an occasional mistake.

I noticed on 4 / 10 boards, the circuit below not behaving as expected:

^{simulate this circuit – Schematic created using CircuitLab}

On the faulty boards, whereas we expect the gate voltages of Q3 + Q5 to be either ~0 V (if NOR output = 1 (5V)) or ~12 V (if NOR output = 0 (GND)), the gate voltages were anywhere from 3-7 V...

Here is why I'm suspicious of the parts:

1. We've used this exact circuit in previous iterations of the PCB, with the same manufacturer, and haven't seen this issue. Only changes are minor difference in PCB layout.
2. After I hand-removed Q1 Q3 and Q5, and replaced them with parts I had from Digikey, the circuit functions as expected. I've done this on 3 boards, and all 3 went from non-working to working.

The relevant NPN + PMOS part numbers are given below, here are datasheet links: DMP3010 MBT2222

Alternatively if something seems fundamentally wrong with the circuit, I'm all ears. But it is a pretty common, simple circuit, and as mentioned one I've used in previous iterations with no problem.

Answer 1

You have to prove the parts are bad. They may be damaged by ESD.

If you had only pulled the Q5 or Q3 and measured V(Q1-C), that would have isolated the part as the problem. Then verify R1 is 10k and not 10M or something else.

The only weakness on the design is that the circuit turn off is slow and load reactance is unknown.

Normally any FET (such as this one rated for 8mΩ @ VGS = -10V) is driven by a gate driver resistance of about 1000x (8Ω as used in datasheet) but your R1/RdsOn ratio is about 10 million. This makes it slow and prone to oscillations with stray inductive/capacitive feedback to gate voltage depending on layout.

• also you are putting ~(4V-0.7)/1k=3.3mA into base and and capable of driving >>100mA into gate capacitance Ciss, until Vce is saturated. But pullup to turn off is only 12V/10k = 1.2mA which results in spurious turn off behavior. More design margin would use 1k for R1 at most.

Conclusion:

Test FETs for ESD damage, leakage in gate as per above. Reduce R1.

There is no assumption as to if/when the ESD damage occurred.

Article on Clone parts.

http://www.sae.org/aaqg/audit_information/2010/Atlanta/Impact%20of%20Counterfeit%20Parts%20NASA.pdf

Answer 2

I suspect they may have substituted a MOSFET that has low voltage gate protection zeners for the DI parts. There is no motivation to sub Q1, they're dirt cheap in China and any similar part would work as well. The MOSFETs, on the other hand, are expensive.

Your circuit switches off slowly so it could put a lot of stress on the MOSFETs if the loads are low impedance, but that would not likely cause the observed effect (though it could conceivably under some conditions).

If you have another misbehaving board try swapping out just Q3 if you want test my theory. You can also contact DI with a high quality photo of the part markings and ask them if they match parts made for sale anywhere in the world. Of course you can compare them visually with the parts you bought through distribution but there are often multiple packaging facilities used and the markings may vary somewhat, especially (but not exclusively) for very different date codes, so a difference is not conclusive. If they look exactly (to a critical eye) the same including method of marking, font, and small features on the transfer molding, it's a pretty good indication the parts came out the same factory.

For such a tiny quantity they probably sent someone down to the market (in Shenzhen at Huaqiang bei lu) and got whatever parts were available from one of the many retail vendors. If you want to be 100% sure, send your own parts to them, especially if the quantities are modest).

Answer 3

I bet you do have a parts issue, However the issue is likely due to your MBT2222. A widely used generic N2222a transistor is likely being swaped and that has been fabricated by hundreds of manufacturers to who knows what specifications .