Electrical Over Stress - Looking your assistance to resolve the EOS issue in Automotive ECU.

Thread Starter

Ganesan Magizh

Joined Dec 24, 2019
1
Dear Friends,

I am working in an Electronics Manufacturer service (EMS) and we do PCB assemblies for Automotive products (ABS).

We are getting many field failures due to components burnt issue. Mostly like diodes, ASIC, MCU and FET components are getting damaged/burning in the field.

Whenever we are sending the defective parts to components supplier for analysis the result from them always is ELectrial Over Stress (EOS).

We have ESD protected manufacturing area and following the ESD standard during the manufacturing process and we ensured that the solderability of the PCBA is good.

I would like to know the possible cause of EOS. Please share your experience and suggestion on EOS.

Thanks in Advance.

Ganesan
 

Tonyr1084

Joined Sep 24, 2015
4,125
Experience: A tool being manufactured had a highly sensitive device on it. It was critical testing was carried out at many points in the assembly process, as you didn't want to spend all that money building the tool only to find out it didn't work in the end. This particular PCB was built without this sensitive device. It was tested as far as could be. No faults detected, they'd install the component. Test it again and assure it was functioning properly. They would then move it into the housing it was to be maintained in. Tested it. It worked. Then they would make some final connections. Test again and there was the failure.

The work bench was properly grounded. So was the soldering irons being used. So was the operator. However, the computer they were using to test the device was plugged into an extension cord that had a ground fault in it. Nobody noticed it until I was inspecting the assembly while being tested. I put my hand on the tool, but didn't notice any electrical sensations. With my other hand I was fondling the power strip which was a painted surface. however, the rivets were not. When I touched a rivet I experienced 67 VAC. I quickly spoke up about this and upon investigation, the cause of EOS was coming from the computer. Remember, the soldering iron was grounded, that included the tip. When touched to the part - the pathway to ground was complete and the component was blown.

The term EOS simply means more current than the device is designed to handle. Imagine you have a component capable of handling 50 amps. Imagine inadvertently exposing it to 55 amps. It's over stressed. This EOS may or may not result in an immediate failure. The REAL danger in EOS is a component that is weakened from the EOS event but continues to work without detection until a mission critical moment (or life critical moment). Many parts make it out into the field where their EOS events become evident. However, lets not overlook ESD. I've seen ESD actually CAUSE problems. Yes. An improperly grounded station - such as one that is supposed to be grounded through a 1 meg ohm resistor - suppose someone thought no resistance would be better - in that case an ESD event would fully strike the component on the bench. Grounding mats are typically through (I believe) a 10 MEGΩ resistor and operators are typically grounded to either the mat or more properly to ground through a 1 MEGΩ resistor. Direct grounding is not only dangerous to the components it's also dangerous to the operator. A proper auditing of the ESD system should be done once a month. However, many businesses I've visited have their ESD certified and then never check it again. One business had a wrist strap test station that could not be relied upon. I reported to quality control that I could test and pass without being plugged in AND without standing on the foot strap test pad. Yes, I could pass with absolutely no grounding whatsoever. That company did nothing about the test station. If it were my purview I would have removed that test station and replaced it with one that could not be fooled. I've also seen people SIGN IN stating their straps and footwear passed and they NEVER SET FOOT ON THE TESTER. NEVER TESTED! There are plenty of examples of ESD violations that happen.

One instance I saw and reported was a young woman carrying a plastic grocery bag through the test department, swinging it as she passed within less than 12 inches of sensitive defense contract PCB's. Upon reporting this she was reprimanded for 1) passing through the test area without authorization, and 2) having ESD hazardous materials in the work area.

There ARE ESD violations. But worse, there's dependency upon an ESD system to be functioning properly when it is in fact in fault.
 
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