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Resonator Cracking

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Resonator Cracking | 16 November, 2007

I'm requesting assistance in developing detection and containment methods for an issue with an SMD ceramic resonator (Murata CSTCE-G-A family). The returns have external visual cracks that originate at the termination and run at approximately a 45 degree angle.

The lab has looked at the parts and classifies the failure as a flex crack. Murata came to the same conclusion and this agrees with literature that I have researched from the internet. The two challenges we have are detection / containment and root cause.

Brief process description: The resontaor is placed onto an FR4 PCB (no adhesive) and re-flow soldered (63/37 tin lead solder Indium SMQ92). After ICT, the opposite side of the PCB is populated and re-flow soldered, so the resonator goes along for a second ride through the oven. After final ICT the PCB is de-paneled using a router machine. It is then sent to the switch assy line where it is programmed and installed into the switch.

Detection/Containment: This is a huge challenge. We have latent failures with as much as 13,000 miles on the vehicle before failure. My understanding is that the original stress event may produce a very small crack that is not visually detectable and may not significantly change the electrical characteristics of the part. If that is true, how can a stressed part be detected in a production environment? Please let me know if you have, or if you know anybody that has, any insight into this.

Root Cause: My undersatnding is that flex cracks can be caused by two stress events, the first being PCB bending and the second being a Coefficient of Thermal Expansion (CTE) mismatch between the part and the PCB (perhaps during the second reflow operation?). If both events produce cracks that look the same, how do I know where to look for root cause? Is there a certain physical feature of the crack that we should look for to determine if it is PCB bend or CTE? Also, what effect does solder quantity have on flex cracking?

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Resonator Cracking | 16 November, 2007

Stress cracks generally happen at wave solder, not reflow. But so you know, stress cracks from heat occur from the body and work towards the leads. They are generally circular. In the past, these cracks where blammed on the nozzle of the placement machine. Now, seasoned engineers know better (well, some do).

Physical stress starts at the termination and works across the body. This generally is the cause 99.9% of time if all you have is reflow. I would start at ICT. ICT can really put some stress on the boards depending on the machine. Also check how you mount the board. A screw/bolt pattern may have to be developed to not create stresses on the board during final mounting. You sure the operators are using the router? You'd be surprised.

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Resonator Cracking | 16 November, 2007

Fractured parts that we've seen at reflow in the past are related to trapped moisture in the part, out-gassing at temp, and fracturing the part. I have seen this on ceramic resonators in the past; don't know why this type of part seems prone to this.

How is the part oriented to the board edge? If it is perpendicular to the board edge, there is a higher chance of stress on the part, particularly at de-panelization. You describe the process as boards being de-panalized using a router; is this truly the process? I only ask because in the past, while we had a depanelizer, my assembler routinely broke v-scored panels out manually rather than use the depanelizer.

Is it possible that the depanelizer could cause the issue?

Minor stress fractures can be detected by x-ray. If you have x-ray capabilities, I'd say perform each step of the process, then x-ray the component to see what's going on in there.

Additional containment: Add a burn-in step in house before the product leaves. Or an environmental stress test. Cycle temperature and humidity. This might not be a bad test to help isolate the occurence, could test multiple parts from throughout the process in an environmental chamber ie. new part, part after first reflow, after second reflow, etc. If you can detect the latent failure using this method, you might be able to find where in the process the issue is introduced.

cheers ..rob

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Resonator Cracking | 17 November, 2007

You mention moisture in the part as a cause of cracking during reflow. What does this type of crack look like? How was it determined that moisture was the culprit?

The resonator is near the middle of the PCB, but is about 5mm from a large hole (about 15mm x 30mm).

There is no way to break the PCB webbing without the router. I've watched the operation numerous times and don't see any opportunity to avoid using the router.

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Resonator Cracking | 19 November, 2007

We determined moisture more as a diagnosis based on omission. We eliminated the other variables, noted that the parts were cracking in reflow, and deducted that it was trapped moisture. As the moisture expands, the component can be cracked/destroyed. The analysis was somewhat confirmed by pre-baking the parts pre-assembly, and observing that the cracks in the components went away.

I can't give you a clear visual that would help you identify moisture as a root cause. I've seen components violently explode, leaving only the endcaps on the board, and have also seen components simply exhibit a crack down the middle of the component.

cheers ..rob

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Resonator Cracking | 19 November, 2007

J Webster: Can you send / post pix of your cracked capacitors?

What do you think about our assumptions? They are: * No flex in end-item use * Supplier has no defective packaging * No hand soldering of these parts * No impact or flex from ATE * No H2O absorption

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Resonator Cracking | 19 November, 2007

I'll e-mail pics to you.

Answers to asumptions: * Flex in end-item use was measured at 410 microstrain * Supplier PPM last 12 mos less than 1 * Part goes through 2 reflow cycles; once on top and once on bottom * ICT microstrain was 1365; reduced to 1050; further reductions in process * Impact from one process step was 1550 miscrostrain; since reduced to 260 * Impact from another process step was 2400 microstrain; since reduced to 305 * No H2O absorption (have not verified or ruled this out)

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