We have generally seen that as volumes increase, quality can sometimes suffer. There are various reasons for this, but, we have found that the majority of them involve rushing and minimal process controls.
How to handle it with your CM? Well, it would all depend on your relationship with your CM. I prefer to begin with a discussion. As a CM, it is my goal to always provide high-quality product to my customers. If something is happening, and I'm not providing that quality, I want to know about it. I also want to perform a full failure analysis and corrective-action.
If any of my customers were experiencing problems, I would first expect them to call me, and advise me of the issues. For the issue that you describe above, I would like the opportunity to examine the boards in question. I would want to be able to see the boards in situ, and see the parts that had fallen off the boards. It's relatively easy to diagnose whether the parts are breaking off of the board (potentially related to handling), or if they weren't properly reflowed in the first place. I would expect my customer to issue me a corrective-action request for the issue. It's possible that true root-cause might not be determinable, but, an investigation should take place anyway.
No one should get defensive; but, that goes back to working relationships, etc. Start with a phone call, describe what you're finding, and request their assistance with determining what is happening. Play on the line that...they're the board assembly experts, and you need their assistance to figure out what the heck is going on. Their goal should be high quality, and, they should want to help you determine what is happening.
I'm a big proponent of working hand in hand with my customers. At the end of the day, we're a team...and need to work together to accomplish both of our goals. The attitude you describe above does not seem like this supplier holds that viewpoint. Or, that there has been a rocky relationship between your company and the supplier, likely for some time now.
If the supplier is unwilling to assist you in determining what is happening with quality, than I'd strongly recommend beginning the search for a new supplier.
cheers ..rob
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