PCB Assembly Services - Screaming Circuits: Footprint Mismatch, Part II


Footprint Mismatch, Part II

A question just came in about the "Footprint Dimension Mismatch" post below:

8mm_on_a_30mil_pitch_footprint_sm "One thing that always makes me wonder is that, despite the footprint mismatch, does this still fall within IPC Class II standards? As far as I know, that's what you guys assemble to, and I heard somewhere that as long as 50% of the lead is aligned over the pad it falls within Class II spec. Is that true?"

It is a good question and does a bit to illustrate the challenge with standards. According to our engineers here who are very familiar with the IPC standards, it does meet IPC Class 2 (IPC-A-610 Rev D, Section 8.2.5 pg 8.41-8.53). So, this does in fact comply with the IPC Class 2 standard.

Our engineer states: "Technically, yes, these parts would pass the IPC 50% rule, assuming the wetting is good and there is wicking / a fillet where lead is on the pad. That being said, the picture indicates a pad pitch issue that amplifies as you move out from the center pads in either direction. I'm not a structural expert, but have to believe this pad-to-part mismatch cannot be good for mechanical integrity. It also increases the chance of shorting between pins if placement is slightly off or the part shifts as the solder reflows." We will still notify the customer in a case like this, but since it does comply with the standard, we would go ahead and build it.

In a prototype environment, that's most likely okay. Since it meets the IPC standard, we can defer to the judgement of the customer and go ahead and build it. Again, if this were a production environment, it would not be okay even though it meets the standard.


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