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Aperture Adjustments and Etch Compensation on Arrays

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SMTA-Jon

#82236

Aperture Adjustments and Etch Compensation on Arrays | 28 March, 2019

As a CM, we don’t have a lot of input on the PCB layouts we receive. Some engineers/software packages output the paste layer 1 to 1, some do an across the board reduction, and some set each part up individually with different reductions per footprint. Thermal pads are also routinely an issue with supplied files applying paste over open, unfilled vias. As such, we have found it preferable to generate new paste layers directly from the copper using a standardized modification profile. That has worked 98% of the time.

Recently, we had a serious bridging issue on a single 0.5mm pitch QFN14. The array files had etch compensation applied and the fabricated PCBs were over-etched slightly. The stencil aperture reduction was done from the adjusted array Gerber, so the stencil didn't gasket properly on the undersized pads.

I would like to know if others have had similar problems and, if so, what changes were made or tools used to prevent them.

This message was posted via the Electronics Forum @ SMTASMTA

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#83447

Aperture Adjustments and Etch Compensation on Arrays | 23 September, 2019

Cannot understood your problem very clearly, please check IPC standard on QFN and its mfg process, also check the QFN supplier's recommendation, so can minimize the bridge.

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#83457

Aperture Adjustments and Etch Compensation on Arrays | 23 September, 2019

I have seen similar things happen with over etching and bridging but frankly the most common mistake that gives us grief is a failure to use the component manufacturer's recommended land pattern and/or aperture designs.

As opposed to generating a new paste file based on a standard recipe, we usually just compare the paste layer to the copper and make adjustments where we think it's needed. It's time consuming and sometimes we miss stuff (resulting in a second stencil purchase) but seems to work best for us.

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