The Importance of Stencils
Stencils are the first piece of custom tooling that the PCBs will interact with during assembly. Thus, it makes sense to make sure it is made correctly. EPCB lists 5 reasons why you need to edit your solder paste stencils.
1. Paste Layers Are Not Always Correct
Although PCB designers follow every specification to set up the components, there could be mistakes on the paste layers. For example, dimensions of the paste apertures could be wrong, paste layer for a component could be missing, fiducials could be missing from the paste layers, etc.
2. Paste Layers Are Not Always Panelized
Often paste layers from the original design are not be panelized. Ask your PCB supplier to provide you with the panelized paste layers and send them to your stencil supplier. This ensures the panelization offset is done accurately and it will match your PCB.
3. Stencil Fiducials Need Special Care
Some fiducials are clustered around other SMT components, while some PCB designers use squares as fiducials. So you’d better tell your stencil supplier which apertures are global fiducials so they can fabricate them differently.
4. Adjust Solder Properly
Larger pads like DPAK, TO, SOT, etc. need more solder, while QFP, TSSOP, QFN need less. Adjust solder volume according to the aperture size is important, otherwise insufficient solder, open, bridging, excess solder would appear.
5. Check Area Ratios
Area ratios on small apertures should be checked and compared against the foil material to be used. Printing performance will suffer if the area ratios are too low.
In short, the solder paste stencil is a critical piece of tooling in the assembly process. Spend enough time to make sure it is done correctly and it will greatly improve the assembly yield.