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I am here to clarify my doubt on paste mask layer for through hole components.
I know the paste mask layer is also called the stencil layer.
It's only used for assembly. I want to know if the paste mask layer (both top and bottom layer) is necessary for through hole components. For smd components I know the pastemask layer is required to solder the components. Is it necessary for through hole components?
Paste mask is normally used only for reflowing surface mount components, while the through hole components are soldered either manually or by wave solder bath at a later stage.
If you have through hole parts and the surface mount components on the bottom side, the paste mask stencil is not required, as the parts will all be soldered at once in a wave solder bath. This process requires you to generate a glue dot coordinates file, which will be used to place a dot of glue under the surface mount component and that will keep the part in place during the soldering process.
It is also possible to reflow through hole components with solder paste, but that requires some tweaking to get the right pad size and stencil opening. I used to have a customer who required that, as the only through hole parts in the boards he made were the connectors. All we did was have a larger pad on the top side, so that there was enough paste on the pad to wick down the hole and ensure a solid solder joint.
But in general you don't normally put through hole components on the stencil, otherwise once you reflow the boards you will find all the holes filled up, and you will really struggle to assemble the TH parts.
If you have SMT components on the bottom side of the PCB and there are no THRU-HOLE parts they you will need to provide a paste mask for the bottom layer.
As described in comments under your question, if you have SMT components on the bottom side and have some THRU-HOLE parts then you may not need a paste mask for the bottom layer depending upon how the THRU-HOLE parts are meant to be soldered.
Best practice is to simply always provide the paste mask for the bottom side of the board and let the the assembly house decide if they intend to re-flow the bottom side SMT components or to glue those SMT parts in place and use a solder wave to handle the bottom side.
Also it is needed to provide the centroid data for all components on the board to the assembly shop. This is needed for both pick and placement of components and also used if glue spot locations are needed for bottom side SMT components. The easiest way to provide this data to the assembly shop is to take your layout CAD file and save it in an ASCII text format. Assembly shops will have the necessary software tools to extract the data that they need from the ASCII format layout file as long as you are using some mainstream CAD package.
