Solder Paste / Mask Sizing - Page 1
Forum section: electronics / general

author: fastbike
author_rank: Regular Contributor
timestamp: August 05, 2023, 11:35:47 pm
content: I've a board currently being produced by JLCPCB that has a QFN48 (SAMD21) and DFN3x3 (MCP2562). As well as the perimeter pins these devices have a centre ground pad. To prevent excess solder paste being applied I have used reduced apertures for the centre pads in the stencils (a 3x3 grid for the QFN and 2x1grid for DFN) to give around 70% opening.JLCPCB have sent a query asking if I want the size of the solder pads increased in the stencil. I've question is this the signal pads or the centre pad but have not received a clear answer.However the Ucamco Gerber viewer shows the same size for the original file I've sent them and the file they have linked to my order.Am I safe to give them the go ahead to proceed ?

author: Smokey
author_rank: Super Contributor
timestamp: August 06, 2023, 03:42:15 am
content: I've never had them push back on stencil apertures. Even when I obviously screwed it up and I wish they would have.I'd ask what their concern is. Have them send you a screen shot or something.Also, there is typically a "recommended paste cutout" or something like that in the part datasheets that's a good idea to follow. If you did that, you are probably good.I've never used the quick turn stencils for anything important though. I've always been spoiled for production stuff since my CM handles the production stencil based on all of their processes and paste and all that.

author: fastbike
author_rank: Regular Contributor
timestamp: August 06, 2023, 05:01:40 am
content: Thanks for the insights. They sent me a screenshot pointing to the two components but that was all, nothing about the size, or even if it was because I used a grid for the centre pad to prevent too much solder paste being applied (which could prevent teh device from seating properly.)Anyway with the pattern I'm using (for the 0.50mm pitch), the pads measure 0.85 x 0.25mm and the layout software (DIPTrace) has applied a shrink factor of 0.05mm, so the finished stencil opening is 0.75 x 0.15mm (0.1125mm sq). I notice that Proto Advantage have a stencil for QFN48 for sale on the digikey website. Their stencil pad size is 0.60mm x 0.23mm (0.138mm sq) so a little more solder is deposited.I will be using a lead based paste with T4 grade solder particles. I'm hoping it will be OK, this is a trial run of 6 boards - I can increase the stencil aperture for the next run.

author: Matt-Brown
author_rank: Regular Contributor
timestamp: August 09, 2023, 06:13:39 pm
content: On many boards the exterior leads are extended out from pads on the board. This is to aid any manual soldering if one or more leads is lifted (makes it easier to get iron onto the pad). Not knowing what you have I suspect it is the outer leads and not the center pad they are questioning (possibly 0.5MM or so extended pads is what the are asking for). Also on stencil it helps get more solder at the edges of the part)

author: NorthGuy
author_rank: Super Contributor
timestamp: August 09, 2023, 09:38:22 pm
content: The amount of the paste deposited on the pad depends on the thickness of the stencil. Hence if your stencil is thicker you would want a smaller opening to get the same amount of paste.

author: fastbike
author_rank: Regular Contributor
timestamp: August 12, 2023, 09:48:01 pm
content: Quote from: NorthGuy on August 09, 2023, 09:38:22 pmThe thickness shows as 0.12mm on the JLCPCB website.I increased the size of the apertures and resubmitted the solder paste gerber, that's what they've agreed to supply now.I can't complain about the service ! Or the price (USD7) !