Stencil shapes... is this normal? - Page 1
Forum section: electronics / beginners

author: crgarcia
author_rank: Regular Contributor
timestamp: April 04, 2022, 12:10:41 pm
content: I sent to JLCPCB my first PCB with stencil ever, and this will be my first SMD soldering everWhen I look at the gerber files they produced for the stencil, I see some weird shapes instead of square pads. Is this normal?This is my kicad:My full project:https://github.com/crgarcia12/electronics-homeassistant-lightscontroll/tree/main/PCB/schematics-v17-safety-jlcpcb

author: Siwastaja
author_rank: Super Contributor
timestamp: April 04, 2022, 12:13:26 pm
content: Not normal. For this reason, you normally output gerbers from your PCB EDA software yourself. Some manufacturers allow you to send PCB EDA design files directly, but it makes little sense as it only adds another factor of uncertainty.Just output the gerbers yourself and do a thorough check using another (third party, not your PCB EDA) gerber viewer. For example, I use gerbv from the geda project to do the final check.

author: wraper
author_rank: Unknown
timestamp: April 04, 2022, 12:21:18 pm
content: For Chinese stencils it's "normal". Even if you provide gerber files to them. These things are added to reduce the amount of solder paste if factory thinks you apply too much of it. Basically if you want for them to not tamper with your stencils because you actually know how much solder paste should be applied, you need to ask them specifically to not do this. And even then they may still add them. It's worse when they don't alter the shape and just reduce the opening size, much harder to notice the tampering.

author: gamalot
author_rank: Super Contributor
timestamp: April 04, 2022, 12:22:44 pm
content: Yes, JLCPCB will change some pads for you, their main purpose is to prevent too much solder paste on the pads.

author: crgarcia
author_rank: Regular Contributor
timestamp: April 04, 2022, 12:29:13 pm
content: Thank you all!There are so many details when building a PCB... Every resistor I place, I discover there are a million more things to learn.It's good to hear that JLC are fixing my mistakes

author: crgarcia
author_rank: Regular Contributor
timestamp: April 04, 2022, 12:41:52 pm
content: Quote from: wraper on April 04, 2022, 12:21:18 pmIf you look at the right of my pictures, they did it for one, but not for the other one. Do you know why?Thanks!

author: gamalot
author_rank: Super Contributor
timestamp: April 04, 2022, 01:35:32 pm
content: Quote from: crgarcia on April 04, 2022, 12:41:52 pmI've seen a via on the upper pad of the rightmost component, and placing a via on the pad is generally not recommended.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 04, 2022, 03:48:50 pm
content: I think that shape is done on purpose to greatly reduce the chance of creating solder bridges when placing the part, also should improve self-aligning when melting.With full-shape pads, part of the paste will expand (squeeze) under it.

author: tooki
author_rank: Super Contributor
timestamp: April 04, 2022, 04:08:50 pm
content: Quote from: crgarcia on April 04, 2022, 12:41:52 pmProbably because the stencil apertures on the rightmost component were already smaller.As a rule of thumb, reducing the stencil apertures by about 15% is a good starting point, and especially for hand application (where it’s extremely easy to apply too much paste), I think reducing them a bit more is a good idea. Apertures the same sizes as pads generally lay down far too much solder, resulting in solder balls and shorts.So it’s not JLC “tampering”, but trying to help. (And this is stated on the website somewhere.) I’m sure one can tell them to not do this if you have already done it yourself.Note that this is in no way unique to JLC. I bet some PCB houses simply do it by reducing the size of the apertures (rather than cutting a shape into it), such that it goes unnoticed.

author: DIYGUY Chris
author_rank: Contributor
timestamp: April 04, 2022, 11:11:35 pm
content: They are not fixing your mistakes, actually there are no mistakes out there, this pad shape is universal for many PCB manufacturers to prevent the excess of solder paste which minimize the risk of getting solder beading, it is all detailed here in their website post:https://support.jlcpcb.com/article/77-solder-beading-treatmentAbout why they did this for Stencil only and not for the other file, since the other file is used probably for your PCB manufacturing order so it has to have the original footprint size and that's thatNow you know some new technical terms like (solder beading) enjoy your assembly journey

author: crgarcia
author_rank: Regular Contributor
timestamp: April 05, 2022, 12:19:59 am
content: Thank you very much all!Quote from: DIYGUY Chris on April 04, 2022, 11:11:35 pmIn there it explains why some pads do not have it. they are diodes! (I'm wondering why they are an exception)QuoteLet's see how it goes my first reflow