Solder Mask / Stencil Oddity from Manufacturer - Page 1
Forum section: electronics / general

author: mathmatic
author_rank: Newbie
timestamp: July 14, 2017, 04:38:34 pm
content: So, I recently had a small-ish batch of boards (100) + stencil made up and they didn't come back quite as expected. I've never used this manufacturer before (although they're one of the popular Chinese web based PCB services), and not sure if I should contact them about it, or if they just decided to do some magic to make my life easier w/o telling me. I double checked my gerbers via an online gerber viewer (just in case my CAD package was being odd), and they appear normal. Thought someone might be able to tell me what's going on here. (Pics attached)First, the boards came with solder mask only over the copper traces, none on the FR4. Pads are all correct. I don't think it should seriously effect anything (and I actually kinda like the aesthetic), but just curious what might have happened / why. Wondering if this is purely because I selected yellow for the mask (and FR4 is yellowish). Just concerned the FR4 might burn / brown a bit.Second, most - but not all - of the stencil pads came with small indents on the inner sides of the parts. Admittedly, this is the first metal stencil that I've ever had fabbed for my personal use, but I've never seen this before. Is there a name for these? I can only figure that it is a "feature" that the manufacturer went ahead and added in to help parts reflow better? Not sure why all the pads aren't like this then though... maybe an odd size?Thanks!

author: kony
author_rank: Regular Contributor
timestamp: July 14, 2017, 06:18:34 pm
content: The soldermask is all over the board as it should be, you'd notice the thickness difference (edges) if there was any additional opening in blank board area.What happend to your stencil is that someone in charge of manufacturing concluded that you sent soldermask gerber instead of ready for production paste gerber - and reduced your paste infill on some pads accordingly. It is usually good idea to include"Do not mess with my gerbers!"for chineese fabhouses.

author: mathmatic
author_rank: Newbie
timestamp: July 14, 2017, 07:58:50 pm
content: Oh, interesting... after closer inspection I think you're right about the solder mask. I tried scratching off a section and it looks almost identical underneath. I can't actually tell unless I put it under a bright light and check the reflectivity, there is a difference though.Re stencil: Noted. Might contact them about that one and see what they say. Thanks. (Was really hoping for the magic feature, lol.)

author: Yellofriend
author_rank: Regular Contributor
timestamp: July 15, 2017, 12:44:26 am
content: Quote from: mathmatic on July 14, 2017, 04:38:34 pmThat PCB maker was well experienced and made you a stencil according to IPC-7525. This is how all stencil should be made.https://www.pcbcart.com/skins/default/en/IPC-7525.pdfQuote from: kony on July 14, 2017, 06:18:34 pmUsing the S/M as stencil gerber will lead to more solder balls and might lead to IC bridges. It will get too much solder paste onto the PCB. I rather have too little, specially for the ICs.

author: Gary.M
author_rank: Regular Contributor
timestamp: July 15, 2017, 01:07:30 am
content: Those anti-balling stencils are a bastard to clean. They tear at the wipes...

author: mathmatic
author_rank: Newbie
timestamp: July 15, 2017, 01:18:17 am
content: Quote from: Yellofriend on July 15, 2017, 12:44:26 amJust to clarify (in case it wasn't clear), the stencil / paste gerber I gave them is not the same as the mask, and the cutouts on it are indeed smaller than the pad size. Are you suggesting that the board house thought them to still be too big and added small inner tabs to create a smaller surface area? Why not just decrease the dimensions slightly and leave them square if that was the case? Or perhaps I am misunderstanding...

author: Yellofriend
author_rank: Regular Contributor
timestamp: July 15, 2017, 04:17:05 am
content: Quote from: mathmatic on July 15, 2017, 01:18:17 amI am curious, did you gave them stencil gerbers? From what software? It seems they were not according to IPC-7525. And I wonder what they used to convert to IPC-7525 format.Yeay, cleaning is a bit tricky. I use alcohol and a hard brush with very short hairs. A toothbrush might be used too.

author: mrpackethead
author_rank: Super Contributor
timestamp: July 15, 2017, 04:33:35 am
content: Quote from: mathmatic on July 14, 2017, 04:38:34 pmThat yellow solder mask is quite strange like that. I almost thought the same thing when i first saw it. It does look like its totally covered your board.QuoteThe reason they are there are an attempt to stop tombstoning or balling. However its quite dependent on your process if it will help or hinder you. A lot of high end stencils dont' have the tabs.i take a LOT of care with my stencil appartures, and they are all carefully considered, calculated and tested. When i send them to the fabricator, i put a big note on them that says " DO NO ADJUST APATURES - MAKE PER GERBER ". I have differnent reductions for different parts.. Its just easier for me to do this myself.