SMD pad conundrum: Solder Paste Stencil Project

Making your own components and patterns, organizing and using libraries.
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KevinA
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Joined: 18 Dec 2015, 15:35

SMD pad conundrum: Solder Paste Stencil Project

#1 Post by KevinA » 22 Oct 2017, 16:50

This all started when I purchased an 'Open Source' HAM radio project. It seems the definintion of 'Open Source' has changed over the years to what-ever those that use the term want it to mean. So I ended up with bare PCB's, schematics, BOM with a parts distribitor part numbers instead of the manufactures part numbers. Around 95% of the parts are SMD in the 2012 Metric (0805 EIA) and there are no Gerbers, Drill or P&P files available. In looking at the PCB I decided to try and make a solder paste stencil using the bare PCB as the starting point. First I scanned the PCB and played around until I got a scan that was 'useable'. Next was importing the image to my CAD program and scaling it within 0.01, using the vertical and horizontal 30 pin 2mm, 20 pin 2.54mm connectors then verifying the pitch of the .5mm processor. Next I was going to create 2012 size pads, group them together then place them over the existing image pads and here is where things got strange: The existing pads are 2mm X 1.8MM with 2.3mm pitch and .3mm gap for 2012 parts... I opened the Murata Capasitor PDF and things got stranger: 1mm X 1mm with 2mm pitch and 1mm gap. A 0805 Diptrace pattern: 1.5mm X 1.3mm with a 1.9mm pitch and a 0.6 gap, the Kemet cap is listed as IPC-7351 with 1.45mm X 1.15mm with 1.8 pitch and a 0.65 gap.

It appears when the PCB was laidout they decided that hand assembly was the build method which defies the recommendation of all SMD manufactures as they state rework requires preheating and post cooling handling to prevent damage to the part being replaced: Heat the PCB and part to xxx, place part and solder (wield as the Chinese call it) minimizing the time the solder tip touches the component, slowly cool the PCB down.
Here are the references:

Alex has stated that Diptrace uses Density Level B of IPC-7351 for DipTrace.
If you can purchase IPC-7351 documentation from IPC, if you have the money.

Diptrace
Image


Kemet
Image
https://goo.gl/JZJnJP

Murata
Image
https://goo.gl/q8ChUv

TAIYO
Image
https://goo.gl/ugjJ2P

TDK
Image
https://goo.gl/KMByQR

My version of IPC-7351C
Image
Notice the rounded corners on the PAD? This is a DXF import but the dimension tools only work on the center of those pads.

Kemet Table
Image

Changes from IPC-7351A/B to IPC-7351C
https://goo.gl/qe2Vut

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