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It's Alive! Part 017

COMPACT

An EAGLE Eye for Detail

Fine Tuning Parts Libraries

I've made plans to produce my own solder paste stencils.

For this I need accurate solder paste masks for the components.

 

The bq51013a part from one of the many EAGLE parts libraries has an incorrect solder paste(tCream) mask - it has a single large rectangle and needs to be divided into 4 separate smaller rectangles.

This can be seen from first PCB layout picture from Part 015.

To most this seems an unimportant issue but it is important because doing it correctly increases the chances of success.

The reason for having 4 smaller rectangles to produce a solder ponds with the right height be compatible with those formed on the smaller pin.

A single large rectangle has a high risk of forming an overly tall solder blob which can throw out the component alignment.

 

Also I've noticed that the parts libraries seem to have inconsistencies between them particularly with the component overlays.

Some have printing over the exposed pads, others are the size of the components others are larger than the components and so on and so forth.

I'll have to select and edit the ones for my PCB to keep them consistent.

 

My First Attempt at a Solder Paste Stencil

Solder paste Stencils are usually made out of staineless steel and there are some organisations that offer Kapton and Mylar substitutes.

This is a picture of my first attempt using self adhesive vinyl. It's not ideal because a single pin divider broke off but it's a great start (and is still useable for this project).

The idea is that the vinyl solder mask is temporarily adhered onto the PCB, the solder paste even spread across it and then the vinyl stencil leaving the desired solder paste deposits.

 

The ruler in the picture is in Centimetres with millimeter graduations to give an idea of the small scale I'm dealing with.

The solder pad sizes are about 0.23mm x 0.7mm.

 

image

  • in reply to DAB

    My 3D printer is not accurate enough for fine pitch SMD components.

    The thinnest resolution is about 0.15mm (6mil) and where possible I use 0.4mm layers.

    The flex of ABS filaments around tight corners is not great at the best of times.

    Then there's the issue of stranding for a single layer of ABS.

    The usual process to produce a 3D item is to print a 2mm disposable raft.

    This raft holds the work to the platform.

    Peeling a single layer successfully would be unlikely

     

    Cheers

    Very Compact

  • Hi Monte,

     

    Have you considered using your 3D printer to make the masks?

     

    It looks like a perfect application for stencils.

     

    Just a thought,

    DAB

  • Hi!

    I'm also getting confused with all the different solder masks; I also found that changing the design rules (.dru) in Eagle can change your soldermask. In my first gerbers I had overlap between the holes of the pads, thus creating one big hole in the soldermask for each row of pins on the QFN. Because I'm smearing the solder paste roughly in place that was unacceptable. After changing the desing rules the soldermask was just as large as the pads. OK for me.

    With regard to the paste stencils: in my previous job (where a lot of PCB's were designed) we always sent files with solder paste size equal to pad size to the assembly company. They would trim the large surfaces in small areas. This was done so the assembly house could adapt the design of solder paste stencils with respect to the stencil thickness or with respect to the properties of the solder paste.

     

    Good luck with cutting more stencils!