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  • Author Author: fmilburn
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Microscopes and Solder Masks

fmilburn

Now that the TI-RSLK robot RoadTest is finished I have turned my attention back to a problem spotted in the last round of PCBs ordered for the AIS alarm project.  But first, I have a new toy.

image

I have been looking for the right used Nikon microscope in budget but finally could not stand it any longer and bought this Chinese model after my friend bought one.  I am amazed at how much better this is than peering through a magnifying glass or loupe.  At low magnification stereoscopic details pops and there is good depth of field.  I bought a version that takes a C-mount camera but don't have a camera yet.  Good value for the money I think but this was a "want" rather than "need" purchase.

 

I went back and put some of my old PCB designs under the microscope and discovered several had a problem I reported in a previous blog - that is, the solder mask did not extend between the pins of smaller SMD packages.  This of course makes solder bridging and the resultant need for rework more likely.

 

I have had the problem occur for two reasons:

  1. The PCB manufactures had a minimum guaranteed spacing specification between the pad and the solder mask.  Check the specification especially with low cost prototyping services.
  2. I use KiCad and the default pad mask clearance is pretty high - so high that there is no mask between some SMD pins.  For KiCad the fix is to go into Dimensions -> Pad Mask Clearance while in pcbnew and change the default to the desired value.  OSHPark has a discussion on the topic and recommends 2 mil (0.05 mm) clearance for 2 layer boards.

 

The MSP430FR2xxx protoboard I put together a while back had the second problem and I did not realize it until I looked under the microscope.  They were made by OSH Park and it is hard to see the problem with the naked eye on that purple background.  Pesky default values....  I have reordered the board with the fix since they are inexpensive and I have another project in mind for that particular microcontroller.

Parents

  • Hi Frank,

     

    That looks like a nice piece of equipment : )

    2mil sound good, or maybe a tiny bit higher may be preferable if you also plan to use prototyping services in China.

    If you look at the fourth photo here:

    Low Cost PCBs - What Can They Look Like?

    (the QFN part photo), the solder mask gap was 2.7mil. Since the mask was offset slightly from the copper (they don't get it entirely accurately registered), some of the solder mask ended up on the pads, but not a lot. So in summary anywhere between 2-3mil sounds like a reasonable guideline. And even if it gets slightly on the pad, it isn't such a big deal for most reasonable sized parts, so 2mil should be good.

     

    If it is a TI part, they may offer information in the datasheet down to mask gap although that might not be always the case (its usually only needed for very unusual parts or very tiny parts).

  • in reply to shabaz

    Hi Shabaz,

     

    RE equipment - I plan to take the grandkids and go looking for bugs and small stuff in the garden next week :-)

     

    Thanks for the tips and the helpful link - I will go back and read that carefully.  This is something I clearly was not paying enough attention to but it is on my radar screen now.  Even with 2 mil clearance, the solder mask strip between pins is pretty thin.  At the moment I am using the TI footprint recommendation and the PCB manufacturer tolerances for the solder mask.

     

    I was wondering about making the pads a tiny bit thinner but still larger than the pins themselves - how bad an idea is that?  Maybe look in the KiCad base library and see if the pads are thinner.

     

    Frank

Comment
  • in reply to shabaz

    Hi Shabaz,

     

    RE equipment - I plan to take the grandkids and go looking for bugs and small stuff in the garden next week :-)

     

    Thanks for the tips and the helpful link - I will go back and read that carefully.  This is something I clearly was not paying enough attention to but it is on my radar screen now.  Even with 2 mil clearance, the solder mask strip between pins is pretty thin.  At the moment I am using the TI footprint recommendation and the PCB manufacturer tolerances for the solder mask.

     

    I was wondering about making the pads a tiny bit thinner but still larger than the pins themselves - how bad an idea is that?  Maybe look in the KiCad base library and see if the pads are thinner.

     

    Frank

Children
  • in reply to fmilburn

    Hi Frank,

     

    That's a nice idea, I might try that with my nephews too : )

    For the smallest parts, I too will sometimes slightly go beyond the PCB manufacturer specified tolerance for low-cost prototype boards, and go with either the manufacturer spec, or tweak a little to make it easier to hand-solder (e.g. bring out the pads a bit more for QFN.