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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,

     

    I'd been looking at various microscopes for quite a while, hoping to find a bargain used one from a high end manufacturer. In the end I did the same as you and went for one of the Chinese ones. I'm not sure which brand yours is, is it an Amscope? I went for one of those in the end as I had seen good reviews about them on various sites. If you follow Louis Rossmann on YouTube you'll probably have seen him explaining why he chose his.

     

    With regards to the camera, I got one in the package I bought but to be honest I will probably replace it eventually. It's not particularly high quality camera and it's USB2 so the frame rate is a little slow so you see lagging between the screen and reality. I'll probably try and get a higher resolution HDMI camera. There are a lot around on eBay/Amazon which all seem to be of Chinese origin and have similar design. I've not seen any reviews of these or recommendations for ones which are considered to be good though. You probably did the right thing getting just the microscope without the camera because now you can just get whatever camera you really want to go with it when you are ready.

     

    The first thing I did when I got my microscope set up was put some of my boards under it and see what they looked like close up. I've not seen the solder mask issues like you and Shabaz have reported but I can see how easy it would be for that to happen. I didn't have to hand any boards from the really cheap prototyping manufacturers though and I usually define all my solder mask apertures in my libraries for anything fine pitch. One thing I did notice on one of my boards though was that I could locate the position of buried vias within a PCB because I could see little bumps (and they are very little in reality) above/below them on the board. I'm not entirely sure the cause of the bumps, maybe air trapped in the buried vias expanding when the boards were reflowed? It's not something I was expecting to see and it makes me wonder if there may be any long term issues with the vias or any traces which run over those vias on other layers in the long term.

     

    Anyway, nice new toy! I'm sure you'll put it to great use and it'll be interesting to hear how you get on with it.

     

    Best Regards,

     

    Rachael

Comment

  • Hi Frank,

     

    I'd been looking at various microscopes for quite a while, hoping to find a bargain used one from a high end manufacturer. In the end I did the same as you and went for one of the Chinese ones. I'm not sure which brand yours is, is it an Amscope? I went for one of those in the end as I had seen good reviews about them on various sites. If you follow Louis Rossmann on YouTube you'll probably have seen him explaining why he chose his.

     

    With regards to the camera, I got one in the package I bought but to be honest I will probably replace it eventually. It's not particularly high quality camera and it's USB2 so the frame rate is a little slow so you see lagging between the screen and reality. I'll probably try and get a higher resolution HDMI camera. There are a lot around on eBay/Amazon which all seem to be of Chinese origin and have similar design. I've not seen any reviews of these or recommendations for ones which are considered to be good though. You probably did the right thing getting just the microscope without the camera because now you can just get whatever camera you really want to go with it when you are ready.

     

    The first thing I did when I got my microscope set up was put some of my boards under it and see what they looked like close up. I've not seen the solder mask issues like you and Shabaz have reported but I can see how easy it would be for that to happen. I didn't have to hand any boards from the really cheap prototyping manufacturers though and I usually define all my solder mask apertures in my libraries for anything fine pitch. One thing I did notice on one of my boards though was that I could locate the position of buried vias within a PCB because I could see little bumps (and they are very little in reality) above/below them on the board. I'm not entirely sure the cause of the bumps, maybe air trapped in the buried vias expanding when the boards were reflowed? It's not something I was expecting to see and it makes me wonder if there may be any long term issues with the vias or any traces which run over those vias on other layers in the long term.

     

    Anyway, nice new toy! I'm sure you'll put it to great use and it'll be interesting to hear how you get on with it.

     

    Best Regards,

     

    Rachael

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  • in reply to rachaelp

    Hi Rachel,

     

    Thanks for all the great information.  Mine is an Amscope but my friend has one that appears identical under a different brand name.  As you say, the reviews I have seen are all good and I have been impressed so far.

     

    Thanks for your thoughts and insights on a camera.  As cheap as cameras are these days, it is somewhat surprising that the ones for microscopes are so expensive although I realize it must be a relatively small niche market.  I have a long time interest in still photography but have never been too interested in video so some study is in order.  Whatever I come up with I will post here on Element14. 

     

    RE:  solder mask - I am not an experienced PCB designer although I have used CAD software quite a bit.  Most (really all) of the errors I encounter can be traced back to me :-)

     

    Regards,

    Frank