Low Temp Solder for special PCB

Discussion in 'General Electronics Chat' started by jjmalove, Oct 26, 2016.

  1. jjmalove

    Thread Starter Member

    Jul 6, 2016
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    Hey,

    One issue with a very custom project I'm working on is the special board material we use doesn't handle higher temperature very well during the reflow soldering process. Anything above 130C starts to cause board warping and delamination. The longer its above the temperature and the higher the temperature the worse it gets. We are currently having the boards made with regular lead solder (I believe 63/37), and they turn out acceptable and we successfully use them, but are noticeably warped.

    Got me to thinking. I know low temp solder exists for specialized applications, but does it exist and is it supported at all by board houses for PCB fabrication. Does anyone have any experience having a board made with a special low temp solder as opposed to traditional lead or lead-free solder options? The plating we use is ENIPIG which is a must, if that matters. The board utilizes a hybrid mix of 0201 caps/resistors and a few BGAs, which we have a board house place. Then we manually place bare dies and wire bond them to the board. We then encapsulate the whole PCB. My point being if a low-temp solder has weaker mechanical strength that is no big deal. Our wire bonds are already orders of magnitude more fragile than solder joints and we have the tools and techniques in place to deal with it.

    Thanks!

    Edit: The ideal melting point would be around 160C. The wire bonding process keeps the board sitting at 130C, so I wouldn't want to be on the hairy edge of the parts reflowing during bonding. Also another step involves a thermoplastic tape to melt and adhere the bare dies to the PCB, which melts at 140C. Again wouldn't want the hot plate to reflow the parts accidentally. I have found at 140C the board doesn't warp worse despite the spec. I'm hoping around 160C would show similar results or at least less warping then we get from the 220C or so reflow process currently done by the board house.
     
    Last edited: Oct 26, 2016
  2. SLK001

    Senior Member

    Nov 29, 2011
    1,169
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    Explain why you are using a PCB material that cannot withstand normal PCB fabrication methods. There are alloys that have melting points in your range. Search indium solder alloys.
     
  3. jjmalove

    Thread Starter Member

    Jul 6, 2016
    50
    1
    Miniaturization. The board height plus components and encapsulation cannot be higher than 0.025". This is also why we are using bare dies and gold wire bonding. It is a 0.003" polyimide flex PCB. Our tallest die (a microprocessor) + wire bond is 0.018". The other dies and components are shorter. We then want that encapsulated so currently with this setup we get a minimum 0.004" of potting on the worst spot, and the rest of the package gets a lot more. The smallest thickness normal boards are usually 0.008", which without any potting puts us above our height limit.

    The X and Y dimensions are a little more forgiving, but the Z is not.

    Here is the literature talking about the temperature:
    Temperature
    Most often the adhesive used is the limiting factor for high temperature rating continuous service. Typical values are 105C (221F) for epoxy adhesive and 120C (248F) for acrylic adhesive. Both systems will withstand normal soldering practices after a moisture driving bake.

    So actually I was wrong and I'm not actually sure which of the two adhesives we use but we're actually rated even less than I thought. The big symptom I see is that the boards have delamination where the pad lifts a little off the adhesive, and the board itself warps a little instead of laying flat. My hope is that having them made with a lower temperature reflow would reduce this.
    Edit: Oh and the reason this matters is because when you try to wind bond to a pad and its delaminated even a little bit, sometimes the bond will fail. This is because the machine touches the top of the pad, thinks its met the bonding area, overtravels into the pad a small amount, then does the bond. It is expecting to overtravel against a solid surface, but instead because the pad is lifted it overtravels into the air and below the pad and doesn't have a good hard surface below it to press against. It would be comparable to trying to cut something against a rubber mat instead of a hard block.
     
  4. jpanhalt

    Expert

    Jan 18, 2008
    6,381
    1,222
    Bismuth-tin is more common for practical, low-temp solders. I have some Kester from several years ago. Didn't search to see if it is still made. Did search very briefly on bismuth solders and came up with this: http://www.indium.com/solders/bismuth/

    Many years ago, I corresponded with an individual at Indium Corp about indium solders for electronic work. He mentioned that joints over gold plated contacts could be a problem (cracking). Besides, indium is expensive, but it is a very neat metal. It is quite soft and is sort of like cooked spaghetti at room temperature.

    EDIT: Just checked the Kester stuff I have. It is 43% tin, 43% lead, and 14% bismuth. I bought it in 1999, so it may not be available

    John
     
    Last edited: Oct 26, 2016
  5. bertus

    Administrator

    Apr 5, 2008
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  6. jjmalove

    Thread Starter Member

    Jul 6, 2016
    50
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    Very cool thanks!
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Back when I was in the hybrid industry we spoke of conductive and non-conductive epoxies. The conductive variety is what you are looking for.

    Unfortunately I am out of that game now and can't easily get a manufacture or part number for you. I still have a few contacts if you need one.

    We had one job where we did a drop shot coating over bare die. It was a disaster as you have coefficients of thermal expansion to deal with, plus the substrate has to be extremely clean of everything. I don't know how the cheap Chinese boards work so well, but they do this in volume so they have the process down.
     
  8. jjmalove

    Thread Starter Member

    Jul 6, 2016
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    The Bismuth one looks really promising. Proper reflow of the one I'm looking at you would set the reflow oven to peak at 180C. 45-60C difference in heat could make a BIG difference in the amount of delamination we are seeing.
     
  9. jjmalove

    Thread Starter Member

    Jul 6, 2016
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    1
    I have never even considered conductive epoxy. That could be easy for the 0201s and hard for the BGA at first thought.

    Are you referring to the black "glob tops" you see on like musical greeting cards and such?
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Your best move is to contact Kester and Indium support directly..
    They will know more than anyone here hands down..
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yes, glob top is how they make the typically black covering over bare die placed directly on a PCB.

    We never used PCB as a substrate, always alumina (ceramic) with conductors and resistors fired in. You can mix wire bond and solder if you use the proper conductive pastes.... Not applicable to you as they fire in at extremely hot temperatures in controlled belt ovens.

    What are now referred to as SMD caps were developed for these hybrids, we conductive epoxied them down as standard practice. They do come with different metals to accommodate this (which is non standard, i expect not available at Digikey, and $$$).

    But epoxy down bare dies and wire bonding was another standard practice. Some needed just adhesion, some needed a potential for the die back so the epoxy was not always the conductive kind.
     
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