Low temperature soldering for ADC chips

Discussion in 'General Electronics Chat' started by cmartinez, May 20, 2015.

  1. cmartinez

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    Jan 17, 2007
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    I've been using Intersil's HI7190 24 bit analog to digital converters for years now. These chips are normally available in 20-SOIC packages and therefore are SMT devices. But, stupidly, I've just recently discovered that they're extremely sensitive to the temperature used when soldering them.
    I've been soldering them by hand using a simple Weller soldering station set at 600 °F (and became rather skilled at it) and it wasn't until I had to use a different ADC that I discovered that they should be soldered below 400 °F . I've tested the low-temp technique on the HI7190, and the devices show a remarkable increase in their performance because of that. The problem is that soldering at lower temperatures is much harder with ordinary 60/40 solder and a soldering iron.

    Anyway, I've got a few questions:
    • Would the pin-by-pin technique used with a soldering iron affect the chips due to abrupt temperature differentials during the process, even if solder was done at low temperature?
    • Would it be better to use an oven and low-temp paste to solder them to the PCB so as to avoid these temp differentials?
    • Are there any 20-SOIC sockets, devices or techniques that would allow me to mount those chips on a PCB without having to actually solder them?
    • Is there low temperature solder out there (such as bismuth alloys, or other) that's available in wire, rather than in paste form?
     
  2. AnalogKid

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    The SOIC sockets I've seen are zero insertion force devices usually used for device programming. They work as a regular on-board socket, but are expensive for that use.

    ak
     
  3. cmartinez

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    Yeah, I've seen them too, but they're outrageously priced
     
  4. dl324

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    Mar 30, 2015
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    Solder paste and hot air tool.

    Use "chipquick" type rework solder; melts at 300 F.
    http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_141305_-1

    Use heatsink while soldering.

    What is your technique for soldering pins? If you don't solder adjacent pins (which might be difficult trying to do SOIC with a soldering iron) you can minimize localized hot spots.

    When I solder SMT devices with an iron (I use a 700F 65W Weller), I use solder paste.
     
  5. cmartinez

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    What I do is I first tin the PCB's traces where the SMT component will be soldered, then I place the component and solder two diagonally opposed pins by pressing each pin with the iron's tip. After I verify that the component is properly aligned, then I carefully press the remaining pins against the PCB. And after that, I add a tiny amount of solder to each pin by melting it at the pin's very tip, right between the pin and the trace.
    This usually results in perfectly soldered pins, but it tends to heat the component a little too much for my taste.
    I do all this work at 400 °F, using 60/40 solder. I've never used soldering paste before.
    How do you suggest I use a heatsink on a 20-SOIC device?
     
  6. dl324

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    If I was going to use solder wire on SMT, this is what I'd do:
    • apply flux to all pads
    • position device
    • apply solder to diagonal pins to hold device in place
    • solder remaining pins
    Applying a heatsink while soldering requires some manual dexterity or a third hand device. What I do is use my forefinger and thumb to hold the solder and the rest of the fingers on that hand to hold the heatsink on the pins. A heatsink can be anything with sufficient thermal mass to stop most of the heat from entering the package. I put a curve in the solder so I can move and feed as necessary.

    Using solder paste is easier.
    • apply paste to all pads
    • position device
    • apply a little downward pressure on the device
    • solder each pin by touching pad, lead, and solder simultaneously
    Flux in the solder paste will facilitate wicking to form a good joint.

    With a hot air tool, the last two steps are replaced with waving the hot air tool along the leads with the air flow set appropriately.
     
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  7. cmartinez

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    I've never used a hot air tool before. Is there a favorite brand of yours you might suggest?
     
  8. dl324

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    I'm going to get flamed here, but I have an Aoyue 2702A+ that does the job for me. It has a hot air wand, soldering iron, and solder remover; but I only use the wand (solder remover a little). Many will swear by Metcal and Hakko but, for my money, the Aoyue gets the job done without breaking the bank.

    I understand when people talk about quality tools lasting a lifetime (and I have some), but I still buy tools from Harbor Freight... In my opinion, something is better than nothing.
     
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  9. Stuntman

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    Mar 28, 2011
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    Never have run into parts (that I know of) that were this sensitive to soldering temp. In fact, I've often been surprised how robust they are. I digress:

    I have the same solder station/multifunction unit that DL324 has. If I were to do all over again, I'd simply buy a hot air soldering unit (no other features). Not super impressed with the solder iron, and I just don't use the desoldering gun. (my $.02) I have also used a dedicated hot air setup from Circuit Specialists that I thought was great.

    My take: when I solder with a heat gun, I feel like I get the part much hotter than simply drag soldering. Hot air rework requires getting a majority of the whole area (and part) up to solder melting temperature. I prefer drag soldering as you direct the heat at the pins, and use the board/chip as a heatsink.

    A video of drag soldering may be easier than any type of lengthy explanation.

    Just use plenty of flux to distribute the heat, and move the iron quickly to wick solder to the pins, but not heat the part more than necessary.
     
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  10. cmartinez

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    That's more or less the technique I use (though I don't like to use flux)
     
  11. Stuntman

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    Flux definitely helps distribute heat, you may consider using it. (and buying a can of flux remover to get the sticky gunk off when finished). I use it extensively with >2 terminal SMT parts

    I've very surprised to hear some of this. For starters, 400F is pretty tight for soldering. From this list, there are only a handful that even melt below 400F:. And of those, all but a couple are leaded. I notice that the datasheet you provided touts PB-Free packages.

    Which begs the question: How are you supposed to use LF solder and stay within 400F?

    Finally, I did find they list a max lead temp (10s) at 300C (572F). Perhaps your thermal gradient idea is not all wrong? Flux could definitely help.

    FYI, I run my irons well into the 600-700 range for drag soldering, and have yet to have a problem. That said, I scoot across the pins.
     
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  12. ian field

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    Used to be possible to get low melting solder, but it contained cadmium so is probably banned under RoHS.

    There is a desoldering product available that dilutes the existing solder with a lower melting alloy to make removal easier.
     
  13. cmartinez

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    Thanks Stuntman and Ian for your advice. I'm definitely going to consider flux now, and also the use of some sort of heat dissipation device. Although at this point I cannot think of a practical way of attaching it to the 20-SOIC device, since ideally, heat dissipation should be placed at the pin's "root", or at the point where the pin meets the chip's main body. This so as to prevent heat from entering the chip.
     
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