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Thread: Help! - with tiny SMD parts

  1. #1
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    Default Help! - with tiny SMD parts

    Well my boards are back (rev2). I had to add resistor networks in the output lines to attenuate ringing.

    Since this is only a purpose-built board (not generic or intended to be used across a wide range of uses) I decided to directly solder resistor networks rather than use pluggable (SIP) style. I went with 0603 (x4) size to avoid increasing my overall board size. Remarkably, I've recently started using 0603 (single-element - e.g. MLCC, LED, TF resistor) parts and not had a problem with assembly.

    I started assembly last night. After soldering these parts, the other SOIC and single-element 0603 seem trivially easy.

    It's really, really hard to hand solder the 0603(x4) networks without bridging across the terminals. Even using my smallest tip (1/32" conical Weller) and tons of flux (from a pen) and thin solder. I damaged one pad but was able to fix it. I've checked for continuity/shorts and all looks good at this point, but I have concerns about reliability in service (temperature cycles, moisture).

    Are there any recommendations on alternatives to hand-soldering (or how to hand-solder more "cleanly")? I'm wondering if I could use solder paste and a hot air station for these parts (only). I've seen cheap air stations on Amazon for about $40, but they also have reviewers claiming they catch on fire or melt in the first few uses. I don't have an old toaster oven (or time to convert to a reflow oven) but could also consider purchasing a cheap one that worked for low-volume (and didn't catch on fire).

    If I try to use solder paste, will I just end up bridging the pads anyway? the pads are 0.45mm (18mil) wide on a 0.8mm (31mil) pitch. Is there a "special trick" for applying solder paste like this, and what type of applicator/dispenser is used? I don't want to spend big $$ on this as it's not something that I will often be doing.
    Last edited by ags0000; 05-03-2018 at 01:07 PM.

  2. #2
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    Default Re: Help! - with tiny SMD parts

    One of the tricks to soldering fine pitch parts that I use, it let it bridge, and the go back with solderwick and clean up between the pins. I do that on the large TQFP parts. Just inspect with an eyeloop or scope to make sure its all cleaned off.

  3. #3
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    Default Re: Help! - with tiny SMD parts

    @Utimgr That's what I ended up doing - but I must not have the right type of solderwick (old Radio Shack stuff). I ended up lifting a pad from the board, and still struggled to get the bridge to clear...
    Last edited by ags0000; 05-03-2018 at 01:40 PM.

  4. #4
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    Default Re: Help! - with tiny SMD parts

    Yes...I have some really old stuff as well that I use just for large thru hole parts. I have some rather small stuff (I don't recall the size right off) and its new from last year...works wonders.

  5. #5
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    Default Re: Help! - with tiny SMD parts

    Hi, Iíve not gone down as small as 0603 the smallest single parts Iíve used are 0805. I have used 28TSSOP which are quite tight 0.65mm pitch. I used solder paste for both, Mechanic brand which Iíve seen good and bad reports about but it seemed fine for me. I bought a small tube (1Ē high and ĺĒ diameter) for $10 about 2 years ago, It is kept in the fridge and only brought up to room temperature for use and is still good today. You hardly use any at all and after 21 x 50mm x 50mm boards I still have 70% remaining.

    To apply it I use a craft knife. I take a small quantity from the tube, normally about half the size of a grain of rice and squish it on a piece of plastic, you can then cut off/scrape up the quantity you need with the tip of the knife and squish it onto the pad and slide the knife away. This sounds harder than it is, if you can place 0603ís you can drop paste solder onto a pad. With the 28TSSOPís I put a thin line of paste towards the outside of pads which bridges all the pads on that side, as long as you donít put an excess of paste it will retract to the pads on heating.

    Melting the paste. Iíve used both hot air and an iron all be it the iron is mostly only used for rework. For a build I use my trusty old Bosch hot air gun it has stripped the paint from all my windows, most of the door frames and skirting boards. The biggest problem is if you get it to close you blow all the parts of the board, 18Ē works well and heats the board at about the correct speed. Having done a few now I start warming the outer edges of the board until the paste starts to look dull and then work into the centre of the board. This sounds counter intuitive but I found that if you start in the middle of the board heating it until the paste melted and then worked outwards the heat applied to the outer of the board kept the middle hot for longer than needed. If you warmed the outer and then worked into the middle the heat from the middle extends outwards and you can flow all of the board within a few seconds.

    Why does paste work well? You need such a small amount of solder to fix these items that it is nearly impossible to wet your iron and not have an excess.

    Just my process and findings.

  6. #6
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    Default Re: Help! - with tiny SMD parts

    Are you going to sell these as kits where the user must solder 0603 resistor networks. If so, I would not use them. To save space I would maybe mount thru-hole components vertically.
    David Pitts
    PixelController, LLC
    PixelController.com

  7. #7
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    Default Re: Help! - with tiny SMD parts

    Quote Originally Posted by ags0000 View Post
    I've seen cheap air stations on Amazon for about $40, but they also have reviewers claiming they catch on fire or melt in the first few uses.
    Funny you should mention that. I had the unfortunate experience of watching my 853D catch on fire. I don't know what kind of plastic they use but it smells awful when it burns. I decided to repair mine and add a cutoff switch. I'm happy with it again. I would say that I got about 400 hours of use before the TRIAC died.

    Something that might help you with solder wick is putting flux on it. To prevent pulling parts or a pad, don't use the very end of the wick. Bend the end up a bit and start using it about 1/4" down from the end. It's the stray wires at the end that grab parts and pads. It's even better to avoid moving with wick at all. Just lay it down and drag the iron over it.

    I've had spotty success with a hot plate. It's great when it works but I think you need practice with it. Some day I'll practice more because I have dozens of PCB's I need to build. If I remember correctly, I ditched all of the 0603's from those designs and went with 0805 and even use a 1206 for one of the higher watt resistors. I got tired of handling the smaller parts and found I was more proficient with larger ones.

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