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Thread: On Regret...

  1. #11
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    Re: On Regret...

    Quote Originally Posted by Th232 View Post
    I'm sorry if I'm reading this completely wrong, but it sounds like you were trying to put the paste on while the board was on the stove? If so, I generally apply the paste and parts before subjecting the whole thing to any heat.
    Most of the pads were covered with at least some solder paste, but there was a lot where it should not have been (where components should have been) and cleaning between the pads would pull up most if not all of the paste (it would rather stick to the metal picks than the pads, which is where a decent stencil was really needed). So, I applied it to as many pads as I could, stuck it on a piece of 1/16" thick aluminum plate with one side coated in kapton tape and put that on one of the heating elements (electric stove with built in fan). I waited for surface tension to clean most of it up and started placing some parts before adding a bit more solder paste for the few remaining bare pads (which melted a few inches off the board and would not behave thereafter).

    Quote Originally Posted by Th232 View Post
    If you do, you could put some solder in the vias first to fill them in and eliminate voids or balling, then reflow the LEDs on top of that. Just make sure that whatever the board's sitting on won't stick to the solder (don't ask...).
    Sounds like someone soldered a board to their heating/work surface.
    I had a feeling I would have to fill the vias before adding the LED, thank you for the confirmation. The layer of kapton tape on aluminum plate has worked pretty well in preventing accidental solder joints during reflow. Some of the excess solder that I managed to pull off the board with a pick simply balled on it. No damage after sitting there for a while, and just a little char/residue from what I would guess was the flux or binder of the paste (came off with a bit of soap and water).

    Quote Originally Posted by Th232 View Post
    This works fairly well for IC packages or other areas where you've got lots of pads sitting in a line, with a few observations:
    * The thickness of the copper layer means solder can gather in the valleys between the pads.
    Which is where soldermask/solder-resist can be quite useful if it is available as an option for the board design and during board manufacture. For some reason the board house messed a little bit with the soldermask of my boards this time around (did not tent the screw holes on the top side or any of the thermal portions of the transistor and LED pads, but they did tent the thermal portion of the vreg, the bottom of the screw holes and avr-isp header).
    Quote Originally Posted by Th232 View Post
    * You need good paste. The stuff we use has a shelf life of 6 months, after around 8 months it's still useable for hand soldering, but other procedures can go funny.
    * Different pastes have different ball sizes. This will limit the pad size you can successfully use this technique on.
    I fail pretty badly on that count. Lead-free manufactured in 12/2010 and stored in its syringe in a ziplock bag in a cardboard box on the floor of my bedroom since 5/2011. Applied pretty well with some portions of the hand cut stencil, but the stencil sucked something awful.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  2. #12
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    Re: On Regret...

    Quote Originally Posted by tician View Post
    Which is where soldermask/solder-resist can be quite useful if it is available as an option for the board design and during board manufacture. For some reason the board house messed a little bit with the soldermask of my boards this time around (did not tent the screw holes on the top side or any of the thermal portions of the transistor and LED pads, but they did tent the thermal portion of the vreg, the bottom of the screw holes and avr-isp header).
    Yeah, I've found soldermasks to be an iffy business. A small warning though, I've seen places where the pitch of some IC leads means there simply wasn't enough space to deposit a mask (certain small and/or leadless IC packages come to mind). Here's part of a board I've had to hand solder before, an FTDI232RL chip (or half of one) sits on those pads.

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    But I digress, that's not a problem for you though.

    Quote Originally Posted by tician View Post
    I fail pretty badly on that count. Lead-free manufactured in 12/2010 and stored in its syringe in a ziplock bag in a cardboard box on the floor of my bedroom since 5/2011. Applied pretty well with some portions of the hand cut stencil, but the stencil sucked something awful.
    I can't speak for that particular variety of solder paste, but for ours we've found that storing it in the fridge really helps with prolonging its life.
    Last edited by Th232; 05-02-2012 at 08:35 PM.

  3. #13
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    Re: On Regret...

    Quote Originally Posted by Th232 View Post
    Yeah, I've found soldermasks to be an iffy business. A small warning though, I've seen places where the pitch of some IC leads means there simply wasn't enough space to deposit a mask (certain small and/or leadless IC packages come to mind). Here's part of a board I've had to hand solder before, an FTDI232RL chip (or half of one) sits on those pads.
    Indeed. There are a number of MEMS rotational rate sensors, accelerometers, and compass ICs available only in leadless packages that permit no soldermask anywhere within a few mils of the exterior of the IC or a proper solder joint will not be created during reflow. If I had been even dumber than usual and attempted to use the MLF-32 package instead of the TQFP-32 for the ATmega88/168/328 on the board, I would have had to be very wary since the default soldermask exclusion area for each pad exceeded the distance between the pads.

    Quote Originally Posted by Th232 View Post
    I can't speak for that particular variety of solder paste, but for ours we've found that storing it in the fridge really helps with prolonging its life.
    Honestly, I have been using the solder paste so infrequently (and only for personal prototypes) that it does not really matter much to me how long it is 'good,' so long as it is 'good enough.'
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
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  4. #14
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    Re: On Regret...

    I just realised I forgot to ask, what exactly are you making? Since you're using 0402s and a TQFP package I presume it's pretty small, so now I'm curious.

  5. #15
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    Re: On Regret...

    The horseshoe shaped board is a possible replacement for the HUV robotics Foot Pressure Sensor board for the Robotis Bioloid foot plate. I used 0402 just because I kinda like the challenge, since it could just as well have been 0603 or slightly larger. The holes on the arms and sides are for reverse mount LED's (eye candy) that I decided against buying because they were a bit expensive and I was not sure the board would even work. If I get it working at all, the next version will have only four with each connected to an 8-bit PWM pin so that each LED will reflect the current loading of the sensor in each corner of the foot. I also need to move the arms about 1~2 mm towards the center to better align with the polyester leads and solder tabs/pins of the 0.1" round FSR's. I should also move the top pads for the FSR's about 2~4 mm towards the front of the board, or go with a through hole connector on the bottom for all the FSR's. If I stick with soldering them flush to surface pads, it might be a good idea to make it three boards that mate together to improve ease of assembling the foot. As is, the length of the board makes it a bit difficult to attach the front FSR's to the bottom of the board (but not impossible).

    The top board was added just because I was already paying for the largest rectangle of the FPS board, so it seemed like a waste to not put something in that big area in the middle of the arms. It is a 6 LED board that will hopefully work better than the predecessor without an avr-isp header (yes, I did place it under the ATmega, but it is meant to be accessed from the other side). If it does work, it may also be useful in providing consistent illumination for the HaViMo2's in the lab so that they do not need to be re-calibrated any time the lighting changes (the 4 M2 screw holes are on the 16mm square common to Bioloid frames).

    The face (Toy Soldier smile) and quote (Gaius Valerius Catullus - Catullus 101) are because I only learned about Doctor Steel's 'retirement' a little while ago (several months after the fact).
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    Last edited by tician; 05-03-2012 at 01:28 PM.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  6. #16
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    Re: On Regret...

    Those look very nice, I'll be interested to see how the pressure sensor board works in particular. Neat idea to provide visual feedback with the LEDs as well, if you don't mind I might use the concept in my own robot.

    I thought "Ave atque Vale" was just a general goodbye, guess I've learnt something new today. Catullus was a seedy guy...
    Last edited by Th232; 05-03-2012 at 08:28 PM. Reason: Need to read more carefully...

  7. #17
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    Re: On Regret...

    I too find the LED brightness indicator to be an awesome idea

  8. #18
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    Re: On Regret...

    "ave atque vale" - "hail and fairwell" to his dead brother. Was thinking of "diruit aedificat mutat quadrata rotundus" or "nostalgia is necrophilia" (two of my all-time favorite quotes - I've got an ever growing text file filled with them), but neither really seemed to fit the board's purpose.

    Pretty sure I destroyed another set of boards today with another attempt at a hand-cut stencil. While more successful than the last attempt (except I may have killed a couple LEDs and vregs instead of just cheap passives), if I cannot get every single pad individually (read: flawlessly) solder pasted and every component placed beforehand then I should not get anywhere near a heat source. Also confirmed that while the kapton does not decompose at the temperature I have been using, the adhesive definitely does and it is not produce a pleasant aroma.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

  9. #19
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    Re: On Regret...

    This just pegged, how are you hand cutting a stencil for 0402s? Sounds painful to me. Maybe a good microscope and a finer needle would work better for them?

  10. #20
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    Re: On Regret...

    Tried 9mil photopaper yesterday thinking it would be easily reused (way too thick and tears a lot), and then the thinnest paper I had on hand today (better but still tore a lot). Printed the cream layer and tried to cut carefully with a utility knife (an Xacto knife would be better). I am pretty badly near-sighted so I can go a while staring at small objects close-up before I go cross-eyed. The problem is that I am using a blade and not a fricken' laser to cut the stencil, so if the pads are too close and I am not careful enough then it tears. If I still had some printable transparency sheets, I would have used them from the start (more likely to be reusable and less likely to tear or contaminate the paste with fibers).

    I did better today with a thinner layer of paste and better cleaning of the excess between pads where the paper tore, but still failed pretty badly. When trying to fix the single bad resistor and clean some of the excess solder off the TQFP pads, something started stinking and I noticed the LED's were pretty badly discolored. Pulled the poorly placed ATmega and the metaphorical plug (then spent a few hours priming the dining room before the fumes finally made me want to spew).
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    [git][mech][hack]
    gives free advice only on public threads

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