Author Topic: surface mount rework fun  (Read 7745 times)

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Offline danmcb

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surface mount rework fun
« on: January 16, 2017, 05:18:21 pm »
I am doing a repair on a board with a dead firewire interface. I suspect the PHY chip, a TSB41AB2. As I don't have a schematic, removing the chip was tricky with the hot air gun - hard to get a grip with tweezers and lift it straight up without disturbing other parts. A repair like this can become a train wreck really fast.

Replacement chip is in tomorrow. I am wondering whether to clean the pads and re-apply paste, or just use flux, align it very carefully, and give it some welly with the heatgun. The less I touch those tiny pads, the better.



Anyone has tips and tricks for this kind of brain-surgery without crazily expensive gear, holler!
« Last Edit: January 16, 2017, 05:20:20 pm by danmcb »
 

Offline helius

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Re: surface mount rework fun
« Reply #1 on: January 16, 2017, 05:35:31 pm »
You must clean (or "dress") the pads to avoid bridging the pins. This means removal of any remaining solder with a wick, cleaning away flux residue, then replacement of the new chip and soldering it on using either drag or side-wipe technique. The solder blob that clings to the tip will be enough to flow onto the legs.
 
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Offline Armadillo

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Re: surface mount rework fun
« Reply #2 on: January 16, 2017, 07:02:10 pm »

Replacement chip is in tomorrow. I am wondering whether to clean the pads and re-apply paste, or just use flux, align it very carefully, and give it some welly with the heatgun. The less I touch those tiny pads, the better.

Anyone has tips and tricks for this kind of brain-surgery without crazily expensive gear, holler!

I would say,

1. Flux and wick the old solder from the pads
2. IPA clean the pads,
3. then apply good reputable solder paste onto the pads
4. Place the IC, "orient it", align it and hot air to solder it.

[the centre looks like a thermal pad? in that case, drag soldering would not be suitable]

The pitfall to watch;

1.0 Discovered that you have installed a fake IC chip, and have to repeat all the steps again. So please ascertain where you buy the chip from. Check and make sure it's not a fake.
2.0 Used expired or cheap solder paste from unreliable sources. Worst, self-adulterated it by adding flux [sparkfun method - do not follow] or IPA. Best to test the original solder paste by placing a small sample onto a flat surface, hot air it and watch if the solder "balls-up and shiny" or spread out and dull.
3.0  "welly with the heatgun" and discover that the faults remain the same and then ponders whether you had thermal heat damaged the chip. This adds complexity to your fault findings.
« Last Edit: January 16, 2017, 07:31:30 pm by Armadillo »
 
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Offline danmcb

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Re: surface mount rework fun
« Reply #3 on: January 16, 2017, 08:57:14 pm »
yes, there is a power pad. the new part is from FEC, never had a problem with their chips. It was only 5€ ... why would I buy cheap?
 

Offline danmcb

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Re: surface mount rework fun
« Reply #4 on: January 16, 2017, 10:12:18 pm »
The big problem here getting the heat right. Doing it with hot air is tricky, because the only way to heat that centre pad enough is via the body of the chip. I am concerned about frying the chip just getting it back on the board. It quite a lot of heat to get the old one off. Plus, you don't actually know if you have gotten the solder on the central pad to melt or not.

I am wondering about actually sending this off to someone with a decent reflow oven. There's a fair chance it would be fine with the gun,  but the trouble is there is no way to know until  you plug it back up. And of course, if it works but the solder didn't flow underneath, there is a possibility of failure due to overheating in future.

I am going to think about it a bit.
 

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Re: surface mount rework fun
« Reply #5 on: January 16, 2017, 10:20:11 pm »
2 things I'd be using for such a job if you hadn't thought about them already:
Preheat, some just use a frying pan.
Lower temp leaded solder.
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Offline danmcb

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Re: surface mount rework fun
« Reply #6 on: January 16, 2017, 10:24:22 pm »
yes, good idea. I could use getting an electric griddle or something for preheat. and low temp solder also not a bad idea.
 

Offline danmcb

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Re: surface mount rework fun
« Reply #7 on: January 16, 2017, 10:44:46 pm »
Unfortunately the back of the board has about a zillion small  R's and C's that mean it's not possible to put it on a skillet. The area around the chip is clear, but the rest of the board has loads of the little beggars.

What would be really handy for this kind of job would be a holder frame with a small heater about an inch square that you can adjust the position of.
 

Offline cdev

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Re: surface mount rework fun
« Reply #8 on: January 16, 2017, 10:49:58 pm »
Just an idea, I have never done this, but - you may want to check out "chip quik" - Using chip quik you likely could prevent the components on the back from coming off by lowering the temperature of the top components quickly using hot air, once it mixes in then the difference (in melting temperatures, if you're careful) will be large.
GET OTHER PEOPLES OPINION ON THIS!

Another idea, blow a cooling fan across the bottom while heating the top with hot air. But that could also cause problems, with mechanical stresses.
« Last Edit: January 16, 2017, 10:51:45 pm by cdev »
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Offline danmcb

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Re: surface mount rework fun
« Reply #9 on: January 16, 2017, 11:08:46 pm »
I wonder if I had something like this:



could I tape some kind of heat insulator on the back of the PCB to shield  the area I don't want to heat?

 

Online tautech

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Re: surface mount rework fun
« Reply #10 on: January 16, 2017, 11:11:31 pm »
Unfortunately the back of the board has about a zillion small  R's and C's that mean it's not possible to put it on a skillet. The area around the chip is clear, but the rest of the board has loads of the little beggars.

What would be really handy for this kind of job would be a holder frame with a small heater about an inch square that you can adjust the position of.
Some use a domestic hot air gun for underside preheat. Thought and care need be applied.  ;)
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Offline danmcb

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Re: surface mount rework fun
« Reply #11 on: January 16, 2017, 11:14:15 pm »
Unfortunately the back of the board has about a zillion small  R's and C's that mean it's not possible to put it on a skillet. The area around the chip is clear, but the rest of the board has loads of the little beggars.

What would be really handy for this kind of job would be a holder frame with a small heater about an inch square that you can adjust the position of.
Some use a domestic hot air gun for underside preheat. Thought and care need be applied.  ;)

You also need a friendly dwarf to hold it while you do the topside! :-)
 

Offline cdev

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Re: surface mount rework fun
« Reply #12 on: January 17, 2017, 12:17:19 am »
Kapton (polyamide) tape is a plastic tape that doesn't melt except at very high temperature which can be used to shield parts you don't want detaching.
"What the large print giveth, the small print taketh away."
 
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Offline valere

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Re: surface mount rework fun
« Reply #13 on: January 17, 2017, 12:43:45 am »
The big problem here getting the heat right. Doing it with hot air is tricky, because the only way to heat that center pad enough is via the body of the chip. I am concerned about frying the chip just getting it back on the board. It is quite a lot of heat to get the old one off. Plus, you don't actually know if you have gotten the solder on the central pad to melt or not.

If you didn't think about it, think about shielding the components (even small ones really next to the IC) around with some kapton tape, you will feel way more comfortable heating to re-solder.

As far as I know it's quite difficult to fry a chip like that (except for very sensitive ones) just by soldering it with hot air ;-) . If you need to be more comfortable to take an old computer board or something like that and tune your Hot Air station and your gesture by training on it.

A way to be sure the under pad is soldered is to, really clean very well the other pads and solder only the under pad by heating on top (and letting cool down, it's bigger it can take a will). If the component doesn't move (and nothing else is soldered) it should be OK. But I'm not sure it's a good idea or useful. Other way is to put melted soldering tin (not sure in english) on the central pad to have a little bump + a lot of flux after, and either put very small amount of it (then clean with soldering wick + add flux) or solder past for the outside pads (not too much), this will be nervously annoying to place the component, but that way when it'll be hot enough the under will melt and outside also, you will be sure because otherwise the component will not be perfectly horizontal and down... However I hink it's doable in one shot with solder past and flux.

I would just solder it using hot air and, solder past around, because normally when the outer pads will be soldered all around (try to heat everywhere especially center of the IC) everything will be OK. I've done it on two boards, an AVR Dragon where I changed the ic of the useless and poor designed boost regulator (to comply with USB 4.25-5.25V  when the integrated uC need at least 4.5V) and with a custom board using an incredibly thin and small H-Bridge, because there wasn't any other low-voltage H-Bridge on the market for my use. -- The kapton tape really really made my life easier on the Dragon, suddenly the capacitors were shielded and I could heat more to get the under pad OK.

I totally understand your fears, I find that frighting when I first began this... and I'm still "ignorant" of this technic despite being able to do a lot of difficult things with it.

You can take a look at my first really tiny nasty component try (not repair, soldering) : https://twitter.com/SikulaRobotik/status/726065207919955968

And I really need the thermal pad soldered otherwise it's dissipation would not be correct. However it's a custom board, it was harder because I had no idea if anything was wrong or not... but the outer pad plan is somehow exposed through the left. On the other (AVR Dragon) it was exactly like you, repair / rework, nasty faulty components, with an insane amount of component around. ;-)

I hope my little experience can help you, if you have any question do not hesitate.

I hope one day to have time to go to the Shenzen maker fair, because, their makers are rebelling and soldering BGA without proper tools, something just basic uncontrolled hot air, or basic hot air station, I really want to master such skills.

I am wondering about actually sending this off to someone with a decent reflow oven. There's a fair chance it would be fine with the gun,  but the trouble is there is no way to know until  you plug it back up. And of course, if it works but the solder didn't flow underneath, there is a possibility of failure due to overheating in the future.

I am going to think about it a bit.
In fact it's not such a good idea, rework stations exist for a reason, this one ;-) . You can't really put the board in a reflow oven without removing all the connectors, plastic LEDs, (seen on the picture you attached), or a least protecting them, which can prove trickier than reworking with hot air.

Heating under the all board at around 100-150°C, is what the quartz pre-heater for rework allows. If you find a way to properly do that, It's a nice way to eliminate mechanical constraint and ease you top heating also. (however you can "pre heat" the the board, by using the hot air flow around all the board with a correct distance and not too hot temp, when you have nothing it's OK, and it's work pretty well).

Just an idea, I have never done this, but - you may want to check out "chip quik" - Using chip quik you likely could prevent the components on the back from coming off by lowering the temperature of the top components quickly using hot air, once it mixes in then the difference (in melting temperatures, if you're careful) will be large.
GET OTHER PEOPLES OPINION ON THIS!

Another idea blow a cooling fan across the bottom while heating the top with hot air. But that could also cause problems, with mechanical stresses.
As far as I'm concerned, I would not do that : rework usually imply "preheating" all the board to a consistent temperature (like with a super pricy quartz heater), this is to prevent any mechanical constraint... This is even why it's such a good idea, when you have limited tools, to get the board temperature rising by heating around up, down everywhere (if you are not too close to the board, it will slowly rise without desoldering anything and melting any connector). The hot air temperature is actually falling when the distance from the output of hot air is rising, try with an apple or something like that, I had fun with my hot air station, and it's incredibly difficult to even burn the surface if the distance rise, even at 500°C max airflow... and it's a decent one trust me.

I just discovered their is an amazing (and $$$) tool, which is a quartz under pre-heating with a top directed & controlled IR hot beam, to rework small BGA, seek Aoyue it's a new tool from them.... well cost too much for me but it's amazing.

EDIT : I was writing when you had kapton like me  :D
Kapton (polyamide) tape is a plastic tape that doesn't melt except at very high temperature which can be used to shield parts you don't want detaching.
« Last Edit: January 17, 2017, 12:48:39 am by valere »
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Offline Armadillo

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Re: surface mount rework fun
« Reply #14 on: January 17, 2017, 03:39:08 am »
The big problem here getting the heat right. Doing it with hot air is tricky, because the only way to heat that centre pad enough is via the body of the chip. I am concerned about frying the chip just getting it back on the board. It quite a lot of heat to get the old one off. Plus, you don't actually know if you have gotten the solder on the central pad to melt or not.

I am wondering about actually sending this off to someone with a decent reflow oven. There's a fair chance it would be fine with the gun,  but the trouble is there is no way to know until  you plug it back up. And of course, if it works but the solder didn't flow underneath, there is a possibility of failure due to overheating in future.

I am going to think about it a bit.

I recommend you to watch this video
Particularly on the temperature profile, thermocouple experiment, distant, hot air speed etc.
If you use the proper tools and technique, it will be a piece of cake for you.
There are tons of soldering station in the market, please get a decent one. I am saying hot air station, not heat gun or those that are used primarily for removing paint work.
You will know when the surface tension of the melted solder spring the chip back into position. "use decent solder paste and decent station"
Experiment with it a few times on other boards, you can do it for sure.
 
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Offline blueskull

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Re: surface mount rework fun
« Reply #15 on: January 17, 2017, 05:13:34 am »
If I were you, I would do the following:
1. Clean (use wick) the pads to get a flat working surface.
2. Apply some solder paste beneath center pad. If needed, add some extra flux.
3. Hot air the chip, remove hot air when you can see it self aligns.
4. Clean remain flux carefully.
5. Apply fresh flux and drag solder the perimeter pads.
6. Clean remain flux trapped in pins.
7. Done.
 
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Offline Armadillo

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Re: surface mount rework fun
« Reply #16 on: January 17, 2017, 06:57:47 am »
Just an idea, I have never done this, but - you may want to check out "chip quik" - Using chip quik you likely could prevent the components on the back from coming off by lowering the temperature of the top components quickly using hot air, once it mixes in then the difference (in melting temperatures, if you're careful) will be large.
GET OTHER PEOPLES OPINION ON THIS!

Another idea, blow a cooling fan across the bottom while heating the top with hot air. But that could also cause problems, with mechanical stresses.

I think you are referring to low temperature chip quik used for "Desoldering". It is not for soldering. It's a No No.!
« Last Edit: January 17, 2017, 07:00:25 am by Armadillo »
 
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Offline android

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surface mount rework fun
« Reply #17 on: January 17, 2017, 07:25:46 am »
It may be possible to drill out the central heat sink pad and solder a custom heat sink directly to the chip from the back of the board. Just a thought  :)
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Offline Cupcakus

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Re: surface mount rework fun
« Reply #18 on: January 17, 2017, 07:35:51 am »
I would just dump flux all over the pads, drag solder some new solder on them, solder up the center pad, drop more flux, and hot air it into place.  You usually know the center has flowed as a small ball of excess will come out one of the corners, which you can just pick up with your iron or drag the pins on that side again.

You can wick clean the pads first, but if your worried about how fragile they are this is inviting disaster IMHO.
 
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Offline danmcb

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Re: surface mount rework fun
« Reply #19 on: January 17, 2017, 08:57:12 am »
thanks all for the suggestions.

No, I am not using a Black and Decker paint stripper :-) I have an 858D.

I have some kapton tape, but it is quite thin. Maybe I have to get a wider roll. I used it a few times put it often doesn't stick that well when the job is being done. Still, some protection is better than none.Did manage to get them down, but a lot of hassle. I since designed that part out of my project! This is easier though, at least the package has pins.

Quite right about not going to an oven. I am just gonna clean the pads, use a little bit of paste and some flux, and give it a go. I think it'll be fine. I will get an electric pancake plate though when I see one! I think that with a piece of scrap aluminium between it amn the back of the board would help - but too difficult to hold it all steady, so I won't try that this time.
 

Offline KhronX

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Re: surface mount rework fun
« Reply #20 on: January 17, 2017, 04:12:59 pm »
For what it's worth, i've replaced several TSB41AB2 chips on a number of various Firewire audio interfaces.

What i can tell you is that in some cases, despite the IC indeed having a metallic "belly pad", there was no gap in the soldermask for the chip to get soldered to the ground plane :)
And at least in normal functioning, it shouldn't need to get anywhere near as warm as to require (that much) heatsinking.

That being said, i've always chosen to do it manually, for the sake of certainty  ;D (as opposed to using hot air to solder the new one on).

Wick the pads (along the direction of the pads; across them is inviting trouble), flux them, align the new chip (note the pin 1 indicator, etc), tack on two opposite corners (with normal solder), and then start soldering :) For me, a 1.2mm-wide chisel tip worked just fine, with my Ersa iCon Nano iron.

PS: Don't worry if the pin 16 pad flies off - it's NC (ie. "Not Connected" to anything)  ;)
PS2: Have some flux and wick ready, in case you happen to bridge together any pins  :D Been there, done that...
« Last Edit: January 17, 2017, 04:14:49 pm by KhronX »
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Offline cdev

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Re: surface mount rework fun
« Reply #21 on: January 17, 2017, 10:08:10 pm »
Indeed I was talking about desoldering it. Didn't realize you had a hot air device either. If you have an "858" get some practice on garbage boards and then "just do it".

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Offline nanofrog

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Re: surface mount rework fun
« Reply #22 on: January 17, 2017, 10:37:00 pm »
If I were you, I would do the following:
1. Clean (use wick) the pads to get a flat working surface.
2. Apply some solder paste beneath center pad. If needed, add some extra flux.
3. Hot air the chip, remove hot air when you can see it self aligns.
4. Clean remain flux carefully.
5. Apply fresh flux and drag solder the perimeter pads.
6. Clean remain flux trapped in pins.
7. Done.
I'd include preheating (say 125 - 150C).

Also, it's possible to create a shield from thin sheet metal as well that goes around the device you're soldering to protect nearby components (JBC calls them protectors IIRC if you'd just rather buy). Brass, copper, or aluminum work really well (braze, rivet, or even just fold seam). Steel too, but the seams are harder.
 

Offline KhronX

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Re: surface mount rework fun
« Reply #23 on: January 17, 2017, 10:42:37 pm »
Preheating's important in the case of BGA chips; QFN's are much less sensitive to such "details" :)

I've never preheated a board before soldering a non-BGA chip onto it, and i've had precisely zero problems.

Shielding the surrounding area IS a good idea, though. Specifically connectors (due to the plastic involved) and electrolytic & tantalum capacitors.
Sheet metal's a bit overkill - i bought a roll of adhesive-backed aluminium tape a couple years ago off eBay, and i haven't even used half of it :)
Double it up if you're paranoid, and stick it into a four-sided "bowl" around the chip-to-be-removed.
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Offline valere

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Re: surface mount rework fun
« Reply #24 on: January 17, 2017, 11:44:27 pm »
Preheating's important in the case of BGA chips; QFN's are much less sensitive to such "details" :)

I've never preheated a board before soldering a non-BGA chip onto it, and i've had precisely zero problems.

Shielding the surrounding area IS a good idea, though. Specifically connectors (due to the plastic involved) and electrolytic & tantalum capacitors.
Sheet metal's a bit overkill - i bought a roll of adhesive-backed aluminium tape a couple years ago off eBay, and i haven't even used half of it :)
Double it up if you're paranoid, and stick it into a four-sided "bowl" around the chip-to-be-removed.

Nice feedback, I keep that in mind to improve my way of working for my next time doing that.
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