surface mount rework fun - Page 1
Forum section: electronics / repair

author: danmcb
author_rank: Regular Contributor
timestamp: January 16, 2017, 05:18:21 pm
content: 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!

author: helius
author_rank: Super Contributor
timestamp: January 16, 2017, 05:35:31 pm
content: 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.

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2017, 07:02:10 pm
content: Quote from: danmcb on January 16, 2017, 05:18:21 pmI would say,1. Flux and wick the old solder from the pads2. IPA clean the pads,3. then apply good reputable solder paste onto the pads4. 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.

author: danmcb
author_rank: Regular Contributor
timestamp: January 16, 2017, 08:57:14 pm
content: 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?

author: danmcb
author_rank: Regular Contributor
timestamp: January 16, 2017, 10:12:18 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: January 16, 2017, 10:20:11 pm
content: 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.

author: danmcb
author_rank: Regular Contributor
timestamp: January 16, 2017, 10:24:22 pm
content: yes, good idea. I could use getting an electric griddle or something for preheat. and low temp solder also not a bad idea.

author: danmcb
author_rank: Regular Contributor
timestamp: January 16, 2017, 10:44:46 pm
content: 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.

author: cdev
author_rank: Super Contributor
timestamp: January 16, 2017, 10:49:58 pm
content: 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.

author: danmcb
author_rank: Regular Contributor
timestamp: January 16, 2017, 11:08:46 pm
content: 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?

author: tautech
author_rank: Super Contributor
timestamp: January 16, 2017, 11:11:31 pm
content: Quote from: danmcb on January 16, 2017, 10:44:46 pmSome use a domestic hot air gun for underside preheat. Thought and care need be applied.

author: danmcb
author_rank: Regular Contributor
timestamp: January 16, 2017, 11:14:15 pm
content: Quote from: tautech on January 16, 2017, 11:11:31 pmYou also need a friendly dwarf to hold it while you do the topside! :-)

author: cdev
author_rank: Super Contributor
timestamp: January 17, 2017, 12:17:19 am
content: 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.

author: valere
author_rank: Contributor
timestamp: January 17, 2017, 12:43:45 am
content: Quote from: danmcb on January 16, 2017, 10:12:18 pmIf 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/726065207919955968And 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.Quote from: danmcb on January 16, 2017, 10:12:18 pmIn 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).Quote from: cdev on January 16, 2017, 10:49:58 pmAs 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 meQuote from: cdev on January 17, 2017, 12:17:19 am

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2017, 03:39:08 am
content: Quote from: danmcb on January 16, 2017, 10:12:18 pmI recommend you to watch this videoParticularly 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.

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2017, 06:57:47 am
content: Quote from: cdev on January 16, 2017, 10:49:58 pmI think you are referring to low temperature chip quik used for "Desoldering". It is not for soldering. It's a No No.!

author: android
author_rank: Regular Contributor
timestamp: January 17, 2017, 07:25:46 am
content: 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

author: Cupcakus
author_rank: Contributor
timestamp: January 17, 2017, 07:35:51 am
content: 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.

author: danmcb
author_rank: Regular Contributor
timestamp: January 17, 2017, 08:57:12 am
content: 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.

author: KhronX
author_rank: Frequent Contributor
timestamp: January 17, 2017, 04:12:59 pm
content: 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 planeAnd 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(as opposed to using hot air to solder the new one on).Wick the pads (alongthe 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 solderingFor 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 pinsBeen there, done that...

author: cdev
author_rank: Super Contributor
timestamp: January 17, 2017, 10:08:10 pm
content: 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".

author: nanofrog
author_rank: Super Contributor
timestamp: January 17, 2017, 10:37:00 pm
content: Quote from: blueskull on January 17, 2017, 05:13:34 amI'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.

author: KhronX
author_rank: Frequent Contributor
timestamp: January 17, 2017, 10:42:37 pm
content: 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 areaISa 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 itDouble it up if you're paranoid, and stick it into a four-sided "bowl" around the chip-to-be-removed.

author: valere
author_rank: Contributor
timestamp: January 17, 2017, 11:44:27 pm
content: Quote from: KhronX on January 17, 2017, 10:42:37 pmNice feedback, I keep that in mind to improve my way of working for my next time doing that.

author: KhronX
author_rank: Frequent Contributor
timestamp: January 18, 2017, 12:00:48 am
content: No problemo, glad to share my experience, as limited as it may beI'm just not a fan of stressing over details that are, in the real world, irrelevant / inconsequentialOn the other hand, if you use a "manual wand" from a hot-air station, there may be cases where you can even get away without bothering to shield the area, if there's nothing sensitive aroundQuote from: valere on January 17, 2017, 11:44:27 pm

author: cdev
author_rank: Super Contributor
timestamp: January 18, 2017, 02:14:11 am
content: flashing copper (thick copper foil) is very bendable and reusable and can be used for temporary heat shielding. You can put Kapton tape around the bottom edge in a U so it wont pick up solder and adhere to the board. Flashing copper is useful for all sorts of PCB stuff.

author: eKretz
author_rank: Frequent Contributor
timestamp: January 20, 2017, 07:04:05 am
content: Quote from: KhronX on January 17, 2017, 10:42:37 pmI would have said the same last week. This week I had to rework some QFNs on a massive ground plane with 9 vias connecting the center pad to more ground plane. That was a #&(:$+:'+'&+:_"&"&-$_++@@!! PITA! And even using an Ersa i-Con 150W iron with a 6mm chisel I was having trouble cleaning that center pad with wick!So anyway, I would amend that statement now toMOSTQFNs aren't that sensitive to such details. For the ones that are, youwillneed a preheat or risk killing the IC. This really applies to any work on components on big ground planes though - it's not really specific to QFNs.

author: tautech
author_rank: Super Contributor
timestamp: January 20, 2017, 07:18:37 am
content: Quote from: eKretz on January 20, 2017, 07:04:05 amI feel your pain.Jobs like that is when I get out some leaded solder with Silver in the alloy....when I bought it years ago it was the lowest temp solder I could find....but the price was ~$40/lb ~15 years ago.Preheat.Preheat.Preheat....................

author: valere
author_rank: Contributor
timestamp: January 20, 2017, 08:23:38 pm
content: Quote from: eKretz on January 20, 2017, 07:04:05 amToo much via and ground plain heatsink dissipation is a pain in the soldering station... I soldered two H-Bridge driver in Power SSOP on a custom design card back when I did not have any Hot Air Station, hopefully we extended a bit the under-pad without soldering mask for this purpose, but there is approximately a dozen via to dissipate thermal... and it took us two simultaneous soldering iron for an approximate cumulated power of 100-120W (pushed at maximum power), and it was barely enough to solder this pad.... I really do not want to know what will happen if need to desoldered those (using Hot Air Station...).This could be a difficult situation... where indeed preheating may help a lot...

author: eKretz
author_rank: Frequent Contributor
timestamp: January 20, 2017, 09:10:27 pm
content: Yep, it always makes me laugh when I see that comment "this iron instantly solders anything, even on heavy ground plane" - they apparently haven't tried soldering on any REALLY heavy ground plane, lol. This one I mentioned was actually pretty tough evenwith150°C preheat. Sure makes soldering on average boards seem very easy though!

author: nctnico
author_rank: Super Contributor
timestamp: January 20, 2017, 09:34:40 pm
content: Quote from: helius on January 16, 2017, 05:35:31 pmIMHO it is better not to clean or mess with the pads in any way. It is very easy to wipe a pad away or dislocate it which makes the repair much harder. What works best for me is to remove the solder on the center pad, clean, apply flux and then use hot air to reflow the chip into place using the existing solder. Touch the pads up using flux some fresh solder and a big tip in the iron. After than turn the PCB over and use flux + hot air and fresh solder to solder the center pad.

author: Smith
author_rank: Frequent Contributor
timestamp: January 21, 2017, 07:22:30 am
content: Quote from: nctnico on January 20, 2017, 09:34:40 pmI never had any problems cleaning QFN/BGA pads after removing components with a preheater. Take your time and be gentle. Use flux and solderwick, and drag the wick in the length of the pin, not sideways.I did have problems cleaning pins without using a preheater. I think a preheater is one of the most helpfull and underrated soldering tools. It can help prevent LOTS of stress on the component AND pcb and even speed up the most simple jobs where you have to place multiple components on PCB's with a bigger thermal mass.When in a hurry, or on a budget (been there) a simple warmhold plate, variac and thermocouple works well. Just preheat to about 100-150 degrees celcius. Ranging from 100 for aiding in some "simple" soldering jobs like small QFN's or big groundplanes, to 150 for big a## BGA chips. I did some soldering jobs in literal seconds, wich took coleagues minutes to do without the preheater. Needles to say my board and component did survive, theirs did not.I never really worried about components falling off the bottom, except for really big FPGA's as I don't have the propper tools, so the bottom of the pcb can become too hot. Normally if the desoldering of leadfree stuff works without dropping components off the bottom, soldering the new component back with the good old stuff won't give any problems.

author: TheDane
author_rank: Regular Contributor
timestamp: January 21, 2017, 12:23:56 pm
content: Quote from: cdev on January 16, 2017, 10:49:58 pmI often replace USB sockets and other components in mobile phones, and have used low melt solder with great success. There are some issues to be aware of:Most low melt solder does not contain flux, so this must be added separately.The low melt solder must be mixed with the regular solder, which means: SolderALLthe joints with a normal solder iron, and make sure it reflows.(This ensures thatifthe connector has a ground pad underneath it, you do not lift (hopefully) any pads when removing the defective plug/component from the board, when doing the final removal procedure)When successfully reflowed, the solder is workable for a much longer time, at normal solder iron temps - and some times hot air is not needed to remove the item. It depends on the ground plane of the PCB.Hot air is great, as it heats the PCB in general, including the ground plane - and reflowing with low melt can enable you to use lower temps, thereby not melting the solder of nearby components.- making removal quite easy.Patience and experience is key, so do some practicing on scrap boards to get to know the mechanics of soldering, the reflow temp of solder, the time to heat up ground planes, the force to apply, etc.Remember to wear googles/eye protection, if the component sticks - and you suddenly jerk it off by mistake from applying too much upwards force and not waiting too long, as molten solder is HOT - just a kindly reminderLow melt solder must be removed after use, as the alloy is not as strong as regular solder!Cleaning with iso-alcohol, and applying a line of solder paste (experience here is great, a thin line and a dab in the holes - and hot air reflows the components perfectly, and it looks as new)Using a microscope can be very useful for tight pitched components, to ensure the right amount of paste is applied. I use tweezers to move the balls around, and to ensure the flux is wetting all pads.Btw - most low melt solders differ, and some contain hazardous metals. I recommend getting solder where you know the 'secret sauce' likehttp://www.ebay.com/itm/Solder-wire-low-melting-point-124C-1m-1mm-Bi55-5-Pb44-5-alloy-good-desoldering-/171173385076Capton tape is your friend, and an ir-preheater can also be great on multilayer boards.

author: valere
author_rank: Contributor
timestamp: January 21, 2017, 02:18:24 pm
content: Quote from: nctnico on January 20, 2017, 09:34:40 pmAs far as I'm concerned, I figured out that at the end of the day what may destroy your pad is more when you don't have enough heating power / large enough soldering tip. I always have to soldering iron, one mounted with a small tip and the second one with a bigger tip, and it's manly to be able to use wick to desolder / clean... since I use a big enough iron tip and a powerful enough soldering iron, I really need to mess things a lot to destroy some pads. With the proper size (for thermal conductivity between iron and wick), the local heating of the pad is way shorter and there is much less risk of "soldering" the wick on the pad thus allowing for an easy destruction of the pad.I prefer cleaning is far easier to do a good safe job after and to avoid short-circuit requiring a re-soldering / a re-desoldering putting more stress on the pad...