Author Topic: surface mount rework fun  (Read 6000 times)

0 Members and 1 Guest are viewing this topic.

Offline KhronX

  • Frequent Contributor
  • **
  • Posts: 323
  • Country: fi
Re: surface mount rework fun
« Reply #25 on: January 18, 2017, 11:00:48 am »
No problemo, glad to share my experience, as limited as it may be :)

I'm just not a fan of stressing over details that are, in the real world, irrelevant / inconsequential  ;D

On 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 around :)

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.
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans
 

Offline cdev

  • Super Contributor
  • ***
  • Posts: 3774
  • Country: 00
  • We're glad you made it, welcome to the future!
Re: surface mount rework fun
« Reply #26 on: January 18, 2017, 01:14:11 pm »
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.
"What the large print giveth, the small print taketh away."
 

Offline eKretz

  • Frequent Contributor
  • **
  • Posts: 570
Re: surface mount rework fun
« Reply #27 on: January 20, 2017, 06:04:05 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.

I 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 to MOST QFNs aren't that sensitive to such details. For the ones that are, you will need 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.
« Last Edit: January 20, 2017, 06:06:31 pm by eKretz »
 

Online tautech

  • Super Contributor
  • ***
  • Posts: 12985
  • Country: nz
  • NZ Siglent Distributor
    • Taupaki Technologies Ltd.
Re: surface mount rework fun
« Reply #28 on: January 20, 2017, 06:18:37 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.

I 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 to MOST QFNs aren't that sensitive to such details. For the ones that are, you will need 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.
:-DD
I 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.  :o

Preheat.
Preheat.
Preheat....................
Avid Rabid Hobbyist
 

Offline valere

  • Contributor
  • Posts: 34
  • Country: fr
    • Instants perdus (photo)
Re: surface mount rework fun
« Reply #29 on: January 21, 2017, 07:23:38 am »
I 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!
Too 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...
Valere - Electronic Engineer - Autonomous robotic designer participating to French Robotic Cup / Eurobot (President of Association Sikula Robotik)
http://sikula-robotik.desbwa.org
http://youtube.com/sikularobotik
 

Offline eKretz

  • Frequent Contributor
  • **
  • Posts: 570
Re: surface mount rework fun
« Reply #30 on: January 21, 2017, 08:10:27 am »
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 even with 150°C preheat. Sure makes soldering on average boards seem very easy though!
 

Offline nctnico

  • Super Contributor
  • ***
  • Posts: 15202
  • Country: nl
    • NCT Developments
Re: surface mount rework fun
« Reply #31 on: January 21, 2017, 08:34:40 am »
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.
IMHO 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.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Smith

  • Frequent Contributor
  • **
  • Posts: 335
  • Country: 00
Re: surface mount rework fun
« Reply #32 on: January 21, 2017, 06:22:30 pm »
]
IMHO 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.

I 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.
Trying is the first step towards failure
 

Offline TheDane

  • Regular Contributor
  • *
  • Posts: 152
  • Country: dk
Re: surface mount rework fun
« Reply #33 on: January 21, 2017, 11:23:56 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.

I 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: Solder ALL the joints with a normal solder iron, and make sure it reflows.
(This ensures that if the 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 reminder  >:D

Low 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' like
http://www.ebay.com/itm/Solder-wire-low-melting-point-124C-1m-1mm-Bi55-5-Pb44-5-alloy-good-desoldering-/171173385076

Capton tape is your friend, and an ir-preheater can also be great on multilayer boards.
« Last Edit: January 21, 2017, 11:25:31 pm by TheDane »
 

Offline valere

  • Contributor
  • Posts: 34
  • Country: fr
    • Instants perdus (photo)
Re: surface mount rework fun
« Reply #34 on: January 22, 2017, 01:18:24 am »
IMHO 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.
As 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...
Valere - Electronic Engineer - Autonomous robotic designer participating to French Robotic Cup / Eurobot (President of Association Sikula Robotik)
http://sikula-robotik.desbwa.org
http://youtube.com/sikularobotik
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf