I need rework suggestions for multi layer PCB with big ground/signal planes - Page 1
Forum section: electronics / projects

author: mcinque
author_rank: Unknown
timestamp: August 01, 2015, 11:33:21 pm
content: Hi all,I've recently encountered a minor issue on a multi layer PCB.Since I'm dumb, I phisically broke a 0402 decoupling cap while cutting an IC pin, luckily leaving its pads intact over the PCB.Normally, on 2 or 4 layers PCBs, I can do this kind of rework quite safely (I've replaced many 0402 caps and resistors with no problems); the standard procedure I guess we all follow is to clean first the pads adding some fresh solder, then remove all the solder with good solderwick (even adding some flux if required) leaving the pads clean, then place the right amount of solderpaste over the pads, place the cap, do a quick pre heat around the area and solder the component quickly with hot air, avoiding to stress the PCB or the near components.The matter is that this PCB was probably a 6 layer PCB with multiple ground and signal planes. The broken IC was a decoupling cap placed between the VCC and GND, both were planes with a huge thermal mass.The quick pre-heat I normally do didn't work at all this time.The matter is that was nearly impossible to solder the cap with hot air like I always do, unless to insist and pop some components or pads.I was forced to use standard "toothpick+soldering iron" using quite high temperature (350°C), and the result is not so perfect like the ones I normally get with solderpaste.In this cases, I fear that the only way is to rework the board correctly is to pre heat the PCB and then use hot air.I guess you can confirm this.I don't know very well what kind of temperatures I should use to pre-heat the board, nor other suggestions to do this kind of rework on this damn multi planes PCBs.What are your suggestions? Which temperature for the pre-heat? And how much time? Do you suggest an hot air gun for pre-heating? What is the maximum temp I should NOT exceed?Sharing your knowledge is really, really appreciated by me, as always

author: zapta
author_rank: Super Contributor
timestamp: August 02, 2015, 12:27:01 am
content: I may missing something but why not soldering the 0402 cap with a solder iron?

author: kaadam
author_rank: Contributor
timestamp: August 02, 2015, 07:25:36 pm
content: I think you are good to go with solder iron. 2-3 weeks ago i replaced a notebook power jack with a bare solder iron, and it's work fine. A more sensitive part can be harmed (or cooked) this way, but a resistor or capacitor will survive it.

author: nctnico
author_rank: Super Contributor
timestamp: August 02, 2015, 07:53:21 pm
content: Quote from: zapta on August 02, 2015, 12:27:01 amMy idea exactly. Using solderpaste and hot air sounds like way to much hassle to me for a 0402 component. Besides that even on a board with less layers you have to heat it for a very long time when using hot air.

author: mcinque
author_rank: Unknown
timestamp: August 02, 2015, 08:08:46 pm
content: Normally I use the soldering iron, but this PCB has really a huge thermal mass around the cap, and was really difficoult to heat the pads connected to the huge plane (without damaging them), even with a thermally controlled soldering station.It seem something like 70um of copper, never worked before on this kind of PCB.I've been forced to use 380°C and a cut cone shape (like those for drag soldering) to transfer the most of heat possible. Never used so much heat in previous repair.I don't think that an 80W soldering station is not enough, of course mine is not a Metcal (it's a Xytronic), maybe I should get something better for this kind of jobs?

author: lod
author_rank: Contributor
timestamp: August 03, 2015, 07:19:01 am
content: The technique you use really has to vary depending what else is on the board. If you are next to an FPGA I would be keeping the preheat temperature right down, they get rather temperamental.You could use a hot air gun to heat the ground plane that it causing you problems, it doesn't have to be at the point you are working, a nearby safer point would let you pump heat into the system.And to reinforce what everybody else has said, use a soldering iron, it shouldn't need to be hugely powered but throw in some liquid flux. Once both sides are melted the cap should line itself up on the pads and leave a pretty shiny job. Once you remove the flux of course.

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: August 03, 2015, 08:56:58 am
content: I'd be curious to see pictures of the board if possibleAnd just repeating what has already been posted - if your iron doesn't have adequate thermal mass then preheat the whole area to a safe temperature (100-150C) with hot air so the iron doesn't sink all its power before reaching reflow temp

author: mcinque
author_rank: Unknown
timestamp: August 03, 2015, 09:47:20 pm
content: Thank you all for your suggestions.Yes, reading your comments it's better to use soldering iron, and if difficoult to solder, the best way is to only preheat the area with hot air and then use soldering iron, indeed.Good suggestion about adapting, using different tecnique based on what you have near the soldering zone.Maybe I must practice, because with that damn small 0402 and the soldering iron I can't get the same, typically chamfered connection I get when I use soldering paste and hot air.Unfortunately I can share any pics because the pcb was repaired and returned to the ownerThe repair is indeed ok, but I'd like to improve my tecnique to achievesimilarresults to a standard reflow process (it's achievable?).