BGA soldering quality issue - Page 1
Forum section: electronics / general

author: roverider
author_rank: Newbie
timestamp: November 05, 2020, 03:20:37 pm
content: Hello,I am trying to sort out quality issues in soldering 0,5mm grid BGA. Im making inspection by Dye and pry test and I am seeing in some samples suspicious golden tinted surfaces mainly on the pryed off balls from pads of the PCBs, but sometimes on the pads on the pcbs. And also ridges and non uniform color on the surface of the pryed balls. Most time without the flown dye. I think the golden tint is due to thin oxide layer on the solder balls (which could cause issues of reliability of the solder joint) but it could be also residues of gold (the pads are electrochemically gold plated) and also residues of the solder flux. Solder paste is Pb free paste Indium 10.1 or Indium8.9, both in N2 atmosphere.normal color solder jointgodenn tint-suspicious oxidationgolden tint and ridges - mainly on right bottom ballI also made an experiment- i desodered the BGA and then I overheated it to induce oxidation and it seems that the golden tint is heat related.-freshly desoldered normally looking but with some faint golden tint areafirst overheating-golden tint with some blue- purple partsSecond overheating-darker golden color.Could you please tell me you opinion if the solder joint is oxidated or not and if the ridrges and non uniform color indicates poor quality or not and why?Thanks.

author: helius
author_rank: Super Contributor
timestamp: November 05, 2020, 03:56:02 pm
content: A solder bond is like a mechanical weld: the metal atoms from the solder and the pad dissolve into one another and become a single metal object. If you pry that weld apart, there will be traces of both the solder and the pad on both sides. If you are using ENIG plated boards, the pads have a gold flash. Indium-based solders do not eat that gold away, they dissolve only a fraction of it to create the weld. That means that at the base of the weld zone, there is still gold in a relatively pure state and yellow color. When pried apart, some of that yellow gold-alloy will be torn away and stay on the solder ball, depending on the precise depth at which the solder joint breaks. The ridges are the remains of the plastic deformation during the prying process, as stress builds up in each metal joint before it weakens and breaks. Both of these signs are expected and normal. The dye test is to identify cold joints or voids and you do not seem to have either of those issues.

author: roverider
author_rank: Newbie
timestamp: November 05, 2020, 04:18:35 pm
content: Thank you,I understand that there is residue of gold, but, when i left solder on hot soldering tip for some time, it starts to have golden and then even to blue/purple tint and there is definately not any gold, it is just thin layer of oxides reflecting/ absorbing parts of visible light depending on the oxide layer thickness.I am confused , if the golden tint on the balls is just parts of the ENIG or or oxide layer like on the soldering tip or both. And more confusing for me is that the tint only is visible only in some joints and also the tint is getting more visible and darker after overheating the balls on air (as showed my crude experiment).

author: helius
author_rank: Super Contributor
timestamp: November 05, 2020, 05:32:39 pm
content: Surface oxidation can indeed create those colors. As can chromates, nitrides, sulfides, and other surface reactions. But the way you described your project, I don't see how these could form in an inert atmosphere. Do you see similar coloration on the balls of new components prior to reflow?

author: roverider
author_rank: Newbie
timestamp: November 05, 2020, 07:14:31 pm
content: No, because I test only final product, because all the assembly is made in our supplier in China.There is some background.By the DnP test, Xray and CT scan I discovered issues with voids (up to 80% pad area) and cracks which caused malfunction of our product. After some adjustments of the soldering process at supplier it gets better, reliability of our product gets better. We got some pre production samples with good quality soldering and also good reliability, but large percentage of production pieces fails, so we expect non consistency of quality with the soldering, mainly BGA- sometimes it is cracks sometimes voids and now the golden tint occured. The tint is only on some pads and on somel samples. The inconsistency seems suspicious me.I saw the golden to purple tint on some components such as pads on ceramic capacitors - some pieces from same batch have classic shiny silver some got golden tint some even purple/blue. But as i said it was on final assembly after reflow.

author: asmi
author_rank: Super Contributor
timestamp: November 05, 2020, 09:18:03 pm
content: Quote from: roverider on November 05, 2020, 04:18:35 pmThat "overheating" looks to me like burned leftovers of the flux. This is what I saw a lot of when I was tuning my soldering profile and repeatedly reflowing the same board over and over.

author: SMTech
author_rank: Frequent Contributor
timestamp: November 06, 2020, 10:11:43 am
content: I would focus more on what you can see from X-Ray. Could be an idea to get a sample of the BGAs in question (if you didn't supply them) maybe China has sourced a crappy batch or has been storing them badly.If you are seeing significant voiding and cracking in X-rays I'd be looking at the whole process, printing, paste choice, paste handling, storage, profile but equally there comes a point when you are spending too much time and effort fixing a false economy. If your contractor can't diagnose and fix the issue on their own, you should take your business elsewhere.

author: Matt-Brown
author_rank: Regular Contributor
timestamp: November 06, 2020, 03:07:18 pm
content: Prying bga off a board should either damage the bga or the pads on the board. The solder joint should not fail between board paste and ball.I cannot see the whole board from the pictures, but depending what is around the bga for thermal mass (large smd parts), I am betting they need to increase soak time in the reflow