Electronics reflow soldering process appraiser needed - Page 1
Forum section: electronics / general

author: szlovak
author_rank: Contributor
timestamp: August 18, 2021, 08:15:42 am
content: HelloDoes anyone know who in Poland or abroad can professionally deal with checking the correctness of the reflow soldering process of an electronic circuit on already produced electronics? Specifically, I am concerned with the correct behavior of the times at which the board can be in the oven, etc. There are components that are sometimes more sensitive and baking in an oven and waiting for everything to melt nicely may cause damage to the most sensitive elements. There is a reason why there are diagrams that give maximum times, temperatures, generally a window where the process should run. We have a solid reason to believe that the process was incorrect and as a result there is a big rejection from the production, but we don't know how to go about it, because the cooperation with the contractor is very difficult, basically according to them everything is to blame, only the not made board and one component damaged (we know this for sure, like 99%). To be more specific we think that quartz crystals are damaged. When we lower the temperature by 5-10 degrees , then resonator refuses to start up. They are started many times a day, for periods like 10ms, it's low power application. All capacitors are correct, problems goes with individual resonators when switched between boards, wrong board starts to work flawlessly, good one before switching crystals starts to fail. On a newer design it is exactly the same when switching crystals, so crystal circuitry is not to be blamed. We can improve generation of that 26MHz clock by adding some small capacitors, but two problems arise: we shift from the generated RF frequency too much (we could compensate it by tweaking some registers probably, but it shouldn't be like that), secondly not all boards start to work anyway in lower temperatures. These boards should work even at -40°C and fail already at +15°C. Does anyone knows how to prove we're right? There is a small chance we are wrong, but we have some evidence of reflow temperature graph they did provide to us, where we can see that boards were too long at temperatures higher than 230°C, like for a minute, graph is slightly coarse, and the maximum temperature was probably in the maximum allowed area as well. What is more , they don't have any other graphs but they produced these boards I think not in on shot, so they should have more of them than only one.... I will appreciate any help leading to solve that problem, including paid ones.

author: SMTech
author_rank: Frequent Contributor
timestamp: August 18, 2021, 10:36:50 am
content: Quote from: szlovak on August 18, 2021, 08:15:42 amProfile graphs are generated in a dedicated profiling process, different people will have different procedures, but unless you are in a high volume or very specific highly regulated industry* its very likely the contractor has a selection of "standard" profiles they have worked out for their oven that match various board types/sizes. It's very unlikely they re-check or redo that profile for every batch, modern ovens are very stable and repeatable and a true board specific profile technically requires a sacrificial board to attach probes to, monthly or even annual verification should be perfectly adequate.For lead free what you describe seems perfectly reasonable, our paste specifies a TAL (time above liquidous i.e 220C) of 45-75 seconds and a peak temperature of 240-260C to make sure everything properly got hot, activated etc.*In those industries the reflow process can be fitted with all sorts extra sensing and logging that runs all the time and will tell you exactly what was happening in the oven at any moment which you could then correlate against a known profile and set of zone targets.

author: szlovak
author_rank: Contributor
timestamp: August 18, 2021, 10:47:26 am
content: Not at all, time window for these ones is only 30s. Now I looked at the graphs again and it was ~82s in the oven, my mistake. Even if it was "only" 60 , then it would be 200% maximum time. I understand that it should be like you say, but apparently it wasn't like that and I need a way to prove it professionally. Numbers of boards goes into many thousands . Test series didn't have that issue, nor newer series of boards. Problem follows individual crystals.

author: SMTech
author_rank: Frequent Contributor
timestamp: August 18, 2021, 12:52:32 pm
content: I'd be looking at the crystal before the process..

author: kylehunter
author_rank: Regular Contributor
timestamp: August 18, 2021, 03:34:52 pm
content: Quote from: szlovak on August 18, 2021, 10:47:26 amI don't think you're reading the specs correctly, or at least relating the info to us correctly. The max time window is 30s for TAL (Time Above Liquidus)? Then you're saying that they showed you the profile that the board went through, and it was 82s TAL? Standard SAC305 reflow parameters state TAL should be between 30s-90s. So, while 82s would be on the long side, it is still in the recommended window.I tend to agree with SMTech that the issue lies elsewhere, but, let's assume that it is an overheating issue. When you design a board with such a temperature sensitive part, it really is on you to relay that info to the CM. While in an ideal world, all CM's would go through each and every datasheet to see the max temp/times for each part, that really isn't realistic to expect.

author: szlovak
author_rank: Contributor
timestamp: August 18, 2021, 08:23:58 pm
content: Quote from: kylehunter on August 18, 2021, 03:34:52 pmSo how long has to be for you to be "too long" when manual says clearly 30s ±10s and that 365°C region for 10s?? I don't know what are you referring to by saying 30-90s for SAC305.Preparing a reflow process is a producer task. He has machines for this, he has experience (or not like in that case probably). He compares graphs for all the parts in circuit and if it's not possible to make soldering, he has to find such problem and not allow production, for instance when part was designed only for leaded solder and somehow was going to be used by mistake.I'm afraid this conversation is heading nowhere.

author: kylehunter
author_rank: Regular Contributor
timestamp: August 19, 2021, 12:29:53 am
content: Quote from: szlovak on August 18, 2021, 08:23:58 pmUhh... The reason I mentioned about you not relaying the specs is based on you saying:"Now I looked at the graphs again and it was ~82s in the oven, my mistake."I initially took that literally, that the board was in the oven for 82s, which doesn't make sense. That's why I asked for clarification to see if you actually meant TAL, which you didn't clarify, but I'll now assume.Now with your additional info you gave. If you and the CM discussed back and forth, and they were aware they were responsible to check every datasheet for max temperatures, that's a different story. If that was my shop, once I saw that a part was a max of 30s TAL, I would immediately come back to you and look for substitutions, or it would be additional costs.The reason me mentioning the IPC standard profile for SAC305, is that this part would require a reflow that would basically not follow that standard. Since 30s is the bare minimum TAL for SAC305. Hence the added time/costs my shop would apply. Unless of course, you're fine with a SAC305 alternative (leaded/low-temp/etc).One last thing:"Preparing a reflow process is a producer task. He has machines for this, he has experience (or not like in that case probably)."I'm really not trying to stir you up, just using my experience in the industry. The "producers" task is to follow IPC standards. Since they presumably used SAC305 solder, that profile follows the industry standard. Unless otherwise told or found different, that would yield a quality product. Like I said, in an ideal world, every part datasheet would be check on every board built (or manufacturer contacted in the case of ambiguous specs), but that's not the norm. IPC standards are gold.A great CM would profile the first board with thermocouple on 4-8 parts on the board. They would confirm that all areas of the board are following the IPC and paste manufactures spec. That graph could then be provided to you, the client, for review.

author: szlovak
author_rank: Contributor
timestamp: August 19, 2021, 07:35:58 am
content: The whole process is of course longer. That 30s window is really nothing unusual when compared to other similar crystals. In general crystals are fragile but usual THT ones had longer pins that prevented them from overheating during soldering.I think I can do some more testing and determine serial resistance that can run resonator with and try to determine oscillator margin and compare to good ones. Then if it was below 3 for bad ones and significantly more for good then we have some better argument, maybe.

author: kylehunter
author_rank: Regular Contributor
timestamp: August 19, 2021, 10:49:03 am
content: Quote from: szlovak on August 19, 2021, 07:35:58 amWhat is the crystal part number?

author: szlovak
author_rank: Contributor
timestamp: August 19, 2021, 12:08:38 pm
content: You've got PM, I have to keep it that way. OK, I've tried serial resistor, adding even 150 ohm keeps oscillator still running at room temperature. Resistance of that crystal is 50ohm, so safety factor is 4 in that case, so it's good. The thing is that if I add some capacitors it is more stable, but the question is if it is OK to go outside the exact frequency, that is well trimmed , add some extra ppms, just to keep it running more reliable? Like I said, it will help to some of them, but not for all. The goal is to have exact value and not something off. And finally there can be anyway some damage to crystal that was already before soldering, or after.

author: szlovak
author_rank: Contributor
timestamp: August 19, 2021, 09:44:24 pm
content: I'll ask and will have to remove some data, probably yes. I don't know what sensors they use, I didn't know that these K couples are so coarse, interesting. I can say now that it was a coarse measurement in general, you will see. I read about those old fashion vapor ovens. I think now IR is used mostly. We definitely will use other firm for the next production and visit them and look what they use and like you pointed out, to have better measurement, documentation of each one process. There are also markers that detect the maximum temperature , in form of some papers etc. The more questions we will ask them , the better.

author: iesn
author_rank: Contributor
timestamp: August 20, 2021, 12:13:43 pm
content: I'm sorry but all my messages was deleted, I can't argue. Seems that my approach was bad.I'm sorry about that. But... you can stay informedthe internet is BIG. I write to moderators to ask for an explication, I think that will never come.Best regards, Dan

author: szlovak
author_rank: Contributor
timestamp: August 20, 2021, 01:55:10 pm
content: Who was asking for the graph ? I didn't use quote

author: iesn
author_rank: Contributor
timestamp: August 20, 2021, 02:05:31 pm
content: I ask for the graph but all my messages was deleted, seems that here the knowledge are prohibited. you can send me a PM or a email until I'll be banned

author: iesn
author_rank: Contributor
timestamp: August 20, 2021, 07:10:38 pm
content: The mystery was solved, an amputated brain ape, mark / report all my posts as spam and a moderator... well... delete them.Back to our business, @szlovakshow me a reflow profile that you have.Best regards, Dan