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We have a board with two BGA chips. Due to the current shortage of many parts, one of the chips is next-to-impossible to find; except that we found a supplier with a good stock of the non-RoHS / leaded version of the chip (which is a discontinued product, but this supplier happens to have a pretty good stock of them).

Problem is: the other BGA chip is only available in lead-free balls.

I know that using leaded process for lead-free BGA chips is not acceptable, because the solder balls of the BGA will not melt and the joint will be unreliable.

My question: is it ok to solder leaded BGA chips with a lead-free (thus, higher temperature) process?

For context:

  • This is for a research project; the boards will be deployed in indoors environments (most likely places with air conditioning, etc.)
  • We do not need to worry about long-term reliability (the boards will be operating for less than a year).
  • Also, the boards are not part of a safety-critical or mission-critical system; sure, we want them to work reliably for the purpose of the quality of the research data; if a small fraction of the boards fail, it is still acceptable.
Cal-linux
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    Among other things, you need to find out if the materials (like plastics) are of the same type as lead-free since lead-free requires better materials to withstand the higher temperatures. They could just be standard now though (or whenever the ICs were produced) rather than two separate supply chains. – DKNguyen Aug 11 '21 at 15:26
  • Google "lead contamination in lead-free process" – DKNguyen Aug 11 '21 at 15:52
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    Years ago I had a brief interaction with Prof. Pan at Cal Poly SLO who was doing work on mixing lead and lead-free solder...I can't find his publications list online, but he might be willing to talk to you for a few minutes or refer you to an appropriate paper. – The Photon Aug 11 '21 at 15:54
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    Here's an [old post](https://electronics.stackexchange.com/q/136542/6334) where I cited a book chapter by Pan (unfortunately the link is dead). Also searching the term "Backward and Forward Compatibility" leads to some app notes at Maxim and TI that might be helpful. – The Photon Aug 11 '21 at 15:59
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    Ask the assembly house that will be doing the soldering. They are the experts. Some have a strict no cross process policy, others will do it as long as they know. Or, they can place it by hand. I all depends on the assembly house chosen. – Aaron Aug 11 '21 at 16:02
  • @ThePhoton -- found the book chapter; very informative, and unfortunately, it seems to agree with JonRB's answer, in that it sounds a bit pessimistic about it (if I can't find alternatives, I may need to estimate what the risk might be, what sort of percentage of defective boards might I expect, then decide if I should go for it) – Cal-linux Aug 11 '21 at 17:22
  • Re-balling a BGA used to be a thing ... I wonder if a leaded BGA can be re-balled to work with a lead-free process? –  Aug 11 '21 at 17:36
  • Are you even allowed to sell those boards? – Simon Richter Aug 12 '21 at 08:47
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    If these are prototypes never to be put on the market, then just go with leaded solder. Particularly if you will take them out of use after a year. If you wish to be very formal, mark them up as non-conforming to RoHS etc directives. – Lundin Aug 12 '21 at 09:21
  • @SimonRichter ‒ not sure whether your question was for me; but no, we won't be selling those boards; like I said, this is part of a research project, and we have no restrictions on using leaded or lead-free process in this context. For that matter, my understanding (this may be outdated) is that in North America we have no obligation to do RoHS for commercial electronics (of course, no-one in their sane mind would do non-RoHS for a commercial product anyway, given the international nature of today's markets) – Cal-linux Aug 12 '21 at 11:49
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    @Cal-linux, IIRC the rule is "if you sell electronics, you need to offer to recycle them." Few places can do that on their own, so they contract out to a recycling company, and usually these contracts stipulate that products must be lead-free. So effectively, the rules are the same. If the products aren't sold, then it's a lot more relaxed, because recycling those is a one-time expense rather than an ongoing contract. – Simon Richter Aug 12 '21 at 11:54
  • Mass production or very small run? – winny Aug 12 '21 at 13:42
  • @SimonRichter Why do you think the recycled junk must be lead free? For such contracts you just packet and label everything as "electronics junk" and that need not necessarily just involve the products that you are selling yourself. Could as well contain some random broken electric device, which may be pre-RoHS. Or your own old products from the pre-RoHS era (made before year 2002). It would be incredibly stupid if they didn't accept such junk. "Ok this one contains lead so I'll throw it with the regular garbage instead..." – Lundin Aug 13 '21 at 10:17

4 Answers4

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I would talk to your assembler and see if you could use two different temperature processes.

Do the Lead free profile first (that goes to a higher temperature) then after those parts are done solder the leaded parts on with a leaded temperature profile. This satisfies the requirements of both. (you don't need to worry about stenciling as BGA's have solder balls)

Another option would be to have the lead free BGA's installed with a IR rework station (which also support temperature profiles) which is more difficult but possible.

At the end of the day it would depend on who is doing the rework and what they are capable of and their experience with BGA parts.

Voltage Spike
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    How would you stencil lead solder with lead-free parts already on the board? A manual process with a mini stencil and a jig to get it in the right place? Sounds expensive and error prone. – Scott Seidman Aug 11 '21 at 16:11
  • @ScottSeidman -- at first, I understood the suggestion as: put all components, each with the corresponding paste; reflow with lead-free, and the leaded parts will have brittle joints; now reflow at leaded profile, so the brittle joints will melt and "reattach", this time correctly; but on second thought (after reading your comment), I see that my initial interpretation is most likely incorrect (I now see multiple issues with it). But yea, I also see the issues you point out. It would be ideal if we had CNC machines that "print" the solder paste on the board (as an alternative to stencils) – Cal-linux Aug 11 '21 at 17:19
  • I think this may be a real option for low count, for for any quantity it might not be practical – Scott Seidman Aug 11 '21 at 17:50
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    My bad, you can do bgas without paste! You would need flux, though. – Scott Seidman Aug 11 '21 at 18:20
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    @ScottSeidman That is what the balls are for, as stenciling would provide different amounts of solder for each pad and potentially make a bad connection (or intermittent with impedance). Flux is probably not as much as an issue, some BGA rework vids just brush on flux and try and keep the thickness even. – Voltage Spike Aug 11 '21 at 19:44
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    Is this the case regardless of surface finish? For example, with ENIG finish (as opposed to HASL, which provides a thin layer of material equivalent to the solder paste), can BGAs be placed just on top, without applying solder paste, and will the amount of solder in the balls be enough? I've always been curious about this; as an example, KiCAD's library footprints for BGAs do have pads on the PASTE layer, which means that a stencil will provide the openings to apply paste on the BGA pads. – Cal-linux Aug 11 '21 at 20:09
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Be very careful with this...

Lead-free BGA's tend to be Sn-Ag-Cu and that copper content will react with the lead in SnPb solder causing very brittle joints.

http://thor.inemi.org/webdownload/newsroom/Presentations/APEX08/Papers/SAC_BGAs_in_SnPb.pdf

--EDIT--

Just to add some clarification to this, it is risky and it does cause cracks, but as the OP stated, it is for a short period of time. At the start of 2019 I ended in a similar situation and as long as you speak to the assembly house that there is a mix-tech process and they the reflow is at appropriate levels then it does work.

The only additional comment is I had Pb and Pb-Free BGA's (due to sourcing) and a test card then went through a RoHS type flow. Post reflow there were inspections performed and there were signs that the Pb balls started to collapse because obviously the temperature and duration was longer and thus tin-lead was more fluid

  • I wonder if that's what plagued early Playstation consoles (the common repair was to re-work the BGA.) – rdtsc Aug 11 '21 at 17:59
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It's possible to get BGA's reballed. When my last employer was switching over to RoHS, we had a batch of BGA's from our fab with balls incompatible with the process (can't remember which way round).

We sent them to a company with a laser reballing machine.

SiHa
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2

Component Prep is Required if Reliability is Important

A double-solder component prep removes the lead contamination.

If component prep is not done, the joint will fail prematurely, as this causes embrittlement. There is a metallurgical incompatibility.

If component prep is done, the issue is mitigated, and the assembly can be conforming and acceptable for class 2, and with process testing, class 3.

This process can be called double tinning or, for BGA parts it is less common but can be dubbed double reballing. Although you may consider some variations or hybrid approach as suggested in the comments.

  1. reball to dilute lead.
  2. reball for mounting

This is a prototype process, There is a target concentration level that is considered acceptable. As long as your process reasonably expects to hit this concentration that is what matters.

The process for cleaning lead and cleaning gold and the applicable standards are largely the same and may already be familiar to your contract manufacturer/assembler, so check with them.

Note: The lifetime reduction depends on several factors, if the product can tolerate higher failure rates or appropriate testing is done. You may opt to pursue a more basic mitigation, like basic reballing, or no mitigation altogether.

crasic
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  • Double-reballing seems a bit excessive. Would it not be good enough to deball, tin with SAC, clean then reball? – SiHa Aug 12 '21 at 16:06
  • @SiHa , potentially, depending on what the assembly house recommends. – crasic Aug 12 '21 at 16:08
  • Note that if you have engineering resources to spend (e.g. this is a continuous occurrence or several 10KU), there are empirical ways to determine how much prep you can get away with – crasic Aug 12 '21 at 16:13
  • Yeah, I suppose the assembly houses leadfree processes are a bit more mature now. When I was doing this, we were having to tell them what to do! The 'black pad' phenomenon was not widely understood, for example. – SiHa Aug 13 '21 at 07:46