With OSP coated circuit boards, what is the maximum allowable time interval between reflow of the first and second sides to insure proper wetting. Board Talk
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With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis.
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He is a pioneer in the science of reflow.
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, Pick and Place, coming to you from high atop Mount Rialto in New Hampshire at Board Talk Headquarters. Today we have a materials question about OSP.
It's on surface finishes. "With OSP coated circuit boards, what is the maximum allowable and recommend time interval between reflow of the first and second sides to insure proper wetting." Well, first off we must say that the subject of OSP is a very emotional and subjective issue. Some people do not believe that OSP is a good finish, even for single side reflow. They just don't like it and that's their opinion. Other people have no problems.
OSP materials themselves have come a long way. In the early days if you were doing double sided reflow you wanted to process to second side as soon as possible because the original OSPs dissipated in the presence of heat So those original materials required a very, very strict regimen in terms of nitrogen atmosphere. If I recall, below 100 PPM?
But I think the question is more significant. I think most people who have these concerns. The question regards the un-soldered pads, second side pads. Being exposed to the first re-flow cycle, are they still solderable? I think most people believe that damage to those un-soldered pads occurs during the reflow cycle. Hence, the mitigation strategy to use nitrogen.
The idea here is that the material itself becomes more porous and of course, heat - which you're doing in an oven - is a catalyst for oxidation and so the pores open up a little bit. But by the same token, we see a lot of people doing OSP's without nitrogen atmosphere.
That's absolutely true, Phil. And that being the case, I don't know that the interval between the first and second reflow is that important. If you talk to the manufacturers, most of them will say, there's no damage occurring during the first reflow or the second reflow if you're going to do two reflows, and wave soldering. So you really don't have to worry about any of these issues.
As long as the pads are not damaged or oxidized during that first reflow, once they're out and down to atmosphere temperature, I don't think it matters whether it's an hour or a day or maybe a couple of days. Certainly, you would e concerns if exposed for weeks or months. They don't last forever.
The more important concern is about the quality of the coating process. OSP is an organic material. It's got to be put on right. Proper thickness, proper quality controls, proper curing cycles and so forth. All of that contributing to allowing it to survive that first re-flow cycle and give you good solderablity and wetting. You probably have a pretty wide window in terms of the timing to your second re-flow.
So if you are concerned about perhaps a longer time frame than we're talking about here, you should consult your supplier of OSP materials and get the finite answer in terms of their specs because they know their chemistry. Well that's it for today. I hope we answered the question and what can I say? Beyond that, this is Phil Zarrow.
And Jim Hall.
Saying whatever you do and however you do it.
Don't solder like my brother.
Don't solder like my brother.
As to the reverse, I have a question. Can an OSP finish be deactivated (board-wise) by heating? That is, if you had gold plating (so little worry corrosion-wise), and wanted to "ditch" the OSP (it's caused me fits in the past), could you pre-heat the board, clean, and treat as any gold plated board?
Alan Knox, Knox Associates Design
We are about to publish data showing that controlling the thickness of the OSP is critical for second side reflow and spread. Newer technology developments have allowed us to help PCB manufacturers to controll and measure this OSP thickness. One more thought- If the delay between first side reflow and second side is due to delays in component deliveries, why not just wait for the components to come in before the board build? This takes time between reflows out of the equation.
Mitch Holtzer, Alpha Assembly Solutions
Bob Willis touches on a good point, the condition of the copper prior to being coated with the OSP. I would like to add another couple of points; there are various Organic Solder Protectant products, and some are better than others. Ask your PWB fabricator which one they use, and ask them how they control the time from fabrication until the OSP is coated. This is critical because bare copper can oxidize enough to cause solderability issues in just a few hours under the right conditions (humidity and room temperature are two factors).
But a more important point, at least to me, is the fact that any good assembly house will pre-bake their PWBs just prior to assembly and quickly reflow to prevent delamination and other issues. A circuit board is a bundle of fiberglass strands, much like a sponge, and it can re-absorb its saturated moisture content from humidity in the air within 6-8 hours @ 55% RH. So the second side should be reflowed as quickly as possible, or the boards should be kept in a dry box if the time between the first and second reflow is going to be more than perhaps 4 hours.
This practice will not prevent any exposed copper from continued oxidation, but it certainly helps. To summarize; if you are going to use OSP, make sure your fabricator uses one of the better OSP products, and they control the time of the initial copper treatment prior to OSP coating, and then you should keep the boards drypacked until ready for production. These steps will all help.
Richard Stadem, Analog Technologies Corp.
I got some boards with an OSP finish a while back. I do hand soldering and found it a horrible finish. One of the only times a customer ever had issues with solder joint quality. I'd never specify it for any of the boards I design.
Allan Knox II, Knox Associates Design
Phil, you nailed it in your last comment. Consult the supplier of the OSP you are using for the time between reflows.
I have run OSP successfully with a double side reflow process in air, however I did minimize the time between top and bottom reflow due to a difference in the wetting characteristics.
Normally I sized the build to run top and bottom in the same day, but did have success up to about 3 days.
When doing this the second pass reflow needs careful attention to hit proper temperatures. The longer OSP sits after opening the package, the more critical this appears to become, and the first pass heat cycle adds to the challenge when setting up the profile for that second pass.
Bruce Webster, Iridium Communications
It's really a question that cannot be answered unless a user works with a supplier to confirm the material and the process used. All finishes in terms of wetting are changed when you reflow them one and a second time and the hold time does have an impact.
In addition, its the surface of the copper prior to coating & the coating process. Then its the soldering temperature, time at elevated temperature and how quickly the cooling is conducted. slow and fast cooling have a major impact on wetting. Then it's also impacted by either air or nitrogen reflow and the levels of nitrogen. After two sided reflow the hold time is even more critical on a good OSP when wave or selective is used.