|Ask the Experts|
July 8, 2019
Reflow Profile for Mixed Lead-Free and Leaded
What changes are required for using tin/lead paste on lead-free circuit boards? Does the oven profile have to be adjusted for lead-free boards, or does the tin/lead solder paste determine the oven profile?
|Expert Panel Responses|
For best overall results, I would profile to the paste.
Technical Support Engineer
The changes you will need to make depend on the type of board finish such as Enig, IAg, OSP, ISn, NiAu.
Either way you should profile the board with an oven profile platform like ECD oven rider or similar software. This way you can verify what temperatures various areas of the board are seeing throughout the reflow process. Because you are using tin/lead paste, I would suggest starting with that manufacturers recommended profile.
Manufacturing Applications Specialist
It all depends on the surface finish of the PWB. If you are using a non solder finish such as ENIG, I Ag, the SnPb reflow profile for the solder is the correct profile to use although we recommend a slightly elevated peak temperature for ENIG with SnPb solder, 220C instead of the more typical 212 peak.
If you are using a Pb-free HAL as the surface finish and are using a reflow profile peak temp below its Liquidus point, then you will be soldering to the surface finish rather than combining two molten alloys. Depending on the profile used and the thickness of the HAL deposit, you might not form the most reliable of joints. For Pb-free HAL and SnPb paste, the profile should be in excess of the Liquidus temperature of the HAL deposit.
S T and S Testing and Analysis
The solder paste used is the primary determining factor for the profile you develop.
Esterline Interface Technologies
This is a great question. Processing lead free boards with Tin lead has been done for many years. The difference now is we need to know what components are on your board. If you have PBGA components that have lead free balls the profile will need to have a peak temperature high enough to melt the lead free BGAs.
For example, the SAC305 lead free ball becomes fully liquidus at 221C. Typically the leaded profile peaks at 215C degrees. This must be elevated to the range of 225C - 230C degrees. This will insure the Lead Free BGA collapse decreasing the void percentage in the BGA ball.
I suggest you contact your solder paste supplier and ask for a solder paste that is designed for this application.
Sr Fiald Applications Support Engineer
The big change is the move from the fusible SnPb HASL finish to a non-fusible surface finish; since you cannot use SnPb solder with a Pb-Free HASL finish, the move to a non-fusible finish is a requirement. The wetting behavior of the new finish will be different. Most likely, you will see slightly higher wetting angles and less spread, which may cause questions from inspectors who are not used to these finishes.
You may see more skewing or movement of small discrete components due to differences in the timing of wetting, which can be greater on the non-fusible finishes. You are also changing the total solder volume by eliminating the SnPb HASL finish, which can have a visible impact for some joints, particularly those on fine-pitch leaded components.
On the positive side, non-fusible (flat) finishes will typically enable more robust printing processes. If your assemblies require multiple soldering processes, e.g. double reflow and subsequent TH soldering, then you should carefully consider what non-fusible finish will provide the best solderability retention through the entire process.
Finally, when choosing a non-fusible finish, be sure to consider packaging and storage requirements to ensure solderability on the line. All that said, back to your original question; does any of this impact the reflow profile? The answer is, very possibly yes. You may find that you need to tweak the profile to re-optimize the transition from soak to spike in order to manage thermal gradients more effectively, to minimize shifting/skewing of discretes.
You potentially could need to introduce nitrogen to the reflow process to enhance wetting (though this is uncommon; most modern paste flux formulations provide good wetting in air). You may find that minimizing peak temperatures and liquidus times helps to preserve solderability through multiple soldering processes.
Those are the main impacts that switching surface finish can have on the reflow process. If the process is currently well-optimized for the paste and the assemblies being processed, it is quite possible that no changes will be required.
In most cases the solder paste determines the reflow profile used. One exception to this may be the lead-free hot air level surface finish. Lead free hot air level melts at lead free temperatures, therefore a normal tin-lead reflow profile may not melt the surface finish. The tin-lead solder in the solder paste alloys with the lead-free hot air level finish which reduces the overall melting point. This lowering of the melting point may be enough to allow use of a tin-lead reflow profile.
Another exception to this is when lead-free BGA components are used with a tin-lead solder paste. Lead free BGA components require a lead-free reflow profile in order to melt and merge with the solder paste. Some tin-lead solder pastes can withstand a lead-free profile, but some cannot. Your solder paste supplier should be able to give specific details on this.
The lead free board does not care about the thermal profile (temperatures on solder joints) or the oven recipe (zone) settings. The lead-free board is simply one half of the solder joint, a solderable metallic surface (pad), plated with: gold; silver; nickel; tin; etc, or even bare copper.
The other half of the solder joint is the component's lead. If you want to use a tin-lead solder paste on these pads that is fine. You should make sure your solder paste flux is able to clean away any oxides and do it well because your paste alloy must wet the board's pad during reflow.
Remember a tin-lead board is already plated with tin-lead, but your lead-free board has no tin-lead on the pads, so your solder paste must supply the tin-lead.
As for the board's thermal profile, you must meet the needs of the solder paste alloy and its flux, in your case a tin-lead solder alloy, NOT a lead free alloy. Make sure you run a thermal profile logger to show that you are meeting the thermal needs of your chosen paste and it's flux.
Senior Project Engineer
Electronic Controls Design Inc
Solder paste determines the reflow profile. So if the paste is tin-lead and bare PCB is lead free (that is the case these days), you will use tin-lead profile.
However, if you have lead free components like BGA and you are using tin-lead paste, you need a very different profile - also known backward compatibility profile to make sure BGAs do reflow without overheating tin lead components on the board.
Hope this answers your question. You should get a copy of IPC 7530 on reflow profile development that I chair.
Ray Prasad Consultancy Group
It is highly recommended to follow the profile for the solder paste being used, so in this case one should use a tin-lead profile when processing tin-lead paste. If you were to use tin-lead solder paste on a lead-free board you would potentially experience contamination issues since the product could no longer be designated as lead-free.
The product could also experience long-term reliability issues since the lead-free solder pads would not melt uniformly at the lower tin-lead reflow temperatures. A better alternative would be to use a lead-free solder paste preserving the entire assembly as RoHS compliant.
Mixing leaded solder with lead-free components can be detrimental to the reliability of the solder.
Regarding profile it depends on the components you are using. For example a lead-free BGA requires the lead-free profile to insure full collapses of the ball. For non balled devices the tin lead profile would be run. However a full review of the board and finishes needs to be completed before choosing the proper assembly profile.
Feel free to contact me for a copy of a paper that I wrote many years ago regarding the reliability of mixed alloy solder connection.
Deck Street Consultants
Use the recommended solder profile for the alloy & flux chemistry and it should work. Just remember that your PCBA will no longer be RoHS compliant.
Senior Manufacturing Engineer
The paste and the components are usually the key elements to create the oven profile. Using lead free boards, the oven profile needs to be adjusted to assure proper soldering (adhesion to the board). The risk is to have incomplete reflow and/or non-wetting. It is recommended to run a test board before you begin to run production
Engineering and Operations Management
|Submit A Comment|
Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.
Insert Your Email Address