|Ask the Experts|
June 23, 2020
Mixed Process Solder Joint Appearance, Smooth or Grainy?
We are using RoHS components that are placed using tin/lead solder paste and soldered in oven using a tin/lead reflow profile. What appearance should we expect for these solder joints, smooth or grainy?
|Expert Panel Responses|
In most any soldering application the solder (paste) will dictate the appearance of the joint. The small metallurgical contributions from the component lead will have a negligible effect on the overall joint appearance...UNLESS you are referring to BGA's with collapsible solder balls.
In that case, the solder of the ball will alloy with the Pb'd solder paste and become a significant fraction of the solder joint and the resultant solder joint will look somewhat grainy. This situation will have more influence on the solder joint appearance than other RoHS-compliant components such as QFPs, passives, etc. Sn-Pb solder joints, if reflowed properly, have a smooth appearance whereas SAC solder joints tend to be grainier.
There are lots of factors that can affect either. Rarely does the outward appearance of the solder joint have any bearing on the resultant reliability of that joint. Of course there are exceptions to that statement, hence "Rarely".
I would expect a smooth finish as the very thin plate of the RoHS components shouldn't effect the melting point greatly as long as you are not on the low end of Leaded Profile say 205C. We have always advised our users when doing this to peak a little higher at 230C just for good intermetallics and just in case.
Technical Sales Manager
BLT Circuit Services Ltd
The tin/lead profile will not get hot enough to reflow a tin based lead free finish on the components. The appearance of the unsoldered finish should not change dramatically. Some of the lead free finish will dissolve into the tin/lead solder joint and could affect the final appearance of the joint. Dulling or graininess is possible. The appearance depends upon the relative amount of lead-free finish that dissolves into the tin/lead solder.
If SAC305 BGA spheres are used with a tin/lead profile, then I would expect the profile to cause some dulling of the spheres. Some of the silver and copper from the spheres will dissolve into the tin/lead solder paste causing a dulling effect on the solder joint.
The appearance of the final joint depends on the make-up of the non-SnPb finish on the part, and the amount of said finish if it fusible or soluble. Components with a Sn finish over a Ni barrier will typically show a smooth finish in the final solder joint. Parts where there is a substantial amount of a Pb-free alloy, e.g. Pb-free BGAs used in a mixed-alloy-assembly process will have a less smooth finish.
Also, some bare thick film end terminations on chip components may be more subject to leaching in the SnPb solder, due to a lack of silver in the solder alloy. Joints on these parts may look grainier than with a silver-bearing alloy, although a lot depends on the reflow profile (hotter is worse, longer is worse).
I would expect them to be smooth and shiny, assuming your profile is correct. We currently build both RoHS and Leaded assemblies, with nearly 100% of the components used being RoHS compatible both from a material and a processing standpoint. I believe that the vast majority of component manufacturers are only producing RoHS compliant components at this point.
Esterline Interface Technologies
Without knowing the component type and plating it's difficult to make any general statements. For example, NiPd plated parts (TI parts) have considerably different solder joint appearance than a tin/lead coated lead. No necessarily bad, just different. Similarly, RoHS compliant BGA will not only look different, but also require profile modifications to ensure a robust solder connection.
Solder joints on ENIG pads will also have a different look than a Sn63 HASL surface finish. Therefore, the best answer is that joint appearance will be component and plating specific. Some will look very similar to their leaded counterparts and others will look quite different.
Technical Marketing Manager
If you are using tin/lead 63/37 solder paste you should see a smooth & shiny surface.
Senior Manufacturing Engineer
It is hard to answer because such words are relative and the joint appearance can also be impacted by process parameters such as the cool down rate of the reflow profile.
Generally speaking a "RoHS" solder joint will not be as shiny as a SnPb joint. For images of Pb-Free solder joints, the latest revision of IPC-A-610 (Section 5.1) can be consulted.
Technical Support Engineer
The ROHS compliant finish on your component leads is most likely tin. The relative amount of tin that becomes part of the solder joint is minute compared to the volume of tin lead from the solder paste. I presume you are asking because you are observing grainy joints. They should be smooth and shinny.
You may want to look at increasing your cooling rate after reflow, this reduces the amount of disturbance as the solder joint goes through the liquid to solid phase change.The next thing to try would be to check the paste vendor's recommendation for the time to peak temperature and the time above liquidus. If you are exceeding the paste's process window, you could also create grainy joints.
Global Director of Customer Technical Support
Alpha Assembly Solutions
Nearly all components today have a RoHS-compliant finish. You should expect shiny well-wetted solder joints using a Sn63 solder and Sn63 profile except for BGAs as the experts here have noted. But please ignore any advice to speed up the cooling rate to achieve this; it is not really important to have shiny solder joints.
Also, ignore advice to increase your reflow profile peak temp or Time Above Solidus, as that is also not necessary whatsoever. If you are using a Pb paste and profile, chances are you are soldering components and/or circuit boards not intended to be soldered at 235 deg. C, and the warranty for those components/pwbs will be voided and possibly reduced reliability could also occur if you do so.
Accelerating the cooling rate is also a very bad idea, as the material properties of the finished solder joint will drastically change (ductility/brittleness), not just the appearance. Stick with the temperature profile and generally- accepted cool-down rate of 2 deg. C per second or whatever your solder paste vendor recommends on the Technical Data Sheet. If you are not experiencing any issues, don't try to fix a qualified process that is not broken.
The ideal appearance of the slder joints is smooth. Based on what you are doing I expect to see issues with the components and solder reflow like incomplete solder joints, "head in pillow", etc.
I have seen a ROHS component (SOT-23) going through a full leaded process - the only way the part got proper solder joints was to send the parts out to be re-tinned with leaded solder before use as the part was just "sitting" on top of the reflowed solder.
Engineering and Operations Management
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