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June 4, 2018

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".

Gary Freedman
Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.

Iwould expect a smooth finish as the very thin plate of the RoHS componentsshouldn't effect the melting point greatly as long as you are not on the lowend of Leaded Profile say 205C. We have always advised our users when doingthis to peak a little higher at 230C just for good intermetallics and just incase.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Thetin/lead profile will not get hot enough to reflow a tin based lead free finishon the components. The appearance of the unsoldered finish should notchange dramatically. Some of the lead free finish will dissolve into thetin/lead solder joint and could affect the final appearance of the joint. Dulling or graininess is possible. The appearance depends upon therelative amount of lead-free finish that dissolves into the tin/lead solder.

IfSAC305 BGA spheres are used with a tin/lead profile, then I would expect theprofile to cause some dulling of the spheres. Some of the silver andcopper from the spheres will dissolve into the tin/lead solder paste causing adulling effect on the solder joint.

Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

Theappearance of the final joint depends on the make-up of the non-SnPb finish onthe part, and the amount of said finish if it fusible or soluble. Componentswith a Sn finish over a Ni barrier will typically show a smooth finish in thefinal solder joint. Parts where there is a substantial amount of a Pb-freealloy, e.g. Pb-free BGAs used in a mixed-alloy-assembly process will have aless smooth finish.

Also, some bare thick film end terminations on chipcomponents may be more subject to leaching in the SnPb solder, due to a lack ofsilver in the solder alloy. Joints on these parts may look grainier than with asilver-bearing alloy, although a lot depends on the reflow profile (hotter isworse, longer is worse).

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Iwould expect them to be smooth and shiny, assuming your profile is correct. Wecurrently build both RoHS and Leaded assemblies, with nearly 100% of thecomponents used being RoHS compatible both from a material and a processingstandpoint. I believe that the vast majority of component manufacturers areonly producing RoHS compliant components at this point.

T.J. Hughes
Manufacturing Engineer
Esterline Interface Technologies
Mr. Hughes has been in the electronics manufacturing field for 20 years. Operating the processes and as a manufacturing engineer for the last 14 years. He is also a CIT as well as an SMTA Certified Process Engineer.

Without knowing the component type and plating it's difficult tomake any general statements. For example, NiPd plated parts (TI parts)have considerably different solder joint appearance than a tin/lead coatedlead. No necessarily bad, just different. Similarly, RoHS compliantBGA will not only look different, but also require profile modifications toensure a robust solder connection. Solder joints on ENIG padswill also have a different look than a Sn63 HASL surface finish. Therefore,the best answer is that joint appearancewill be component and plating specific. Some will look verysimilar to their leaded counterparts and others will look quite different.

Tim O'Neill
Technical Marketing Manager
Tim O'Neill is the Technical Marketing Manager for AIM Products. AIM is a global supplier of materials for the PCB assembly industry including solders, fluxes and thermal management materials. Tim has a B.A. from Assumption College and post-graduate studies in education. He has 20 years of experience in the electronics soldering industry, beginning his career in 1994 with EFD and was key in business development of their fine pitch solder paste dispensing technology. Tim joined AIM in 1997 and has since assisted many clients with assembly challenges, specializing in Pb-Free process development and material selection.

If you are using tin/lead 63/37solder paste you should see a smooth & shiny surface.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

It is hard to answer because such words are relative and thejoint appearance can also be impacted by process parameters such as the cooldown rate of the reflow profile. Generally speaking a "RoHS" solder joint willnot be as shiny as a SnPb joint. For images of Pb-Free solder joints, thelatest revision of IPC-A-610 (Section 5.1) can be consulted.

Kay Parker
Technical Support Engineer
Indium Corporation
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.

The ROHS compliant finish on yourcomponent leads is most likely tin. The relative amount of tin thatbecomes part of the solder joint is minute compared to the volume of tin leadfrom the solder paste. I presume you are asking because you are observinggrainy joints. They should be smooth and shinny.

You may want to look at increasing your cooling rate afterreflow, this reduces the amount of disturbance as the solder joint goes throughthe liquid to solid phase change.The next thing to try would be to check the paste vendor'srecommendation for the time to peak temperature and the time above liquidus. If you are exceeding the paste's process window, you could also creategrainy joints.

Mitch Holtzer
Global Director of Customer Technical Support
Alpha Assembly Solutions
As the Global Director of Customer Technical Service (CTS) for Alpha, Mitch sets direction and provides coordination for the Alpha CTS group in a global capacity. A major focus of this position is to provide strategic support to OEM, CEM and Automotive customers and target accounts. Mitch joined Alpha in 1998 and has progressed through positions of increasing responsibilities in Marketing, Product Management and R&D. He is a graduate of Purdue University with a degree in Chemistry and holds an MBA from Temple University.

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.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

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.

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.