Selective Solder Pot Temperatures

Selective Solder Pot Temperatures
What temperature should the solder pot in our selective soldering machine be? Should the temperature be different from our wave system? Jim Hall and Phil Zarrow, also known as The Assembly Brothers, address these questions and share their expertise.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
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Phil Zarrow
Phil Zarrow
With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.


Welcome to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting, the Assembly Brothers coming to you from ITM headquarters, high above Mount Rialto.

We're here to talk about electronic assembly, materials, equipment, components, practices and procedures, with the emphasis on practice because you have to keep practicing until you get it right.

Jim, what is our question today?

Our question comes from R.W.

Can you suggest a temperature range to run the solder pot in our selective soldering machine? We use a no-clean flux with 63/37 solder and with four to ten millimeter nozzles.

Our boards are FR4, 1.6 milimeter and with either an ENIG or a HASL finish. Should the temperature be any different from our wave soldering system?

Basically you are talking about a tin-lead alloy and solder pot temperature, and again the molten solder the same temperature as what we are doing with wave solder.


Right, but one of the advantages of selective soldering is that you can tailor the process to the individual solder joints.

Whereas a board going through wave soldering sees one process, one pre-heat for all of the joints, one immersion time and immersion depth in wave parameters for all the solder joints.

My instinct with all soldering, Phil, is never get anything any hotter than you have to, because high temperatures, other than promoting wetting, high temperatures do no good to any of the materials and components on our circuit board.

But I think with selective soldering the more pertinent questions are, what is the configuration of your machine, specifically how much heating do you have to do with the solder immersion into the solder nozzle?

Do you have initial pre-heat that brings the board up or are you relying on immersion in the nozzle to give you all of your heating? I would think that would be the most significant factor in determining what the temperature should be.

If you don't have any pre-heat and you're going to have to do all of the heating for each your joints from the immersion of the nozzle you are probably going to have to run at a higher temperature to get any kind of through-put.

There may be copper erosion issues and other things, of course with tin-lead that shouldn't be too bad of a problem. And then there is a trade-off between higher temperature and immersion time versus hole fill.

The advantage that I spoke of is being able to customize the process literally on a per pin basis on your board. The simple thing is that a very thin lead in a small hole can be soldered faster than a big heavy lead in a big hole.

So you are looking at your through-put and that is going to drive your temperature decisions.

It would be nice to not have to get it any hotter than a solder wave of 240-250 C, but to get reasonable timing, reasonable heating in the joint if you are doing heating by immersion, it may be desirable to take the temperature up to 260 C. I couldn't say.

It has to be driven by the machine configuration and the nature of the specific joints in the board that you are trying to solder.

You might also want to check with the applications people at the manufacturer of your particular selective machine and see what guidelines based on their experience.

Having worked for a lot of equipment companies, I have a lot of faith in most of the service people because that is what they do. They just solve people's problems.

They hear about the bad stuff and the difficulties every day. So they are usually a pretty good source of information.

Well, you have been listening to Board Talk.

Note that Board Talk has not been FDA approved and possible side effects may range from enlightenment to absolute bewilderment.

And on that note, thank you for listening to Board Talk and whatever you do, whether you're selective, wave, reflow, hand, laser soldering whatever you do.

Don't solder like my brother.

And please don't solder like my brother


Selective pots using common Pb-Free alloys (SAC305, SAC0307 etc) are typically run at higher temperatures as the mass of molten solder is minuscule compared to a full wave, therefore, thermal transfer is reduced. Increased contamination, usually copper or nickel, is common due to the higher temperature of the molten solder. This is due to increased dissolution at higher solder temperature, longer contact times and the small volume pots used in selective equipment. Some second generation Pb-Free alloys can be run at slightly lower temperatures, thus reducing some of these issues.
Timothy ONeill, AIM
Todd, I wasn't aware that the Japanese call out the lower temperature, but they are right in terms of assembly reliability. Heat is temperature plus time. A lower solder temperature requires longer dwell time in the solder. Many companies like to use a high solder temperature to achieve faster throughput. However, PCBs and components experience less thermal stress with lower temperature even if longer.
Jim Smith, Electronics Manufacturing Sciences, Inc.
We are using 280C for most of our selective solder machine programs. A few programs may run slightly hotter to attain better hole fill, but none run any cooler. Raising the solder temp can lead to more copper and/or iron dissolution into the solder pots, so testing solder chemistry on a regular basis is very important.
Terry Ruszin, Lutron Electronics
Jim has listed many key considerations for the SS equipment. I would argue that it is the product or PCB that determines the amount of heat in the solder pot. The critical SOLDERING temperature is measured in the solder fillet on the PCB, not the pot. The critical temperature for the FLUX ACTIVATIION is measure on the PCB BEFORE the solder fountain.

The only consistent temperature to use as a goal is these two locations, unfortunately the requirements for these two points will change with each PCB AND each fillet on that PCB.
Ike Sedberry, ISEDS
Don't forget to have the solder in the pot analyzed for impurities regularly. The alloys from the lead and board finishes leave remnants in the pot and performance can change over time.
Bradley J Fern, Datacard
Well done!!!!
Steven Reinstein, The Waveroomplus
We have a single pot selective using SN100C at +60'C above Liquidus. Low mass boards (1/2~1oz Cu) no pre-heat (but are indirectly heated when fluxing). However we also have high mass boards 2.5kg ea (4oz Cu). These are brought up to 70'C during the pre-heat stage, with PCB underside around 85'C. Seems to work. My general rule is to have about +20'C head room before you get a nozzle stick on any high mass object e.g. bus-bars.

However nozzle size, wave height(i.e. flow rate) and speed all have a bearing as to how much energy is available at the joint.
Robert Hills, Tait International
Thank you for the feedback on selective soldering 63-37. We have had a lot of experience with Japanese Automotive companies using SAC305 alloys and wanting to solder at 260. Many of the selective solder companies recommend 290-310. I have been surprised to see positive results at 260. It slows the process down a bit, but seems to work. I am curious what your thoughts are on this solder temp for SAC 305 and why the Japanese seem in lock step with this requirement.
Todd Gilmore, AGI Corporation

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