Idle Wave Solder Temperature

When our wave solder system is not in use, including after the workday and over the weekend, should we maintain the wave solder pot at a specific temperature?

If we allow the solder to cool to room temperature, then reheat to soldering temperature through many cycles, does it change the characteristics of the solder or the solder properties? What is the most common practice?

P.H.

Expert Panel Responses

Our experience with both wave pots and Hot Air Solder Levelers has been that when we intend to have them idle for an extended period, such as a weekend, we allow them to cool to room temperature.

We have not experienced adverse effects from this, and have been following this practice for decades. We do regular analysis of the pot to ensure that the solder alloy is controlled.

Nick Koop
Product Manager
Minco Products Inc

Nick has been in the circuit industry since 1985, in technical and management roles. Nick is the Vice Chair of the IPC Flexible Circuits Committee, and Chair of IPC 6013 Flexible Circuits Perfomance Subcommittee. Nick holds a Bachelors Degree in Mechanical Engineering and a Masters Degree in the Management of Technology from the University of Minnesota.

There are two significant downsides to cooling the pot below the solidification temperature:

Repeated solidification and re-melting puts mechanical stresses on the immersed parts of the system, and may stress heaters due to repeated periods of 100% duty cycle on start-up.
The time required to re-heat the pot is time lost to production.

The benefit of turning pot heat off is, of course, the reduced energy used. The alternative is to cool the pot to just above the melting point. A set point 5 C above the melting point of the solder alloy in use may result in some solidification at the coolest points of the pot, but will keep the bulk of the solder liquid, easing start-up. A good rule of thumb is that if the machine will not be used for 1 week or more, turn the pot off.

For nights/weekends, keep it just above the melting point.When starting up, first bring the pot back to normal operating temperature, then start the pump and allow the pump to run for 10 minutes prior to running any production. This is necessary to ensure that the pot isre-homogenized; there will be some stratification during the cool-down period,and there may be some collection of solid crystals of IMCs (intermetallic compounds).

It takes 5 minutes or more to re-homogenize the system. At the end of the start-up process, while the pump is still running is an ideal time to pull samples for solder pot analysis, which should be done on a periodic basis. The required interval depends on your specific situation.

Fritz Byle
Process Engineer
Astronautics

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.

It depends (ka-ching) on several factors.

Some wave solder machines contain 1800 lbs of solder, whereas others may only hold 200 lbs, and the amount of energy required to keep the solder molten is directly proportional to the amount of solder in the sump.
The more molten solder exposed to air, the more dross (waste) is created overtime. If it takes an hour just to get the dross out of the sump on Monday morning, but only takes two hours to get up to temperature, then you might as well shut it off over the weekend.
What is the required build schedule? If you miss a couple of hours Monday a.m.while the machine comes up to operating temperature, will it have a significant cost/schedule impact? There may be times when it pays to shut the machine down for the weekend, other times where you need to have it up and running at 5 a.m.sharp Monday morning, come heck or high water.
Some wave solder machines have the capability to "idle down" when not running to a temperature just hot enough to keep the solder molten. Others may not have such an energy-saving option.
It also depends on how long it takes for the solder to come up to working temperature from a cold or idle ("warm") state described in number 4 above.
It usually is best to have someone come in an hour or two earlier than the rest of the assembly team just to get the wave solder machine turned on, as well as any reflow ovens, etc. on the line. If that person goes home an hour or two earlier, no extra cost is incurred. Learn to stagger start times and lunch/break times so your pick and place machines never stop moving, your conveyors never quit rolling, and the dollars never stop rolling into Accounts Receivable.

I never liked the idea of leaving 1000 lbs of hot molten metal in an empty building. I have known of chimney fires caused by the flux inside of the exhaust igniting because for whatever reason the exhaust blower shut off while the pot remained on. Most local electrical codes require wave solder machines to have the exhaust hard-wired to the machine's solder heater circuit.

If the exhaust blower shuts off, it is easy for the temperature inside the exhaust to increase due to the heat from the pot not being exhausted fast enough. If it reaches certain temperatures, and the flux residue typically lining the inside of the exhaust flue is flammable, it can ignite.

Not to many things are so bad as to come to work Monday morning, only to discover work isn't here anymore.

In answer to your second question, no degradation of the solder alloy itself occurs due to temperature cycling, as far as I know. However, over time the alloy can change due to drag-out from the thousands or even millions of component leads being soldered. Typically the tin content will drop down, and impurities such as gold, silver, nickel, copper, etc. increase.

It is a very good idea to have a sample removed from all of the solder pots in the building (wave solder,selective solder, tinning pots) and analyzed by your solder supplier to make sure it continues to be compliant to the limits listed in J-STD-006B section 3.3, either quarterly or after X number of hours of machine run time (they almost all have an hour meter inside). This should be part of the preventive maintenance plan (process control plans). You may occasionally need to add a small portion of a bar of pure tin for Sn63 alloy, which typically falls down below the limit.

Calculations based on the amount of solder in the sump and the alloy percentage readings are included in J-STD-006B to help you determine how much to add.It is important, very important, to make sure the bulk alloys you solder and tin with are within specifications, or all kinds of solder defects and reliability issues can result. The periodic analysis will help catch issues such as wrong solder alloy being dumped into the pot (lead-free instead of leaded, or vice versa), and many other little things that cause each day to be filled with joy and optimism for today's solder process engineer.

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.

I would recommend the system be shut down and turned on prior to the start of the shift. Automatic timers can be used to shut down and start up the system. The system can also include the venting system shut down and start up.

As to your question regarding changes in the metallurgy or characteristics of the solder from the system being shuAt down, is an anecdotal position and I've never found any evidence of the alloy changing from the shut down of the system.

As being responsible for 23 wave solder lines while at Digital for over 20 years, this was never an issue.I would say the savings in energy would be substantial over the period of a year from shutting down the system daily and over the weekend. Hope this answers your question.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation

At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

Reader Comment

You should also make sure that the solder pot is covered prior to shutting the heaters off. The reason for this is there may be trapped air in the solder and when being reheated the solder can have a explosion which in turn could cause burns to an employee at worst or a large solder spill at least.

David Morse, Whelen Engineering

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