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October 12, 2011

Components Jumping Out During Wave Soldering

We recently started having a sporadic problem.

When components reach the wave solder pot, the components jump out of their holes. The problem doesn't appear to be caused by vibration, and the wave machine fingers have been checked.

Any ideas of what might cause this?

C. C.

Experts Comments

It is not uncommon for through-hole parts to rise up out of their holes when coming into contact with the molten wave of solder in a wave solder machine or selective solder machine. There are a number of reasons for this, but the main (root cause) reason is simply oxidized component leads that resist wetting in the molten solder.

If solderability is poor, the leads resist immersion into the molten solder, and if the resistance is high enough, even components that have a fairly large body mass will simply "float" up out of the plated-through holes due to the lack of wetting tension from the solder.

This can happen on components that have a lead-free finish more so than a tin-lead or gold finish. The reason is that lead minimizes oxidation somewhat, and gold oxidizes very little. Lead-free finishes that consist of SAC305 or similar alloy (nearly all tin) on the component terminations can oxidize very quickly (read: shelf life of less than 1 year).

The original RoHS requirements went into effect in 2006. It is now 5 years later. With the number of older through-hole components aging, the phenomena of components not wanting to wet is also increasing.This is not limited to through-hole parts, it is also true of SMT parts as well.

There are other factors that can cause components to "float", but these are usually secondary causes. They include the aspect ratio of the hole diameter to lead diameter, mass or weight of the component body, lack of flux to promote good wetting (flux helps overcome the surface tension of the solder), and other causes.

Keep in mind that SN100C solder has greater surface tension than 63/37, also.

Some suggestions:

  1. Identify the component type that tends to float the most. Usually it is smaller glass-bodied diodes, smaller axial resistors and capacitors, radial leaded SIP packages, etc.
  2. Tin or re-tin those component leads prior to assembly. Even if you simply re-tin, the fresh solder may be all that is needed.
  3. Clinch the leads and trim to the edge of the pad. Yes, I know additional labor is involved, but if it is just one or two part numbers, put them in first and bend the leads on the back side and trim, then stuff the rest of the parts into their holes. This little bit of extra labor is usually cheaper than reworking them later.
  4. Be sure you are getting plenty of flux INTO THE HOLES of the CCA just prior to wave or selective soldering. This means having a good fluxer, not just one that vaporizes flux onto the bottom of the PWB, but preferably a good foam fluxer or a SPRAY fluxer that is better than ultrasonic. The key here is to get a good flux coating inside the plated through holes. Flux reduces the surface tension and promotes wetting, thus it helps reduce the number of components floating.
  5. You may wish to evaluate a different flux, but be sure to fully qualify it before using it on production CCAs.
  6. Make sure the operators are not touching the through-hole component leads when placing the parts into the CCA. Simple transfer of finger oils is often enough to reduce wettability such that the parts will float.
  7. If the lead diameter to hole diameter aspect ratio is too high, ie, the plated through holes are much larger than the lead diameter, it is easier for the parts to float. You may need to consider changing the finished hole diameter on the problem components. Keep in mind that other parts on your list of approved component vendors may include parts with a larger diameter lead. Be sure to check this before making any changes.

You can test for solderability using a meniscus wetting tester, but wetting tests on individual components do not always correlate well with the components that float during wave or display poor wetting. Passing the wetting test does not always mean they will solder up fine later.

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 suspect it is probably a combination of things. Until the solder wets the leads the component has a degree of intrinsic buoyance because of the density difference between the component and the solder.

If the leads are slow to wet the this will tend to cause the components to rise up but it is unlikely that this would cause the component to "jump out".

If there is thin air trapped under the component this will expand rapidly with the heat of the wave further lifting the component, this expansion can be rapid. The combination of the two effects slow lead wetting and rapid expansion of trapped air could conceivably cause a very light to "jump out".

Neil Poole
Senior Applications Chemist
Henkel Electronics
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.

One possible problem could cause this:

Leads are longer than past. Could be a set up problem with their insertion tool and the trim set up of the leads after placement. They should check their mechanical set up of the insertion system.

Gregory Arslanian
Global Segment Manager
Air Products & Chemicals, Inc.
Mr. Arslanian has been involved in electronics packaging processing and equipment since 1981 including flipchip, TAB, wirebonding and die attach. Current responsiblities include R&D, applications, marketing and customer interaction.
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