Wave Soldering Problems

In our circuit assembly production line, we are seeing many process defects that occur after wave soldering operations. The defects include; insufficient fill of PTH barrel and large voids within plated-through holes. We cannot determine what causes of these defects and don't know how to prevent them. Can you provide suggestions for avoiding these conditions and recommend corrective actions related to these kinds of defects?

E.W.

Expert Panel Responses

Insufficient hole filling and why? There are many many reasons for this and I would not know where to begin, but a more information would have been useful, such as board thickness, finished hole size, board plating material, flux type and flux application method and finally the type of solder being used in the wave solder machine, is it lead-free or leaded solder? Not knowing any of these items, check the following:

Flux application and its penetration through the board,
Top side board temperature going into the wave or coming off the preheaters, is it high enough to allow the molten solder to rise through the board.
Depth of the board in the wave, try running the board at least 1/2 of the board thickness in the wave to use the hydrostatic pressure of the wave to push the solder up into the plated through hole.

The second item is voids in the solder joint. This can also be caused by a variety of reasons and the primary one which comes to mind is bad plated through hole barrels. Check the plating in the barrels to make sure there are no voids. See IPC-600 for the criteria. Voids will have a tendency to create outgassing and this creates voids in the solder joints. The other issue which could be causing this is if VOC free fluxes are used and the application is putting on too much flux and it is not drying off during the preheat stage of the wave solder system. As this product hits the wave solder system, the liquid will volatize and this will also create voids and blow holes in the solder joints. I hope this helps, if more information is needed please get in touch with me.

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.

There are several things that could cause insufficient fill of the PTH.

Insufficient/ incomplete flux coverage. Check your flux application to be sure that there is sufficient volume and area coverage over the complete PCB.
Insufficient pre-heat. Solder flows to the heat. If the PTH's have not come to temperature, the solder will not flow as well. Exact temperatures depends on your operation. As a rule of thumb, the assembly should reach approximately 150C to 170C just before contacting the wave. If a profile of the wave process is taken, it should look very similar to a reflow profile of the same assembly.
Contamination. Probably the most common cause of insufficient solder coverage is contamination. Verify that the boards have been handled properly before the assembly process and during the process. Any oxidation, oils, foreign materials...etc... will cause soldering problems. Hand clean several boards and then put them through your assembly process. Make sure the circuit boards are out of the sealed packages for a minimal amount of time before they are processed.
Wave height. Capillary action accounts for some of the solder flow through the PTH. Another part of proper fill is wave height. The wave should be sufficient to "push" the solder up through the PTH without over flowing the PCB.

Kris Roberson
Manager of Assembly Technology
IPC

Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

Lack of Dedrossing can be a key factor in the type of defects you describe. Dross is formed on the surface of the solder pot as it comes in contact in the air that we breathe. Similar to the tarnish on sterling silver turning dark, only this happens in a matter of hours in a hot solder pot. Performing dedrossing multiplet imes per 8 hour shift may reduce or eliminate the problem you describe. Visually looking at the surface of the solder pot you should see a clean shiny surface, if you see clumps of dull silver or particles floating you need to dedross. If the dross gets on to the solder joints you are likely to have failures.

Ken Bliss
President & CEO - Retired
Bliss Industries, Inc.

Retired - Mr. Bliss has 20+ years experience creating process methods that improve profitability by maximizing hidden unused capacity and throughput. Ken has expertise in all areas of manufacturing specializing in electronics assembly.

Voids and insufficient top side fillet happened in PTH due to one or more of the following causes: Materials:

Improper curing or plating process of the PWB (which causes out gassing),
Poor solderability of the pcb and/or the thru hole components (could be due to oxidation),

Machine set up:

Insufficient flux penetration from the spray fluxer,
Insufficient preheating, esp on the pcb top side (if there is heat sink on the PCB topside)
Insufficient wave contact time (2 to 4 secs recommended)
Insufficient waves heights (for chip wave and main wave), (waves should meet half the PCB thickness)

Our ECD WaveRIDER and Fluxometer will be able to check and monitor all the soldering parameters in the wave soldering machine, and to narrow down the possible root cause(s) of the problem. Thus, if the machine setup is good, then you can just look at your incoming raw materials quality, and their storage process.

EH Lim
Managing Director, Asia Pacific
ECD

EH Lim has been in the PCB Assy industry since 1985, starting at Thomson/Singapore for 5 years before moving to Electrovert Asia Pacifc. Lim was Sales Director for Vitronics Soltec prior to joining ECD in 2007 as Managing Director for Asia Pacific.

It appears that there is probably not enough flux that is wetting the entire barrel including the top side. Better preheat may help reduce the voids that are forming. Also, if a flux with increased activity level is used, it could improve the capillary flow and wetting.

Bjorn Dahle
President
inspīre solutions LLC

Bjorn Dahle is the President of inspīre solutions LLC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.

Poor topside fillet can be caused by many points. I suggest the following:

If using spray fluxer the flux has to penetrate through the barrel. If using alcohol based flux, thermal fax paper can be place on top of a board and it will give you a visual that flux has penetrated the hole.
Assembly temp at exit of preheat section. Topside laminate should be 200 deg F - 240 Deg F just prior to entering the wave.
Dwell time in the solder ( lead based ) average 3-6 seconds for an .062" assembly and solder pot temp at 480-500 deg F. If lead free 8-10 seconds dwell time at 510 - 525 Deg F solder temp.
Typical conveyor speed lead based for average wave will be 4-6 FPM and 2-3 FPM for lead free alloy

John Norton
Eastern Manager
Vitronics Soltec

John Norton started his soldering career in 1983 for Hollis Engineering. He has also worked with Electrovert as a technical training manager and Vitronics Soltec for the last ten years. He has held various technical development and sales positions.

Reader Comment

If you have already verified the WS parameters and setup and the materials used in the process are OK, you might want to take a look at the board design. There are some design constrains that make it difficult to achieve a good barrel fill:

SMT components on the bottom side located near TH pads
TH pins connecting to multiple inner layers, i.e: GND pins
Drill diameter is less than required

Any of these issues or worse, two or three issues combined on the same board will give you a hard time filling up the TH barrels.

Guillermo Flores, Jabil Circuit, Mexico

Reader Comment

I am surprised no one mentioned baking theboards to expel moisture trapped in the PCB. We have had great success eliminating voids at our LEAD FREE WAVE SOLDERby pre-baking boards and storing them in vacuum sealed bags. Keep in mind thatonce a board is baked, if left out in the open air, it will reabsorb 75% of themoisture you just baked out of it within 48 hours. Once you bake the boards, you must build themthru the WAVE SOLDER process. Don't letbaked boards sit out all weekend in open air. They will absorb moisture andcome MONDAY you will see voids all over again!

Jon Fischer, Genesis Mfg, USA

Reader Comment

One problem we encountered was caused by the PWB fabricator. Solder mask was getting into the thruholes and preventing the solder from wetting up to the top of the hole. We found small amounts of what looked like uncured solder masking at the top of thruhole solder plugs. We analyzed it and sure enough it was uncured masking. We were fortunate enough to be able to slow down the PWB thru the wave bath and raise the wave pump to push solder up the thruholes. The solder mask was not cured in the holes, if it was, it would have been impossible to "wash" it out. Point is, verify the thruholes are clean.

Jerry Wiatrowski, General Dynamics

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