MAINTENANCE OF WAVE SOLDERING PROCESSES

The procedures outlined on this bulletin will assist you in developing a reliable, reproducible, automated wave soldering process and maintenance procedures suited to your particular needs.  These procedures were modified from a number of documents produced by Alpha Metals who sponsored a number of wave soldering video and defect guides available on CD ROM. Further details on the video tapes and CD ROM are included on the Bob Willis Internet Home Page www.bobwillis.co.uk

FLUX POT

A flux pot usually consists of a wave, foam or spray fluxer and some auxiliary tool, such as a brush or an air knife, to assure uniform coating of the board, and to remove excess flux from the bottom of the board. The spray fluxing system has taken over from the foam fluxer as the most popular for no clean fluxes.  Regular maintenance of the flux pot is essential to achieve satisfactory results in your soldering process.  The flux pot should be level to insure uniform coverage of the board.  The height of the flux pot should be set so that the wave or foam head can easily contact the underside of the board, without excessive pump pressure in the wave unit or air pressure in the foam unit.  The flux level should be as high as possible over the fluxing stones to make the foam as consistent as possible.

Support brushes if used should be cleaned regularly by soaking them in alcohol or flux thinner.  They should be adjusted and checked regularly to insure that they uniformly contact the underside of the board, and that they wipe the board, and that they wipe the board without lifting it out of the track or pallet.  Brushes should be replaced at regular intervals. The use of brushes is less common today due to loss or lifting of components.

A foam fluxer consists of a porous ceramic or plastic stone at the bottom of the flux pot.  The stone is contained in a structure called a "chimney", which has been designed to hold and shape the foam head such that it is at the correct height for proper fluxing action.  Air is supplied to the centre of the stone from a low-pressure air source.  To prevent contamination of the flux, the air line must contain an oil/water trap and a filter for foreign particles. It may be necessary to include a chiller unit to remove water from the air line. Both trap and filter should be cleaned and/or replaced at regular intervals.

When starting up the foam fluxer, the inlet pressure and the needle or "throttle" valve must be opened so that air is introduced slowly into the flux.  The foam head should rise to the height of the board.  Improper air adjustment will destroy the fine bubble structure of the flux and increase solvent evaporation from the flux pot.  A suitable balance must be found to insure proper air flow at the minimum workable line pressure.

The specific gravity of the flux is a parameter which determines the non-volatile solids percentage in the flux.  The specific gravity should be checked periodically during an eight-hour day, and flux thinner should be added periodically, as necessary, to maintain the proper solids content should be discarded and the flux pot should be cleaned at least once every 30 to 40 hours of operation. In high volume the flux may need to be changed more frequently so rather than time it may be board number that is the reference.  A maintenance schedule suitable for each particular operation should be established to determine the proper time to change the flux.

As part of the regular maintenance cleaning of the fluxer, or prior to introducing a new flux into the system, the stone must be cleaned to remove solids which might cause the stone to clog.  The procedure for cleaning is as follows:

1 After draining the old flux from the system, refill the system with isopropyl alcohol, ethyl alcohol or the proper flux thinner.  The flux pot should be filled to at least 1 in over the top of the stone. The higher the better the performance.

2 Bubble air through the stone for 10 to 15 minutes.

3 Drain the thinner and blow thinner out of the stone.

4 Shut off the air and immediately add new flux to the system.  (The flux level should be about 1/2 in over the top of the stone.)

A full procedure for wave solder process set-up is available from the FREE process documents page on the Bob Willis Internet Web Page www.bobwillis.co.uk

CAUTION

Always keep the stone covered with flux or thinner.  DO NOT leave any portion of the stone exposed to the air for any length of time.  Such exposure may cause dry spots to form on the stone which will lead to the deterioration of the stone the plastic stone are less of a problem but this procedure should be used for all stones.

PRE HEATERS

The functions of the preheat stage of an automated soldering process are to evaporate excess solvent from the flux on the board, thermally activate the flux, and reduce heat shock.  To do this properly, the topside of a plated-through-hole board should be heated to 100-120C.  It is important that the temperature on the topside of the board be checked periodically, and the heat input of the pre heats adjusted accordingly.  Two types of pre heats are generally used in automated wave soldering.  They can be used either separately or in conjunction with each other to achieve the desired result.

Connective pre heats force a controlled quantity of hot air against the bottom of the board.  They are most effective in removing large quantities of solvent from the flux, since they force solvent vapours away from the board area. This hot air heater has become popular for VOC products in the industry due to the difficulty in drying water carriers.

Radiant pre heats are the second type.  They consist of large flat plates or Calrod units maintained at very high temperatures which heat the flux by radiant heat transfer.  Proper maintenance and cleaning of pre heats is important to avoid fires from flux drippings.  Consult the operating manual for the proper cleaning method for your system.

SOLDER POT

Temperature in the solder pot should be checked periodically, with an independent thermometer, to insure accurate measurement.  Do not rely strictly on the controls of the solder pot. When the solder temperature has risen to the desired operating temperature, turn on the solder pump and allow the alloy to mix for at least five minutes before processing boards.  Add solder to the pot to maintain the proper level of molten solder in the system.  Use only stainless steel tools to make adjustments in the solder pot (i.e., only a stainless steel skimmer should be used for removing dross and only a stainless steel ladle should be used to empty the solder pot). During operation, a solder pot will pick up metallic impurities which may affect the quality and appearance of the solder joints. Solder should be sent to an independent testing laboratory every 4 to 12 weeks depending on volume of boards processed to check the level of metallic impurities. 

If the solder is contaminated it must be removed.  Use a stainless steel ladle, as previously outlined.  Do not discard this material as it has considerable value.  If the precious-metal content of the solder is high, the scrap value may be great enough to equal or exceed the cost of a new charge of solder.  Alpha Metals offers a Reclamation Program for dross and pot metal.

After emptying the pot, do not scrape, wire-brush, or otherwise destroy the integrity of the inner surface of the pot, as this will recontaminate the new solder.

A new Wave Soldering Defect Guide is available from EPS. The interactive CD-ROM shows all the most common wave soldering defects seen in manufacture and the possible reasons for the defects occurring.

GENERAL MAINTENANCE

Correct maintenance is a major factor in obtaining a reliable reproducible, automated soldering process.  All rails, tracks, fingers and pallets should be cleaned regularly.  Pumps should be pulled and cleaned at least once each month.  Refer to the machine manufacturer's manual for complete instructions on proper cleaning and maintenance.

Speedline Technology offer training courses on wave soldering, machine maintenance and process se-up in the USA and in Europe. A list of process courses available are listed on the EPS Web Site.

There are further documents on the Bob Willis Internet Home Page to assist you during wave soldering. How about taking the wave and Reflow soldering quiz sponsored by Alpha Metals.

In house wave soldering workshop are just one of the topics covered by EPS training schedules further information is also covered on the Web Site.