I forwarded the contents and Steve's images of this recent thread on to our wave solder expert, John Goulet, and he has offered his two cents worth (usually adds up to at least a dime).   

Bruce Tostevin
Benchmark Electronics
Hudson, NH



I have three Electra's and one Ultrapak.  One Electra is still using OA. We use everything from selective solder pallets to snap-on plows or snap masks. We do not see dross as exhibited by your picture when the solder pots are maintained.

The temperature profile of a board must be known to assure you not overheating the board and getting enough flux. 
The No-Clean fluxes have a narrower process window and need faster conveyor speeds with appropriate top preheat to maintain the same temperature profile.  Machines with OA flux, a good foam head, low flux air knife (barely feel it) to prevent blowing the flux off and cool pallets usually run very well when the operators clean the surface dross every 4 hours and where a full solder pot PM is done weekly.

The points in the E-mail that are important and not mentioned are:

1.The plate over the solderball heater comes in different widths. The wider the plate the less nitrogen leaks out however the less room you have for solder to return to the pot as it exits the nozzle. If the pallet plows the solder over you get an extended solder contact time and dross splashed up from areas that have been collecting dross. There are two dross collection areas.

The section just ahead of the air knife where the heavier solder drains out 5 ports, leaving behind dross along the whole interior of he A/K assembly. Then the nitrogen and Air knife (actually hot Gas Knife) will bounce off the board/pallet and distribute dross all over the hot board. The second is the nitrogen diffusers themselves.

2. Often people concentrate on the nozzles and dross around the pot but don't clean the diffusers. The porous stone like material collects dross floating on top. Every time the wave height goes down as when in Standby Mode and especially when the pumps are off during break time. The pot level rises, coating the diffusers in dross. Most should be blown off upon startup if kept relatively clean. Since you can't remove them with out cutting the ferrules off, cleaning is not easy. The caked on dross can blow onto the board.  Simple Test is to turn on the nitrogen high before running the first board then reset to your normal setting.

Shut Standby OFF so its not pulsing. With a clean solder pot the dross should be gone. If in Standby the dross comes back then the diffusers are the cause. Obviously another test is with new diffusers.

- Stainless steel diffusers worked better for us. After 6 months they still look like new.

3.  Cleanliness is key. Never let the dross get into the area around the impeller shields. The shields don't do very much when
both pumps are running the level is at the bottom of the shields. You should perform a surface dedross every 4 hours of operation. Make sure the operators are lifting the two shields and removing the black and yellow dross. You don't want this in the impeller area.

4. The oscillating chip wave on the Ultrapak's have to be remove at the end of each shift. You just can't clean them any other way. Cleaning the sides just looks good. There is lots of dross trapped underneath. The rotary chips are much better. At start-up set it to 880 RPM's for 30-40 seconds then reset to the desired setup height. This will clean most of the dross in it. 

5.  The turbulence you mentioned is normal on the contour/hybrid nozzle. Its critical not to have more lead clearance than you need for the reasons you mentioned. The spatula should slide under the pallet and the highest point/nozzle and have just a little extra for any bowing .  If you go to 10mm pallets then you'll have to go from 0.400" Lead Clearance to .450" and you may have to grind off another .128" of the solder ball heater mentioned in paragraph #1 so the solder can flow smoothly and not plow solder everywhere.

6. Pallet Design: Solder reliefs or gas grooves if using nitrogen are very helpful for even solder flow and to prevent dross from being collected and trapped in the thru-hole pockets. Solder Reliefs are grooves 1/2" wide and 0.020" deep milled, along the length of the pallet, to the very edges. They are spaced every 1.5 to 2 apart but connecting the PTH-hole sections of the pallet. It substantially reduces the plowing and squirting the solder onto your preheaters. If you lower the solder pot, increase lead clearance then you will have to increase the RPM's to reach the board in the pallet. The extra RPM leads to picking up more dross and more turbulence in the wave that should be smooth.

7.  No-Clean Flux: The machine has to be perfectly clean and when switching to No-Clean the PM's for the solder pot have to be increased.  Dross is noticeable and even when its not immediately visible you can see the drop on SIR values on SIR boards. Actual assemblies are in spec, especially if the are cleaned anyway. Cleaning is typical again to improve test probe contact and to  remove untrapped solderballs.

Hope this helps.  John G.

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