Hot Air Knife

19 January 2011

Example of one air knife produced and supplied originally by Invicta Engineering in the UK
Example of one air knife produced and supplied originally by Invicta Engineering in the UK

There will always be designs that stretch a wave soldering system’s capability...

...And that is why, back in 1977, the Hot Air Knife (HAK) was originally designed by Hollis Engineering to remove solder shorts from closely spaced terminals.

HAK technology was (and is) a method of projecting a thin sheet of hot air, or nitrogen, approximately 0.018in, at the base of the printed board assembly as it exits the solder wave.
As the joints leave the solder wave and when they are still in a liquid state, a HAK removes the excess solder from the terminals. Hot air knives have been mainly featured on equipment offered by Electrovert and Soltec. The Soltec system, however, was a moving head as opposed to a fixed nozzle located directly behind the solder wave.

A system consists of a control unit that feeds air through to the air knife at very low flow and then at high flow when required to remove solder shorts. This reduces the drop in temperature if the system was simply switched on and off as required. The air passes through a tube and out through a series of holes into the head through a screen which produces a more even pressure as it exits the slot aperture. Thermocouples monitor the air and heater temperature which will be monitored at the control panel.

The process works due to the wetting forces which form a reliable solder fillet. At the solder to pin/pad interface the wetting forces are at their highest. As we move away from the pads the forces acting on the solder between the pins are weaker.

Remember the totally unorthodox method of removing excess solder from a board assembly when a de-soldering tool is not available. We have all done it or seen test engineers do it. Heating the solder with an iron and then quickly knocking the board on the side of a bench removes the solder short but not the solder from the joint areas. An air knife is a more elegant and non destructive technique to remove shorts.

The angle set for air contact will depend on the type of board: surface mount, plated through-hole or single-sided assembly. The angle of the air knife can range between 45-80°. All plated through-hole boards are less sensitive to set-up than single-sided boards as the volume of solder on the joint section is much less between the lead and the pad. It is slightly more difficult to set the process for single-side boards than for plated through circuits. With plated through boards the solder at the top of the hole can be seen to shimmer as the air contacts the base of the board directly after separation from the last wave.

An air knife is like any process tool - it must be correctly set for a product or optimised for a group of different boards. If it is switched on and left to operate who is to say if it is working and justifying its cost. Correct set-up for an air knife is through experience and production trials. Incorrect set-up will not remove solder shorts and it can even create soldering defects by blowing solder onto the top of the board.

During the description of the knife and its operation we have referred to it solely as an air system. It has also been used with nitrogen when associated with a nitrogen soldering process. In a sealed unit where the wave has a tunnel, the knife is also used to introduce nitrogen into the system. Only a small bleed of gas is introduced through the knife when boards are not being processed.

Hot air knifes are also associated with the solder leveling process as they are used to ‘squeegee’ off the excess solder on the surface and from through-plated holes in a PCB.

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