Technical: Vacuum soldering with H.F. heating


Vacuum soldering with H.F. heating

In 1976, there was a need for better soldering methods. Conventional soldering no longer met the increasingly high requirements in terms of connections of different materials, the cleanliness of the connections, the inability to rework and the vacuum tightness. It is very important that contamination of the workpiece be soldered and the soldering surfaces is prevented.

One method to achieve these requirements is vacuum soldering: the soldering takes place in a vacuum environment. This requires a vacuum pump installation. The vacuum in this method prevents attack on the solder and the workpiece by oxygen and other gases. It also serves to remove the oxides from the metals to be joined.

The workpiece must be preheated several times under vacuum to a temperature lower than the solder melting temperature before soldering can begin. The removal of the oxides of the various metals takes place under different conditions. At R.V.S. it already happens at a vacuum of 10-4 Torr, with aluminium at 10-6  Torr and with copper, the effect can already be achieved if the workpiece is heated and soldered in a shielding gas (Argon).

A major advantage of this soldering method is that, after sufficient cooling under vacuum, the workpiece is just as smooth and bare as before soldering. There is also no problem with gas or flux inclusions because these are not used.
Before starting this part of the soldering process, the workpiece must be prepared. Important is:

  1. Choosing the correct shape of the solder joint to be made (fig. 1 and 2).
  2. Degreasing the metals to prevent unwanted gas formation. The best result is obtained by cleaning in a solution of 50% alcohol and 50% acetone.
  3. Application of the correct gap width (0.01 mm) and the required amount of solder of the required composition (no flux).
  4. Setting up the workpiece in the coil of the HF generator under the vacuum bell.

Fig 1: Before and after soldering
Fig 1: Before and after soldering

Fig 2: Before and after soldering
Fig 2: Before and after soldering

The first soldering test was chosen for the workpiece Fig. 3 made of stainless steel 316. In the first phase solder seam A and the second phase solder seams B and C were soldered with gold-copper solder (80-20) at a temperature of 890 oC and a pressure of 10-4 Torr. The result was very good and completely vacuum-tight after testing; see photos below.

Fig 3: Design of first test piece
Fig 3: Design of first test

The solder for high vacuum soldering should not contain zinc or cadmium due to the high vapour pressure, which in vacuum causes these elements to evaporate from the solder composition and contaminate the bell. This also applies to the workpiece. For each type of metal, one must select a solder that wets well. The solder is therefore usually on a gold-copper base or silver-copper base. Information about this can be found in e.g. ”Technik die Verbindet” by Degussa No. 19. This contains a table of various solder compositions, their melting points and for which metals to be joined they are most suitable.

Soldering problems that occur with a complicated workpiece must be solved experimentally.

For example:

Heating difficulties are examined by heating the work without solder in the vacuum bell to see if all parts are heated sufficiently. The temperature can be easily estimated by the starting colour. The heating is maximum when the H.F. coil is as close as possible to the workpiece so that maximum coupling is achieved. The workpiece in the bell is heated using an exchangeable high-frequency coil, which is connected to the terminals of the RF generator via a coaxial passage in the vacuum bell bottom. The connections should preferably be strip-shaped and as short as possible so that the fewest high-frequency losses occur. This specialized and time-consuming soldering method is only necessary if high demands are placed on the workpiece, such as vacuum tightness and/or a clean solder seam.

H.F. heating

High-frequency heating is a heating method in which electrical high-frequency energy is converted into heat. For this purpose, the high-frequency energy (e.g. 4MHz – max. 6 kW) from an HF generator is sent into a water-cooled coil.
When placing an electrical conductor (workpiece) in the field of the coil, local heating will occur in the workpiece due to inductive coupling.
This method of local heating, which can be easily controlled in temperature, can be used for many applications. Some practical applications are: – hardening – annealing – cementing – soldering.

The vacuum pump installation

When putting the vacuum pump installation into use, see Fig. 4, open tap I and close tap II and the butterfly valve, so that the diffusion pump is sucked into its working vacuum by the pre-vacuum pump. The heating of the DIFF pump can then be switched on, the heating time being approximately 1/2 hour. If the diffusion pump is at pre-vacuum, valve I is closed and valve II is opened so that the bell is sucked in pre-vacuum (the butterfly valve remains closed).
Once this has also happened, valve II is closed and valve III and the butterfly valve are opened, after which the diffusion pump, in series with the pre-pump, sets the clock to the desired vacuum (max. 10-8 Torr).

Fig 4: The acquired vacuum pump installation
Fig 4: The acquired vacuum pump installation


Fig 5: The diffusion pump
Fig 5: The diffusion pump

The diffusion pump already works at a pre-vacuum of 10-1 Torr. Higher pressure increases the risk of the diffusion oil burning. In the diffusion pump, see Fig. 5, the propellant vapour jet (7) flows at supersonic speed from an annular baffle system (A-D) and expands in the form of a screen on the wall (the oil is condensed on the water-cooled wall (3). Due to the expansion, the vapour pressure in the jet is relatively low. The diffusion of the air or the gases to be pumped away in the jet is therefore so rapid that, despite its high speed, the jet is almost completely saturated with air or gas.

The propellant is evaporated by electrical heating. If the condensed propellant returns, it must be cleared of air and gas before it is taken into the reservoir, otherwise, the propellant will become too contaminated. Under the last shot, the oil has cooled to approximately 130 °C, causing the air and gaseous particles to escape from the propellant and be extracted by the vacuum pump. The gas pressure there has been increased to above 10-2 Torr.

Photo 1: Vacuum pump with the HF-generator
Photo 1: Vacuum pump with the HF generator


Photo 2: The soldering line-up
Photo 2: The soldering line-up


Photo 3: Placement of the vacuum bell
Photo 3: Placement of the vacuum bell


Photo 4: The end result
Photo 4: The result
  • An internal report from October 1976