The use of paste requires different techniques and general practices to those employed for other forms of brazing or soldering consumables. The basic principles always apply.
Location of paste deposits
Whilst no particular joint design is excluded from brazing or soldering with paste, the suitability of a component is dependent on there being a place to deposit the required amount of paste. Ideally the component should have a shoulder or ledge on which the paste can be adequately supported.
Components should be assembled and the paste applied as near to the mouth of the joint as possible. Avoid placing paste inside the joint as it may not burn off completely, restricting the flow of the flux and alloy.
If the component does not have a convenient shoulder or ledge then the paste should be applied above the joint. Any movement of the paste during heating can be controlled to direct the flux and alloy flow towards the mouth of the joint. In this case a flux binder system with minimal slump characteristics is recommended.
Dispensing of paste onto components
A major advantage of using a paste is that it can be dispensed in accurate and controlled volumes, ensuring that a particular joint receives the same amount of paste on each application.
Paste is often supplied in cartridges, which can be fitted to accurate air operated dispensing machines. Where larger volumes of paste are required a reservoir connected to a paste gun can be used. Paste can be supplied in hand cartridges for manual dispensing although this reduces the close control over paste volumes achieved with even the most basic dispenser.
Contact us about the most suitable method for dispensing paste onto a particular component; we can then formulate the paste accordingly.
The size and shape of paste deposits
The shape and design of the component govern the size and placement of the paste deposit. It is often possible to calculate an approximate amount of paste required per joint. However, in most cases a simple production test will quickly establish the correct amount to use.
If possible, paste should be applied as a single deposit. This method needs the minimum of dispensing equipment and the least time spent on the dispensing operation.
On long joints, it may be necessary to apply more than one paste deposit. This increases the area covered by flux and alloy and is advisable particularly if the alloy is not free flowing.
Paste can be applied as a number of dots, or as a continuous stripe. In applications such as this, movement of either the dispensing nozzle or the component itself may be required during paste application.
Heating methods for brazing and soldering paste
Our paste products can be used successfully with most of the standard heating methods.
- Fixed burners – accurate and reproducible heat patterns can be set electronically using a range of fuels. They may be arranged as single or double burners on basic shuttle or rotary indexing machines.
- Furnace heating - uses a reducing atmosphere or vacuum to prevent or break up surface oxides during the brazing process.
- Induction heating - quick and easily controlled heating well suited to paste products.
- Resistance heating – works well with brazing and soldering pastes. Resistance heating with a separate filler metal and flux can cause problems because the flux acts as an insulator. Using paste will help to overcome this as the finely divided powder offers a conductive path through the flux.
- Hand torch - pastes should be heated indirectly to ensure the paste deposit reaches the correct temperature at the same rate as the rest of the component. If heated directly, the flux in the paste will not have sufficient time to clean the parent metals and the molten filler metal alloy will not wet out onto the joint area.
Heating brazing paste
The behaviour and flow characteristics of a brazing paste when heated will depend upon its formulation, whether it contains flux and the heating method being used. A typical fluxed paste for brazing in air with a silver brazing filler metal based around the 'B1' binder system will go through the following stages:
- On initial heating, the paste deposit will increase in size. During this early stage of heating the paste deposit should be heated slowly and indirectly to cause it to 'set'. Heating it too rapidly or directly can cause it to 'explode' or spit.
- With further heating, the paste will either start to smoke, or if a flame is present ignite. Where ignition of the paste does not occur, for example when using RF induction or resistance heating, local exhaust ventilation should be used to remove the fume from the working environment.
- As the heating continues the paste's appearance will change from a shiny, glossy deposit, to a dull, dry one. The binder has now been lost from the paste which has become 'set'. Once 'set' it is stable and can be subjected to a more rapid rate of heating.
- Next the flux starts to melt, 'wetting' the parent metals and reducing surface oxides on them. Initially this takes place locally around the base of the paste deposit. As the temperature continues to increase the flux becomes more fluid, spreading out and flowing into the capillary gap present within the joint.
- Finally, as more heat is applied the filler metal will begin to melt and then flow completely. The rate of heating at this stage needs to be sufficient to prevent liquidation of the filler metal. This is where the filler metal only partially melts leaving behind a skull of solid material and resulting in a joint that is only partially filled.
Both the binder and the flux in a paste can be changed or modified to match the needs of a particular brazing job. For example, binder systems can be supplied that collapse and spread onto a component where paste is needed over a large area.
Removal of brazing paste residues after heating
The flux residues of brazing pastes are corrosive and therefore their removal after brazing is essential. They are similar to those generated in other brazing operations and may be removed by the same methods - soaking in hot water (> 40oC for 30 minutes), soaking in 10% sulphuric acid or by mechanical removal (e.g. grit blasting). The method should depend on the type of flux present. Brazing pastes often leave a 'footprint' or mark on the component that is difficult to remove after brazing. For advice on the best method of removing the flux residues please contact us.
Heating soldering paste
Solder pastes are formulated around totally different binder systems and flux types to those used for brazing and they therefore react completely differently when heated.
Solder paste when heated initially will exhibit some slump depending upon its formulation. With further heating the deposit may bubble and heave (foam) and begin to smoke. Local exhaust ventilation should be used to remove the fumes. If heated too rapidly the paste deposit can boil and spit. Gentle, indirect heating (such as hot air heating) is recommended throughout the soldering process.
The solder paste becomes more fluid as the temperature is increased but it should remain colourless throughout the soldering operation. If the flux starts to go brown it is an indication that either it is beginning to become exhausted or that the paste has been overheated.
As the solder alloy within the paste begins to melt, the flux is displaced from the joint and floats on top of the molten solder. Fluxes and solders will always flow to the hottest point of a joint and further flow of the solder into, along or around the joint can be encouraged by creating a thermal gradient across it.
If the solder fails to wet and flow as required or forms a molten 'ball' then this is an indication that the flux has not been able to remove the oxides present on the surface of the parent materials.
This could be due to exposing the paste deposit to too high a temperature or exhausting the flux before the filler metal becomes molten. Alternatively, an inappropriate fluxed binder system was selected for the application.
Removal of soldering paste residues after heating
Flux residues from soldering paste are classified as non-corrosive, intermediate or corrosive. Non-corrosive residues may be left on the completed joint. Intermediate or corrosive flux residues should be fully removed. Warm water, mildly alkaline solution, or in the case of rosin based flux residues an organic solvent, should be used. For advice on the best method of removing the flux residues please contact us.
Storing and shelf life
Brazing and soft soldering pastes have limited shelf life and strict stock rotation should be exercised during their storage. The binder system and the filler metal powder can separate and the paste may dry out during storage. Separation or drying out of a paste is more likely if the product is not stored correctly.
Braze and soft solder pastes should not be stored below 5°C or above 25°C. The ideal storage temperature for pastes is between 10 and 15°C. The viscosity of brazing and soft solder pastes is affected by temperature. They are designed for use between 18 and 24°C (Viscosity measurements are taken during production at 20°C). If subjected to temperatures below 15°C the paste viscosity will increase making dispensing more difficult. If stored at temperatures above 25°C the paste will become very fluid and may separate and dry out. It is therefore recommended that pastes should be withdrawn from the storage facility some time before use and placed in the workshop environment to obtain a normal working temperature. Storing at temperatures below 5°C can result in damaging the products irreparably.
- Paste should ideally be stored in cool dry conditions away from direct sunlight and other sources of heat.
- Pastes supplied in buckets or pots should be stored in closed containers and stood with the lid upper most.
- Pastes supplied in machine cartridges or hand syringes should be stored standing up in racks with the 'follower plug' upper most and the nozzle facing down. Storing cartridges / syringes on their side can result in separation of the paste.