Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
3.3.2 Soldering and brazing
Soldering and brazing are processes whereby the basis metals are wetted by
the filler metals, with subsequent filling of the joint gaps by capillary action.
If the process is carried out in air, oxidation of the metals will occur and,
therefore, fluxes are commonly used in both techniques to dissolve the oxide
films and to ensure wetting by the filler metal of the metals to be joined.
Soft solders are based on the Sn–Pb system. The eutectic 60% Sn–40% Pb
alloy melts at 183°C, thus enabling joints in electrical and electronic
components to be soldered with minimum danger of thermal damage to the
components. The joints may be readily reheated where necessary to disassemble
the components and resoldered, as when making repairs and alterations to an
electronic assembly.
The shear strength of a soft-soldered lap joint depends on the local state
of stress, but is usually in the range 20–40 MPa. Alloys of lower Sn content
than the eutectic are used wherever it is possible to use a higher soldering
temperature, with 50% Sn used in copper plumbing and 40% Sn in lead
plumbing, where the longer freezing range allows the solder to pass through
a ‘mushy’ stage as it cools, enabling smoothing of the joint by ‘wiping’.
Solders with tin contents between 30 and 5% (with Sb and As additions) are
used as filler metal in repairing or shaping automobile bodywork.
Lead-free solder
With the indisputable evidence of lead toxicity, the electronics industry is
embracing environmental issues that will involve the take-back of products
and the move to an appropriate end-of-life treatment for them. The reason
for concern is that a great number of products are being disposed of in
landfills and solder from some of them is leaching into municipal water
supplies. In response to the wave of regulations, the following list of desirable
attributes has been compiled for lead-free solder:
1. The selected element will have no negative environmental impact
2. Sufficient quantities of the material must be available for the foreseeable
future
3. Melting temperatures similar to 63/37 Sn/Pb, preferably below 200 °C
4. Equal or similar thermal and electrical conductivity
5. Adequate joint strength and thermal fatigue resistance
6. Easy repairability
7. Low cost
8. Compatibility with existing processes.
There are viable candidates for replacing Pb/Sn eutectic solders, usually
based on adding small quantities of a third or fourth element to binary tin-