These alloys have exceptional melting and flow qualities and are used in electronic and spacecraft applications. Cold solder, used for filling cracks in metals, may be a mixture of a metal powder in a pyroxylin cement with or without a mineral filler, but the strong cold solders are made with synthetic resins, usually epoxies, cured with catalysts, and with no solvents to cause shrinkage.
Epoxyn solder is aluminum powder in an epoxy resin in the form of a putty for filling cracks or holes in sheet metal. It cures with a catalyst. The metal-epoxy mixtures give a shrinkage of less than 0. In spite of the sustained efforts of researchers and technology leaders in packaging, to date there is no lead-free solder alloy that is a drop-in replacement for tin-lead solder in assembly processes.
On the other hand, optimism that adhesive-type solders will prove to be a serious alternative to metallurgical materials continues to grow.
These polymer-based conductive adhe-sives are being used in various applications previously "reserved" for tin-lead solders. Regular production equipment and traditional assembly processes are producing high-quality assemblies with demonstrated long-term reliability using the new solders.
And for some products, polymers are considered an enabling technology. One major market is the polyester-based flexible circuit market, particularly those built using polymer thick film. Polymer solders are also known as "conductive adhesives," or materials that provide the dual functions of electrical connection and mechanical bond.
The adhesive components of a typical material are some form of polymer, i. Already used extensively as electrical insulators, as solders their necessary conductivity is accomplished by adding highly conductive fillers to the polymer binders.
The most common polymer solders are silver-filled thermosetting epoxies supplied as one-part thixotropic pastes. Silver is used not only because it is usually cost-effective, but also for its unique conductive oxide. A blend of silver powder and flakes achieves high conductivity while maintaining good printability.
Because the mechanical strength of the joint is provided by the polymer, the challenge in a formulation is to use the maximum metal loading without sacrificing the required strength. Polymer solders do not typically form metallurgical interfaces in the usual sense. Electrical integrity requires that the metal filler particles be in close contact to form a conductive path between the circuit trace and the component lead. Ideally, the silver flakes will overlap and smaller particles will fill in the gaps to form a conductive chain.
In the past, polymer solders were successful only on circuits using precious-metal conductors. This was because junction resistance was seen to increase to unacceptable levels when ordinary printed circuit boards and components were joined with these materials. Tin whiskers can be mitigated by using newer annealing techniques, incorporating additives such as nickel, and using conformal coatings. Lead free solders generally have a higher melting point than conventional solder.
The flux is released during soldering and reduces reverses oxidation of metal at the point of contact to give you a cleaner electrical connection. It also improves the wetting properties of the solder. In electronics, flux is usually rosin. Acid cores are for metal mending and plumbing, and should not be used on electronics. Remember the three main materials found in solder: lead, tin, and flux?
There are countless varieties of solder available on the market based on the relative ratios of these materials. To complicate matters further, there are also additives and other metals that may be added to give solder certain properties or enhance its conductivity.
Here are just a handful of examples of alloy additives and what they do:. Antimony increases mechanical strength without reducing wettability while preventing tin pest. Bismuth significantly lowers the melting point and improves wettability. Inhibits growth of tin whiskers. Indium lower the melting point, improves ductility, and is used for soldering to gold or for cryogenic applications due to its high resistance to temperature swings. Indium alloys are expensive and prone to corrosion.
Nickel in solder alloy can protect UBM under bump metallization layer from dissolution. Silver provides mechanical strength, but with lower ductility than lead. It can improve resistance to fatigue from thermal cycles in lead-free solders. The wide variety of solder blends can help you find the right combination of properties for your electronic device.
Often it is the specific combination of elements in a solder blend that can be used to get a certain result. In70Pb30 is compatible with gold contacts low gold leaching and has high resistance against fatigue under thermal cycling. If it is used for normal soldering then the thin solder can be slightly less convenient because longer lengths are used making a normal joint.
With the drive to reduce the amount of lead being used for environmental and health reasons, lead free solder is now being widely used. A European Directive mandates that solder containing lead shall not be used commercially. This means that for any soldering to be undertaken by the hobbyist the traditional solder containing lead will not be available.
The traditional tin lead solder is being replaced by other types of lead free solder. A variety are appearing on the market. One type contains This has a very similar melting point to tin lead solder and melts at around C. Another type is being sold and contains a small amount of silver. Although slightly more expensive than the tin copper solder it has a lower melting point at around C. This second type of solder is sometimes sold as "Lead free silver solder". However it should be remembered that it only has a small amount of silver, and there are other silver solders on the market.
These solders are sold in reels or in dispensers in the same way that the traditional tin lead solder was sold. In use these new lead free solders perform in a very similar fashion to that of the traditional solder. Although the melting point is slightly higher, this is not normally noticeable when used with ordinary soldering irons. There are also not believed to be any other differences in the method of use.
It should therefore not present a problem to the home constructor or hobbyist. When starting to build electronic circuits it is essential to choose a solder suitable for the job, and not a type of solder used for plumbing, etc. With the new directives on the use of lead free solder, these new forms of solder are now widely available, and should obviously be used if possible.
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