Solder



A solder is a fusible metal F), used in a process called soldering where it is melted to join metallic surfaces. It is especially useful in the fields of electronics and plumbing.

The word solder comes from the Middle English word soudur, via Old French solduree and soulder, from the Latin solidare, meaning '‘to make solid’'. In North American countries the word solder is pronounced with a silent l. Most other countries pronounce the l.

 

Lead solder

eutectic mixture, which means:

  1. It has the lowest melting point (183 °C or 361.4 °F) of all the tin/lead alloys; and
  2. The melting point is truly a point — not a range.

At a eutectic composition, the liquid solder solidifies as a eutectic, which consists of fine grains of nearly pure lead and nearly pure tin phases, but in no way is it an intermetallic, since there are no tin/lead intermetallics, as can be seen from a tin/lead equilibrium diagram. [1]

In plumbing, a higher proportion of lead was used. This had the advantage of making the alloy solidify more slowly, so that it could be wiped over the joint to ensure watertightness. Although lead water pipes were displaced by copper when the significance of silver often added, and the proportion of tin was increased (see Lead-free solder below).

Hard solder

As used for brazing, is generally a copper/zinc or copper/silver alloy, and melts at higher temperatures.

In silversmithing or jewelry making, special hard solders are used that will pass assay. They contain a high proportion of the metal being soldered and lead is not used in these alloys. These solders also come in a variety of hardnesses, known as 'enamelling', 'hard', 'medium' and 'easy'. Enamelling solder has a high melting point, close to that of the material itself, to prevent the joint desoldering during firing in the enamelling process. The remaining solder types are used in decreasing order of hardness during the process of making an item, to prevent a previously soldered seam or joint desoldering while soldering a new joint. Easy solder is also often used for repair work for the same reason. Flux or rouge is also used to prevent joints desoldering.

Flux core solder

  Solder often comes pre-mixed with, or is used with, deionised water and detergent, instead of hydrocarbon solvents.

Lead-free solder

  According to the European Union Tin Whiskers" are another problem with many lead-free solders, where slender crystals of tin slowly grow out of the solder joint. These whiskers can bridge a short circuit years after a device's manufacture.

  • SnAgCu solders are used by two thirds of Japanese manufacturers for reflow and wave soldering, and by about ¾ companies for hand soldering.
    • SnAg3.0Cu0.5, tin with 3% silver and 0.5% copper, has a melting point of 217 to 220 °C and is predominantly used in Japan. It is the JEITA recommended alloy for wave and reflow soldering, with alternatives SnCu for wave and SnAg and SnZnBi for reflow soldering.
    • SnAg3.5Cu0.7 is another commonly used alloy, with melting point of 217-218 °C.
    • SnAg3.5Cu0.9, with melting point of 217 °C, is determined by NIST to be truly eutectic.
    • SnAg3.8Cu0.7, with melting point 217-218 °C, is preferred by the European IDEALS consortium for reflow soldering.
    • SnAg3.8Cu0.7Sb0.25 is preferred by the European IDEALS consortium for wave soldering.
    • SnAg3.9Cu0.6, with melting point 217-223 °C, is recommended by the US NEMI consortium for reflow soldering.
  • SnCu0.7, with melting point of 227 °C, is a cheap alternative for wave soldering, recommended by the US NEMI consortium.
  • SnZn9, with melting point of 199 °C, is a cheaper alloy but is prone to corrosion and oxidation.
  • SnZn8Bi3, with melting point of 191-198 °C, is also prone to corrosion and oxidation due to its zinc content.
  • SnSb5, tin with 5% of antimony, is the US plumbing industry standard. Its melting point is 232-240 °C. It displays good resistance to thermal fatigue and good shear strength.
  • SnAg2.5Cu0.8Sb0.5 melts at 217-225 °C and is patented by AIM alliance.
  • SnIn8.0Ag3.5Bi0.5 melts at 197 to 208 °C and is patented by Matsushita/Panasonic.
  • SnBi57Ag1 melts at 137-139 °C and is patented by Motorola.
  • SnBi58 melts at 138 °C.
  • SnIn52 melts at 118 °C and is suitable for the cases where low-temperature soldering is needed.

Different elements serve different roles in the solder alloy:

  • ductility than lead. In absence of lead, it improves resistance to fatigue from thermal cycles.
  • wetting properties of the molten solder. It also slows down the rate of dissolution of copper from the board and part leads in the liquid solder.
  • Bismuth significantly lowers the melting point and improves wettability. In presence of lead and tin, bismuth forms crystals of Sn16Pb32Bi52 with melting point of only 95 °C, which diffuses along the grain boundaries and may cause a joint failure at relatively low temperatures. A lead-contaminated high-power part can therefore desolder under load when soldered with a bismuth-containing solder.
  • Indium lowers the melting point and improves ductility. In presence of lead it forms a ternary compound that undergoes phase change at 114 °C.
  • Zinc lowers the melting point and is low-cost. However it is highly susceptible to corrosion and oxidation in air, therefore zinc-containing alloys are unsuitable for some purposes, e.g. wave soldering, and zinc-containing solder pastes have shorter shelf life than zinc-free ones.
  • Antimony is added to increase strength without affecting wettability.


See also

  • Solder paste
  • Soldering gun
  • Soldering iron
  • Solder sucker
  • Solderability
  • Welding
  • Soldering
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Solder". A list of authors is available in Wikipedia.