For best contact reliability, you should match the contact material of the SIMM
sockets on your motherboard. Mixing metal Types may lead to contact corrosion, especially
in high humidity environs. Visually inspect the sockets; if they are gold, buy SIMMs with
gold contacts. If they are tin, buy SIMMs with tin or lead contacts. However, this is not
always a critical issue, and either kind usually works. Most PentiumŪ boards have tin
contacts, and almost all SIMMs manufactured today use a tin or lead alloy instead of gold.
Gold can be deposited on the connector fingers of memory modules in two ways. Boards
manufactured using cheaper processes use immersion gold plating that gives a very thin
layer of gold over the entire surface of the board. The actual gold thickness is very
difficult to control in the immersion process. Depositing too much gold can cause the
solder joints on the board fail after becoming too brittle. Boards manufactured to the
PC100 and PC133 specifications have thicker gold plating, deposited only on the connector
fingers using an electroplated process. This process is more expensive, easier to
control and guarantees that the proper amount of gold is present on the surface of the
gold connector fingers.
- immersion plating [Metallurgy]. The process of applying a metallic
coating to a part simply by immersing it in a solution; for instance, by
electroless plating.
An immersion plating process is not the same as an electroless process. In an immersion
process you have a galvanic displacement in which a metal less noble, for example copper
or nickel, is displaced by gold. As soon as the copper or nickel surface is no longer
exposed, the process stops. A true immersion deposit is limited in thickness and typically
does not adhere extremely well to the substrate.
An electroless plating process is essentially an autocatalytic process. In a process of
this type the plating process will continue once it has started and as long as the
plating bath contains the proper components (metal ions, reducing agents, etc.). In
theory, there is no limit to the thickness of metal that can be deposited.
- Electroplating [Metallurgy]. The process of producing a metallic coating on
a surface by electrodeposition - i.e., by the action of an electric current. The principle
of electroplating is that the coating metal is deposited from an electrolyte - an aqueous
acid or alkaline solution - on to the base: i.e., the metal to be coated. The latter forms
the cathode (negative electrode). A low-voltage direct current is used; the anode is
gradually consumed.
Electroplating is normally done with direct current. However, particularly with cyanide
copper baths, improved smoothness and uniformity of the coating can be obtained by means
of the so-called periodic-reverse process, in which the polarity is periodically
reversed, so that the metal is alternatively plated and deplated.
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