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There are many different surface finishes available today that all have their advantages and disadvantages. Below is a rough overview of some of the options and the positives and negatives of choosing them.
This is a solder process that is typically a tin-lead solder. This process is not RoHS compliant and also is not very controllable in the form of spacing, (pitch) and the height of the deposit. The positives are that it is a simple and very economical surface finish that is the oldest and or possibly the first one ever used.
As listed above it is the same process except instead of using a tin-lead solder there are other solders used that eliminate the lead, like Tin-Copper, Tin-Copper- Nickel, or Tin- Copper-Nickel Germanium. One of the negatives over standard HASL is the melting temperature is much higher, so the PCB and it’s components have to be able to withstand these elevated temperatures. The positives are that the finish is RoHS compliant and is still a relatively cheap surface finish that is widely available.
Immersion Tin, or sometimes it’s called White Tin is an Immersion process in which a thin layer of Tin is deposited onto the surface. One of the disadvantages of this process are that the Tin oxidizes fairly quickly, so shelf life is significantly reduced compared to other surface finishes. Another big disadvantage is that there have been many cases where the Tin can develop what is called “tin whiskers” in which the Tin can cause shorting between two conductors. Last major disadvantage is that the Immersion Tin does not withstand multiple rework cycles.
Some of the positives of immersion Tin is the flatness, since such a small amount is being deposited on the board there is a very tight control on flatness. This is a a very big advantage for “press fit pin” style connections. Other advantages are that it is RoHS compliant and is still not a relatively expensive process.
ENIG is probably the most popular choice for PCB’s nowadays. Some of the reasons for it’s popularity is that it has great shelf life, RoHS compliant, and is great for wire bond applications. Some of the negatives are that it is expensive, not reworkable, and can suffer from what the industry has labeled “black pad” which is a phenomenon in which the nickel oxidizes causing wire-bonding issues.
ENEPIG, or some call it “Universal Finish”, is the same process as ENIG except it has an extra step that adds a layer of palladium. The advantages over ENIG is that this palladium layer is added in order to stop the possible corrosion of the Nickel layer from the subsequent gold process. The results are a more dependable wire-bonding process since this process can guarantee no “Black Pad” issues. Another big advantage is that ENEPIG can be reworked. As for the disadvantages…ENEPIG is a more expensive process, and also can slow the “wetting” of a surface mount pad causing some minor hurdles in the assembly process.
Gold/Hard Gold is a process in which the PCB is plated with both electrolytic Nickel and Electrolytic Gold. This results in a very durable finish compared to ENIG. This process is most common for connector fingers on a card, switches, and some keyboards. Rarely is this process used for a soldering application because of it’s poor solderability and higher cost. This process can be quit expensive for a few reasons. First reason is that the amount of gold that is plated is roughly 30 micro-inches thick. The second reason for the expense is that the process is very labor intensive since the manufacturer of the PCB has to mask/tape off the entire area that does not get plated. This taping and untaping is very time consuming process.
OSP is an organic coating, (ant-tarnish) that is applied to bare copper on a PCB to ensure that the copper does not tarnish before populating the board with components. This process has been around for some time now but has never achieved great popularity. The positives is that the material is very inexpensive, RoHS compliant, simple process to apply, and is reworkable. Some of the disadvantages are that it has a short shelf life, no way of measuring thickness to ensure you have the desired amount, and is very sensitive to handling.
Immersion Silver is a direct replacement for HASL. This process was made popular by the Military for various reasons, one being that the strength of the joint is superior compared to Nickel/Gold. Silver is a RoHS compliant finish and is more economical than ENIG. Silver has a good shelf life as long as it is not exposed to acids/salts. It is imperative that anyone that is handling the parts do not do so without gloves. The biggest disadvantage of Silver is the handling, care, and storage since it is very susceptible to environmental impacts like temperature and humidity.