When ENIG Doesn’t Solder.
One of the misconceptions about Electroless Nickel Immersion Gold (ENIG) is that it remains solderable for an unlimited amount of time. This misunderstanding is based in the fact that gold is a noble metal. Noble metals are metals that are resistant to oxidation and corrosion. Hence the belief that ENIG remains solderable since gold is the metal that we see. Some people in the electronics industry are surprised when they learn that ENIG has a solderable shelf life of 6 months.
The shelf life of ENIG is documented in the IPC-4552 “Specification for Electroless Nickel/Immersion Gold (ENIG) Plating for Printed Circuit Boards”. Table 3-1 of the IPC-4552 specification states that ENIG must meet solderabiltiy requirements of J-STD-003 category 3 durability with six months shelf life.
If gold is a noble metal why is the shelf life only 6 months?
To appreciate why ENIG has a shelf life of 6 months you need to understand how the gold surface interacts with the nickel surface. The IPC-4552 specification is the industry guide for ENIG and is based on many years of evaluating and testing the ENIG surface. One deviates from IPC-4552 at their own peril. The specification outlines the basic process and identifies the acceptability criteria for ENIG on finished Printed Circuit Boards (PCB). A copy of the specification may be purchased from the IPC through their web site at www.ipc.org or here.
The electroless nickel is plated onto the copper surfaces of a PCB to a thickness range of 3 to 6 μm [118.1 to 236.2 μin] per IPC-4552. If the nickel is plated below that range the potential for solderability issues increases. If the nickel is plated above that range then the potential for fractures to form in the nickel increases. Another property of nickel is that when nickel is exposed to air it oxidises very quickly. Oxidised nickel does not solder very well. A protective layer is applied to prevent the nickel from oxidising. This protective layer is the immersion gold.
The immersion gold is plated to a thickness of 0.05 μm minimum [1.97 μin minimum] per IPC-4552. An important fact to note is that when soldering to an ENIG surface the soldering occurs between the tin within the solder and nickel layer of the ENIG surface. The immersion gold is actually absorbed into the solder joint as an impurity. The only service performed by the immersion gold is to protect the electroless nickel and prevent it from oxidizing until the ENIG is soldered to.
What happens if the immersion gold layer is below the specified amount?
If the immersion gold is not plated to the minimum thickness then the electroless nickel is not adequately protected and the nickel oxidises. It is important to point out that the gold is plated onto the nickel surface through a galvanic reaction. That is to say that in order to plate one atom of gold an atom of nickel is displaced. As the immersion gold is plated nickel is removed from the surface in exchange. This process results in the immersion gold layer to become porous as a result of the nickel being displaced through the gold layer. This is why ENIG has a shelf life of 6 months. The immersion gold layer is porous. Any chemistry or gas coming into contact with the immersion gold ultimately comes into contact with the electroless nickel. The immersion gold slows down the rate of oxidation but does not prevent it.
Can I increase the shelf life of the ENIG surface by specifying that the immersion gold be plated thicker?
No. Both the shelf life and solderability of ENIG decrease as the thickness of the immersion gold increases. To appreciate why this occurs you need to understand the composition of the electroless nickel that is plated onto the PCB. The electroless chemistry used to plate nickel onto copper contains phosphorus. Phosphorous is a necessary element that is critical for the process. With out phosphorous the electroless nickel cannot plate. The nickel plated onto the copper likewise contains phosphorous. The ratio of phosphorous to nickel is low enough so that the phosphorous is kept in check.
As stated previously the thickness for immersion gold is specified as 0.05 μm minimum [1.97 μin minimum]. As with all manufacturing operations the immersion gold is applied in a range. An upper limit of 0.127 μm [5.0 μin] should not be exceeded. Higher than that and the ratio of phosphorous to nickel increases at the boundary between the nickel and gold. A non-conformance referred to as hyper corrosion occurs which may effect solderability or even form into black pad when soldering.
Is a shelf life of 6 months all I can expect?
Proper storage and handling per IPC-1601 may extend the shelf life of the ENIG finish up to a year or more. Shelf life beyond what is specified per IPC-4552 should never be assumed. Solderability should be verified on boards pulled from storage. Select a representative sample from the boards pulled from storage. The sample should be processed through the assembly operations that the boards from storage shall be exposed to. The sample should have solder paste applied, processed through re-flow and then visually inspected. The decision to process should be based upon the observed results of the samples processed. If is better to sacrifice one or more samples for evaluation when the value added cost of components are taken into consideration.