Surfaces
The surface chem. Ni/Au = Electroless Nickel(-Phosphorus)/Immersion Gold = ENIG = Sudgold = Flash-Gold is well suited for both aluminum wire bonding and multiple soldering and is therefore the ideal surface for combining both joining techniques. It consists of a 4-6 µm thick nickel layer which, for process-related reasons, usually contains 6-8 % phosphorus. For tarnish protection, the nickel layer is coated with 70 nm – 120 nm gold. The storage time is more than 12 months.
The gold layer dissolves completely in the tin during soldering. The actual solder connection is then made with an intermetallic Ni/Sn phase between the solder and the nickel layer.
Press-fit technology is not primarily possible because of the brittle nature of the nickel. Some connector manufacturers offer press-fit pins with special press-fit zones, which are also said to be suitable for chem. Ni/Au.
Gold surfaces on flexible conductor plates must not extend over longer distances (e.g. over the entire surface or on conductors), since the brittle nickel can break during handling and this can lead to conductor interruptions. Furthermore, the transition area from a gold-plated pad to the covered conductor is a mechanical weak point that readily serves as a predetermined breaking point during handling.
More Gold Surfaces
For gold wire bonding, a palladium layer is deposited between nickel and gold. This ensures good adhesion of the gold bonding wire. Otherwise, the ENEPIG surface has the same properties as ENIG. Compared to earlier layers for gold wire bonding with increased gold thickness (thick gold, reductive gold), there are no restrictions on soldering.
This makes ENEPIG a universal surface for all bonding and soldering processes.
Further Gold Surfaces
Prior to the development of the ENEPIG surface, another surface existed for gold wire bonding: Electroless nickel/thick gold. The higher gold layer is 400 nm – 600 nm. In a special process, this layer thickness can be applied chemically or autocatalytically/partially reductively.
The chemical thick gold also dissolves completely in the tin during soldering. With small amounts of solder, the gold content in the molten solder can become so high that intermetallic SnAu phases are formed, which can impair the reliability of the solder joints.
Furthermore, this gold bath is maintenance-intensive in process control and therefore expensive.
For further questions regarding storage and drying of printed circuit boards, please contact our technology manager Dr. Lehnberger:
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