Solving the Surface Finish Selection Paradox

Jul 14, 2020
| By christinepinto…

By Kunal Shah, Ph.D., LiloTree

Surface finish is applied on to copper (Cu) integration scheme on printed circuit boards (PCBs) and semiconductors wafers. Choice of surface finish is critical because the performance and reliability of the electronic assemblies depends on the right selection. There are several options available in the market and one (OEM, PCB manufacturers, PCB designers, EMS) needs to consider the following critical selection criteria,

  1. Insertion loss: Next generation electronic assemblies require HDI, high frequency PCBs. Right choice of surface finish is critical to attain optimum signal integrity with minimal insertion loss. Materials like Nickel (Ni) in ENIG/ENEPIG have lower conductivity and electromagnetism leading to high insertion loss. Immersion Sn, EPAG/EPIG also show insertion loss. It is critical to select a surface finish with insertion loss equivalent to copper substrate for superior performance.
  2. Solder joint reliability: Few surface finishes options (i.e. ENIG, etc.) tend to form brittle solder joints with lead-free solder. Also, Ni based surface finishes form Sn-Ni intermetallics/solder joints which are less strong to Sn-Cu intermetallics. Can the surface finish participate in ensuring smaller intermetallics and robust solder joints? The choice of the surface finish is critical in ensuring robust solder joints in your final assembly.
  3. Shelf life: Looking at the current pandemic situation going on around the world, longer shelf life is critical  due to supply chain disruptions across the world. Flexibility in assembly, shipping and other logistics is a critical need. Several surface finish options show concerns over longer shelf life, i.e. Immersion Silver (Ag), Immersion Tin (Sn), OSP, etc. However, ENIG, ENEPIG, soft Gold (Au), hard Au, etc. show longer shelf life due to Au as outer most layer in the final finish.
  4. Reflow cycles during assembly: With ever-increasing complexity of the electronic assemblies, it is paramount to have the surface finish that can withstand multiple reflow cycles (3+). There are few surface finishes options i.e. OSP, Immersion Ag, etc. which may compromise soon after couple reflow cycles. Usually surface finishes (ENIG, ENEPIG, etc.) with Au as outer most layer in the finish show good performance after multiple reflow cycles.
  5. Cost-effective: It is also critical to review cost during the selection of the surface finish. There are few options that involve use of precious metals, i.e. Ag, Au, Palladium (Pd). Some options use very thick Au layer (20-30+ micro inches in soft/hard gold) whereas others use multiple precious metals (Pd and Au in ENEPIG/EPIG). Thickness of these surface finishes are critical to ensure cost-effectiveness.

Overall performance and reliability of electronic assemblies depends on the right material selection of every component involved. The surface finishes on the PCBs are critical piece to ensure the optimum performance and reliability of the electronic assemblies. Pay attention which selecting surface finishes for PCBs and wafers.