Surface Finishes – Protect Printed Circuit Boards with RoHs and WEEE compliant finishes
Surface finishes are necessary to protect the solder surface from getting oxidized and becoming useless. A surface finish serves two importance purposes – to protect the copper traces and to ensure solderability by adding an intermediate layer between the PCB and the component that will be mounted on it.
There are many options for circuit board surface finishes, each varying in price, availability, shelf life, reliability, rework-ability, and assembly processing. Each finish type has its own pros and cons; based on your design requirements, the final product, and most importantly the environment of application will play a key role in deciding the type of finish is used.
CCI Canadian Circuits’ extensive experience in lead-free manufacturing of printed circuit boards has been instrumental in successfully converting countless projects from solder to a lead- free alternative following the regulations of RoHS and WEEE industry standards.
HASL (Hot Air Solder Levelled)
HASL is one of the cheapest options for final finish with excellent solderability. A very thin layer of solder is coated into the copper pads and holes. Typical solder thickness is 0.1-1.0 mils on surface features. Solder thickness varies by board geometry, aspect ratio, and hole size and is therefore considered unfavourable in higher technology SMD applications. It comes in Tin/Lead and Lead-free HASL options.
ENIG (Electroless Nickel Immersion Gold)
ENIG is the most popular surface finish in the industry because of excellent Solderability, flat surface, and longer shelf-life. It’s a two-layer metallic coating namely Nickel and Gold. Nickel acts as a barrier to the copper and a solder surface. Gold protects the Nickel during storage. Because of the composition of expensive metals, ENIG is expensive. Approximately 100-150µ" of Nickel is coated directly to the bare copper and then 2-5µ" of Gold are then coated over Nickel.
In this technique, silver is deposited on solderable features. Typically, 5-25μ of silver is deposited, with a nominal thickness of 15μ. Benefits of Immersion silver outweigh its cost, moreover it follows RoHS and WEEE directives. This surface finish is stable with moderate shelf-life up to 12 months however once the board is removed from its packaging, it must go through soldering in a couple of days.
ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)
ENEPIG is similar to ENIG in both uses Electroless Nickel over copper, the difference is while ENIG has Gold coated directly on the Nickel, whereas ENEPIG has an additional layer of Palladium between Nickel and Gold. This added layer of Palladium protects Nickel from corrosion. This method allows for increased package and interconnection density.
Soft Gold (Wire Bondable)
Electrolytic gold deposited to the customer’s specified thickness (typically 30-50μ). Gold deposit has a purity >99.9%, a hardness of 50-90 Knoop. Soft gold is typically applied to selective features but can be employed as a solderable coating.
ENIG gives excellent solderability but due to electroless disposition of gold is too soft/thin (2-5 µ" of gold), it can not withstand repetitive abrasion. The other option is electroplated gold or hard gold. Hard Gold consists of a thick layer (typically 30 µ") of gold is plated over a 100 µ" thick Nickel. It is generally applied to the edge connector fingers.
Typically, a global solderable finish is applied after a selective hard/soft gold finish has been applied. Other solderable finishes are possible, but involve more process steps, which may increase the risk and associated costs.