July - August 1999
eMail the Editor
The Impact of PC Board Surface Finish on
SMT Assembly Process Yield and Reliability
By Vern Solberg
Plating process options in wide use include hot-air-leveled tin/lead (HASL), immersion gold over electroless nickel and electroplated gold over electroplated nickel. Coatings are generally identified as a chemical preservative to prevent oxidation on the surface of bare-copper attachment sites.
Because of the uneven surface condition of the tin/lead finish, many companies are specifying nickel/gold over the copper base. PC board fabricators will typically use the tin/lead-plated circuit pattern as an etch resist, but strip the tin/lead after etching. One of the more popular plating finishes is the gold over a nickel barrier.
In this plating process, following solder-mask application, the exposed attachment sites and holes on the boards are plated with an electroless nickel alloy followed by a thin (flash) coating of gold alloy using a liquid immersion process.
The electroless/immersion plating provides a uniform surface finish. Of all the coating and plating options noted, Ni/Au is the most versatile (as long as the gold thickness is within the range of 0.08 µm-0.23 µm in thickness and nickel plating does not exceed 2.5 µm-5.0 µm in thickness).
The advantage the plating processes offer over preservative coatings is shelf life and a permanent coverage over exposed copper on vias, or other features not exposed to a solder process. Gold over nickel, a dominant plating technology in Asia, will provide excellent solder attachment process control as long as the gold thickness is limited (electroplating Ni/Au process is not recommended).
On the other hand, coating over bare copper is the most economical method of achieving a flat, uniform attachment site for SMT. Although handling and storage are concerns that must be addressed, the organic preservative materials have a unique advantage over other coating options.
When there is a choice between HASL, Ni/Au and OSP finishes for mass reflow soldering of surface mount assemblies, OSP (when handled properly) has proven itself in meeting both cost objectives and improved overall assembly process yield.
Chip Scale Review o 7291 Coronado Drive, Suite 8 o San Jose, CA 95129 o Email: firstname.lastname@example.org
|© 1998 ChipScale REVIEW|