Substrate, printed wiring board (PWB), or “bare board” surface finishes have a rich and varied history of innovation in electronic manufacturing. This has had quite an impact on EMPF endeavors over the years. The latest trend in electronic manufacturing – lead (Pb)-free electronics manufacturing – brings with it the newest innovations and accommodations for both the PWB fabricator and the electronics manufacturer.

Tin-lead (Sn-Pb) based electronics are almost always exclusively specified with one of these common, well-documented, surface finishes:

• Hot Air Solder Leveling (HASL)
  
• Electroless Nickel Immersion Gold (ENIG)
• Organic Solderability Preservative (OSP)

HASL is a process in which a bare circuit board (with its solder mask applied) is dipped into a bath of molten solder such that all exposed copper surfaces are covered by solder. Excess solder is removed by scraping the PWB with hot air knives.While the HASL method has the advantage of leaving a very solderable surface (nothing is quite as solderable as solder itself), it has some drawbacks. First, the hot molten solder temperature may cause warpage of the bare board. Second, the HASL coating has a tendency to be non-uniform in thickness (air knives do not always provide uniform pressure and air flow rate). Third and last, it is easy to incorrectly do HASL, resulting in a thick intermetallic layer of Cu3Sn that is not
solderable in the manufacturing process.

The ENIG finish is not applied hot, the way HASL is. ENIG is a mildly heated electroless nickel bath that deposits approximately 100 microinches of nickel (Ni) onto the solder-masked bare board. Then the board is immersed in an immersion gold (Au) bath that applies a scant three to five microinches of Au over the Ni. This surface is easily solderable in the manufacturing process, (although not quite as solderable as the HASL solder finish). The ENIG finish is also very flat, allowing fine-pitch microelectronics to be more easily manufactured at a high production yield.

Organic Solderability Preservative uses a water based organic compound (usually a benzotriazole compound) that selectively bonds to copper and provides an organometallic layer that protects the copper for soldering. The clear OSP-coated copper surface of the finished bare PWB pads are quite flat, accommodating fine-pitch manufacture. The solderability is acceptable, but not as robust as HASL or ENIG which protect for six months and one to two years of storage, respectively. In fact, OSP is not usually used for military boards because of the inability to store boards for a long time before manufacturing, and use provides a smaller manufacturing window, particularly if multiple reworks are anticipated.

Additional PWB surface finishes that are used with either the traditional tin-lead or the new lead-free solders in manufacturing are immersion tin and immersion silver. Immersion tin is an approximately 40 microinch coating of tin on the solderable surfaces of the PWB. Some of the immersion tin coatings have shown a high propensity for tin whisker growth while others have not. Since the causes of tin whisker growth are not yet fully understood, there is a reluctance to use immersion tin coated PWBs.

Immersion silver is predominantly used for lead-free PWBs because it is a solderable coating that neither causes tin whiskers nor contains any lead. However, some other features of the immersion silver coatings can also cause concern. A phenomenon called “champagne voids” (shown in Figure 1-1) is caused by heating the air entrapped under the coating which bridges the roughness of the copper pad. During high temperature soldering, the expanding air creates a plane of tiny voids at the solder joint/copper interface.

Silver also reacts with sulfur in the atmosphere. Where the immersion silver coating is not soldered (and alloyed), copper sulfide corrosion products are formed. Figure 1-2 shows a PWB with an immersion silver finish that was exposed to a sulfur bearing atmosphere, resulting in copper sulfate corrosion product growth.

The EMPF has contributed to the body of knowledge in this critical bare board surface finish aspect of lead-free electronics processing. Future contributions are anticipated in the areas of lead-free HASL (HASL bare board finishes using the higher melting lead-free solders such as SAC 305 or SN 100C), the immersion coatings of tin and silver, and the failure analysis of lead-free CCAs made using such finishes. Any of these failure analysis (FA) services are available by calling the EMPF Helpline at 610.362.1320.