PCB Surface Finish Breakdown - Pros and Cons
Surface finishes serve a two-fold purpose. One is to ensure solderability and the other to create the base of electronic connection between board and device. Which surface finish you choose will ultimately come down to the design of the PCB, the assembly of its components, and it’s end application/environment. Here is a breakdown of the surface finishes we offer:
HASL (Hot Air Solder Leveling) and Lead-Free HASL
Typical thickness: 1-40um (micron, or 1 millionth of a meter); shelf life 12 months
There are multiple benefits to using HASL as a surface finish. Namely that it is the most common finish and therefore is inexpensive and manufacturers have high familiarity with it (we don’t charge extra for it).
The manufacturing process is nearly the same for Lead-Free HASL as for HASL, thus carries almost all the same pros and cons as HASL. However, Lead-Free HASL is also environmentally friendly and relative to other lead-free finish options, has a less expensive price tag.
A downside that applies to both materials is the difference that appears in thickness between large and small pads, making it not suitable for HDI (high density interconnect) PCBs or <20 mil pitches between SMTs and BGAs.
Another downside comes from the process itself which causes high stress on the PCB and could decrease long-term reliability.
ENIG (Electroless Nickel Immersion Gold)
Typical thickness: 3-6um nickel / 0.05-0.125 gold; shelf life 12 months
ENIG is the best lead-free solution for PCBs (note: it’s also an expensive solution). An excellent option for fine pitch, flat surfaces; it suits the modern-day PCB design trend of HDI and tight pitches between BGAs and SMTs.
It is composed of a double layer of metallic coating. Nickel acts as a barrier to copper and a surface for SMT placement. A thin layer of gold protects the nickel during storage. A couple downsides arise from the metallic composition however, one being undesirable magnetic properties and the other is known as “black pad” where a buildup of phosphorous between the layers causes faulty connections and fractured surfaces.
OSP (Organic Solderability Preservative)
OSP is an organic surface finish that selectively bonds to copper and protects the copper pad before soldering. It is another lead-free finish, so along with ENIG and lead-free HASL is environmentally friendly.
It is well suited for fine pitches/BGA/small components, doesn’t demand high equipment maintenance, and is highly repairable. However, due to its transparent nature, it’s very difficult to test. OSP also requires sensitive handling to avoid scratches and has a limited shelf life (6 months).