A surface finish provides a coating over the outer layer copper that prevents oxidation and provides an electrically conductive surface. This surface has two generic functional requirements.
- The first is to provide a solderable surface for connecting components with solder.
- The second function is to attach a component without soldering, such as a wire bond or press-fit connector.
There are several tradeoffs involved with the surface finishes, including shelf life, cost, reflow cycles, and solder joint flatness. While each finish has its own benefits, in most cases, the process, product, or environment will dictate the surface finish that is best suited for the application. This is another factor that needs the involvement of all stakeholders—designer, manufacturer, assembler, and even end user (environmental issues)—to select the best finish for the specific product design.
Different types of surface finishes:
HASL is the most prevalent conductive protective coating used in electronics. A thin solder coating is deposited onto all exposed copper pads. Boards that have been HASL leveled will have bright silvery pad coloration. However, with the environment-friendly culture of RoHS, this finish cannot be used. The replacement is HASL Lead Free surface finish as RoHS compliant.
Electroless nickel immersion gold (ENIG) process plates a thin coat of nickel covered by a thin layer of gold. The gold provides a very good solderable surface. When components are soldered onto these pads, the gold diffuses into the solder joint. After HASL, this is the most prevalent coating as it increases the shelf life, can withstand six reflow cycles, and increases the flatness for press fir components. Hardness, wear resistance, corrosion resistance and minimal magnetic properties in addition to the high degree of deposit uniformity have made it a premier metal finish satisfying a wide variety of applications. It is a multifunctional surface finish, applicable to soldering, aluminum wire bonding, press fit connections, and as a contact surface. The ENIG finish typically will result in a surface that is cleaner than most other surface finishes. The ENIG finish has excellent contact surface resistance properties and is wire bondable. ENIG is also easy to inspect, it indirectly strengthens the plated through hole and is lead free.
Chemical / Immersion Tin coatings are one of lead free surface finishings which are used in the printed circuit boards technology. The tin coating prevents the underlying copper from corrosion and preserves the solderability during lead-free assembly processes and for a long storage life of boards. The immersion tin is deposited directly on the copper surface by a chemical displacement reaction from acidic bath containing complex compounds. However, under unfavorable conditions intermetallic phases with copper can form, which worsen the solderability of tin coating. Usage:
The use of immersion tin as a deposit suitable for press-ﬁt requirements. It should be noted that changing to a thin immersion deposit such as immersion tin from HASL may require a re-evaluation of the pre-fabrication hole sizes to ensure the correct interference ﬁt required for press-ﬁt applications.
Surface Immersion tin is not recommended as a ﬁnish for soft membrane switch applications.
Electromagnetic Interference (EMI) Shielding: A key characteristic for this application is a consistent metal interface between the PCB and the shield material. Due to the dynamic nature of the immersion tin deposit and the basis metal (Cu) of the PCB, the interface between the EMI shield and the deposit is NOT consistent. The growth of intermetallic compounds (IMCs) will change the electrical characteristics of the interface between the EMI shield and the deposit. It has however been demonstrated to be a suitable interface for EMI shielding for certain speciﬁc applications. Testing for suitability is recommended.