Among all the PCB (printed circuit board) fabrication craft, surface finish plays an extremely role in PCB assembly and utilization. Copper layer on PCBs tends to be oxidized so that generated copper oxide layer will seriously influence soldering quality. However, surface finish is capable of preventing pads oxidizing so that excellent solderability and electric performance will be ensured.
Surface finish, or surface coating, is the most important step in the process between PCB manufacturing and PCB assembly with two main functions one of which is to preserve the exposed copper circuitry and the other of which is to provide solderable surface when soldering components to the circuit board. As is shown in Figure 1, surface finish is located at the outermost layer of PCB, playing a role as a "coat" for the board.
Surface Finish Types
Basically, there are two main types of surface finishes: metallic and organic, each of which has the specific categories as the display of Table 1.
1. HASL (Hot Air Solder Leveling)
HASL is a conventional type of surface finish used on PCBs. The PCB is typically dipped into a bath of molten solder so that all exposed copper surfaces are covered by solder. Excess solder is removed by passing the PCB between hot air knives. Usually, HASL follows the procedure below:
The pros and cons of HASL are shown in the following table.
To reach the standard concerning environment, HASL develops with an environment-friendly category that is lead-free HASL that increases lead-free solderability plate of choice.
2. ENIG (Electroless Nickel/Immersion Gold)
ENIG consists of electroless nickel plating covered with a thin layer of immersion gold, which protects the nickel from oxidation. Among all types of surface finishes, ENIG is the most expensive but provides the best characteristics for PCBs. Usually, ENIG follows the procedure below:
The electroless nickel step is an auto-catalytic process that involves depositing nickel on the palladium-catalyzed copper surface. The reducing agent containing nickel ions must be replenished in order to provide proper concentration, temperature and ph levels necessary to create a consistent coating. During the immersion gold step, the gold adheres to the nickel plated areas through molecular exchange, which will protect the nickel until the soldering process. The gold thickness needs to meet certain tolerances to ensure that the nickel maintains its solderability.
Table 3 below shows the pros and cons of ENIG.
Similar with ENIG, ENEPIG (electroless nickel electroless palladium immersion gold) is the development of ENIG. The difference between ENIG and ENEPIG is the palladium layer that is capable of stopping the nickel corrosion and copper migration to nickel layer.
3. ImAg (Immersion Silver)
ImAg consists of thin immersion silver plating over the copper traces. Usually, ImAg follows the procedure below:
ImAg is a good type of surface finish for soldering and testing. Creep corrosion is its major weakness. Table 4 below shows the pros and cons of ImAg.
4. ImSn (Immersion Tin)
ImSn is mostly the same as ImAg except tin is used in ImSn while silver is used in ImAg. In terms of the advantages of ImSn, it provides an extremely planar finish on the copper pads, making it very suitable for SMT applications. Besides, ImSn provides a surface that is easily detectable by common AOI (automated optical inspection) techniques.
5. OSP (Organic Solderability Preservatives)
OSP is a transparent organic material surface finish. It uses a water-based organic compound that selectively bonds to copper and protects the copper until soldering. Usually, OSP follows the process below:
Table 5 below shows the pros and cons of OSP.
In summary, each type has its own advantages and disadvantages. In the process of selection, some main elements must be taken into considerations including cost, corrosion resistance, ICT (in-circuit test) and hole fill. Based on the features of PCBs and utilization purposes, suitable surface finish type is selected. More considerations concerning surface finish selection will be discussed in the second part of this article.
Comparing these types of surface finishes, generally speaking, in terms of COST, ImAg and OSP are the most inexpensive while ENIG is the most costly. In terms of CORROSION RESISTANCE, HASL and ImSn have the best corrosion resistance capacity while ImAg has the worst. In terms of ICT, only OSP is the worst while others are just similarly good. In terms of HOLE FILL, HASL and ENIG are better than other types.
Surface Finish Selection
Surface finish selection on PCBs is the most important step for electronic assembly since it directly influences process yields, rework numbers, field failure rate, test capability, scrap rate and cost. All the important considerations about assembly must be taken into surface finish selection in order to ensure the high quality and performance of PCBs.
In the overall process of PCB assembly, people with different positions have different opinions about how to select surface finish and different selection standards. No matter what type is selected, it only caters to the requirements and convenience of people with few considerations about the quality, performance and reliability of PCBs and PCB assembly.
Based on the introduction of each type of surface finish above, some attributes are the most important elements as the selection standard.
• Cost sensitive product
• High volume required
• Cosmetics of surface finish
• Lead-free wave solder
• Fine pitch components used
• Wire bonding to surface finish
• High yield ICT
• Lead-free shock/drop
• Possibilities of corrosion failure
The flowing table shows the attributes each type of surface finish HAS and DOESN'T HAVE, it helps provide guidelines in the process of surface finish selection. Based on specific requirements and features of PCB products, optimal surface finish type can be selected.
All in all, as for the type of surface finish selection, an optimal type must be selected and numerous functions can be accomplished. Each type of surface finish has its own advantages and disadvantages. But don't worry. There are some engineering tricks as the solutions to the problems caused by disadvantages of surface finish. For example, as for the disadvantage that OSP has lower wetting force, some solutions are available such as changing board solderability plating or wave solder alloy, increasing top-side preheat etc. The key point is that all the possible elements must be considered in order to obtain ideal performance.
Nowadays, environment issues have become increasingly important in the electronic fields. In order to restrain the generated hazardous substances, RoHS is published by EU. RoHS, also known as Lead-Free, stands for Restriction of Hazardous Substances. RoHS, also known as Directive 2002/95/EC, originated in the European Union and restricts the use of six hazardous materials found in electrical and electronic products. All applicable products in the EU market after July 1, 2006 must pass RoHS compliance. RoHS impacts the entire electronics industry and many electrical products as well. So, surface finishes with lead-free solder will have more followers in the future.