People who place an order of printed circuit board(PCB) prototype or PCB mass production would know that a processing technology documentation is needed. One of these technologies is which PCB surface treatment to choose. Surface treatment, is also called surface finish or surface coating, is an essential processing step in PCB prototyping and PCB assembly.
What are the surface finish and its function?
Surface finish in PCB manufacturing is a coating between components and the bare copper board. The copper layers on the PCB board are easy to be oxidized, which will cause problems such as false welding, empty welding, even the components cannot be soldered successfully. That’s why there is an essential process in PCB manufacturing in which a layer of materials is applied or plated on the board surface to protect it from oxidation and guarantee the solderability electric performance.
At present, PCB manufacturers mainly have the following surface finish types: HASL, OSP, ENIG, Immersion Tin, and Immersion Silver. Of course, there is some other special PCB surface treatment for special applications. Let’s analyze the surface finish listed below, along with their benefits and drawbacks in detail.
HASL (Hot Air Solder Levelling)
HASL is the predominant surface finish applied on PCB manufacturing for its economical. It is a process of using molten tin(lead) solder on the surface of the board and flattening it by heating compressed air to form a coating layer, and the extra tin will be removed by using hot “air knives.” HASL is divided into two kinds: spray tin with lead and spray tin with lead-free. Lead-free HASL is greener and can meet the standard of RoHS, but lead HASL can’ t.
Benefits of HASL
Drawbacks of HASL
1. Low cost
3. Good welding performance
4. Longer storage time
1. Poor surface flatness
2. Not good for fine pitch components
3. Not suitable for double-sided board
4. Big thermal shock during process
OSP (Organic Solderability Preservatives)
OSP is a type of surface finish, which creating a water-based and organic film on the clean bare copper board by using a chemical method. OSP is suitable for low-tech PCBs and high-tech-PCBs, especially in BGA. It is an ideal surface coating process if the PCB has no surface connection requirements or storage period limitations.
Benefits of OSP
Drawbacks of OSP
1. Simple process
2. Flat surface, is suitable lead-free soldering and SMT
3. Reworkable and easy to operate
4. Low cost
5. Environmental friendly
1. Short shelf life
2. Limited multiple reflows times
3. Not suitable for crimping technology and line binding
4. Difficult for visual inspection and measurement
ENIG (Electroless Nickel Immersion Gold)
ENIG is a double layer metallic coating; the nickel layer is a barrier to copper surface and covered a layer of gold to prevent nickel’s oxidation. ENIG is a surface treatment that is mainly used in PCB manufacturing, which needs an excellent surface connection function and a long storage life such as phone keypads, the edge connection area of router housing, and the electrical contact zone of flexible connection in chip processors. ENIG is a selective immersion process.
Benefits of ENIG
Drawbacks of ENIG
1. Flat surface, suitable for SMT
2. Long storage time
3. Lead-free soldering
4. Suitable for fine pitch/BGA
5. High-temperature resistance
2. Sometimes has the problem of “black and syndrome”
3. Signal Loss(RF)
ImSn (Immersion Tin)
Immersion Tin is a metallic finish applied directly over the base printed circuit board. It is a proper surface treatment for fine pitch products and SMT applications with its excellent flatness.
Benefits of Immersion Tin
Drawbacks of Immersion Tin
1. Excellent flatness
2. Suitable for fine pitch/BGA/smaller components
3. Lead-free soldering
5. High reliability
1. High storage requirement
2. High solder mask requirements
3. Very sensitive to handling--need gloves
ImAg (Immersion silver)
Immersion silver is a coat which is the same as immersion tin except that silver is used in ImAg. It is widely used in communication products, automobiles, and computer peripherals.
Benefits of Immersion silver
Drawbacks of Immersion silver
2. Flat surface, suitable for SMT
3. Simple process
4. Suitable lead-free soldering
5. Suitable for fine traces
6. High stability
1. High storage demands, easy to be polluted
2. Easy to occur electro-migration
3. Solder joint cavity problems
There are several types of surface treatment in PCB manufacturing, and every surface coating has its own merits. Customers should consider some factors in your places, such as the budget price, expected production volume, kinds of components, requirements for durability, environmental impact, and other related demands.
Please feel free to consult with us to make decisions if you are unsure of what surface finish you need. Professional engineers in NextPCB will give you the right suggestion with high-yielding and cost-effective to meet your PCB manufacturing requirements.