Surface finish type used on the PCB is one of the very important factor which determines the quality of the PCB. There are several types of surface finish done on the PCB based on various considerations.
There would be some gap between the PCB fabrication and actual assembly. If anyone working in the manufacturing industry, they udnerstand the pain they to got through setting up the machine, verifying the components as per BoM and other assembly related tasks. So, the time gap between the fabrication and assembly should not have adverse effects on the fabricated assembly. Especially, the copper need to be protected. Surface finish helps PCB to protect the surface copper until the PCB is assembled. This surface finish is basically a coating on the top and bottom sides of the PCB on the copper. This coating dissolves into the solder paste during the actual component assembly. The surface finish also improves the solderability.
How do you select the Surface finish type?
1. Cost is defintely the major deciding factor.
2. Contact resistance is another major factor. Surface finish shouldn't induce any significant resistance for the current flow.
3. The finish material should easily dissolve with the Solder Paste.
4. The surface finish should not induce any unevenness in the PCB (critical for BGA soldering).
5. Less oxidation
The different types of surface finish used on the PCB are:
1. HASL (Hot Air Solder Levelling)
2. Immersion Tin
3. ENIG (Electroless nickel immersion gold)
4. Organic Surface Protectant (OSP)
5. Immersion Silver (IAg)
For boards with BGAs:
If your boards have BGAs, then your PCB surface should be even as any unevenness creates improper ball sodlering. ENIG is the most preffered surface finish in this case. This is one of the costliest finishing.
For boards which are targetted for low cost:
HASL is the most preffered finish.
HASL and ENIG are the most preffered type of surface finish in the market.
Advantages of HASL over ENIG:
1. Less Costly
2. Robust
3. Most of the EMS vendors support HASL
Disadvantages of HASL over ENIG:
1. HASL is not environment friendly, as more LEAD emission is involved.
2. ENIG has good corrosion resistance than HASL
3. ENIG is good for BGA boards (AS HASL creates unevenness)
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