PCB Basic Knowledge - Application Of Various Surface Finishes
What changes should be made to the surface treatment of PCBs for the relevant environmental directives of the European Union? What package is available for the surface treatment of the PCB?
1. HASL hot air leveling (we often say tin spray)
Spray tin is a common treatment in the early days of PCB. Now divided into lead-containing tin and lead-free spray tin.
The advantages of spray tin:
-->Long storage time
--> After the PCB is completed, the copper surface is completely wetted (completely covered with tin before soldering)
--> Suitable for lead-free soldering
--> Suitable for visual inspection and electrical testing
The weakness of tin spray:
--> Not suitable for line bonding; due to surface flatness problems, there are limitations in SMT; not suitable for contact switch design.
--> Copper will dissolve when tin is sprayed, and the board will experience a high temperature.
--> Special thick or thin plate, spray tin has limitations, production operation is not convenient.
2.OSP (Organic Protective Film)
Organic protective film applications have also been used relatively early, and OSP films suitable for high temperature and multiple reflow soldering have been developed.
Advantages of OSP:
--> Simple process, very flat surface, suitable for lead-free soldering and SMT.
--> Easy to rework, easy to produce and operate, suitable for horizontal operation.
--> The board is suitable for multiple processing (for example: OSP+ENIG)
--> Low cost and environmental friendliness.
-->Restriction of the number of reflow soldering (multiple soldering is thick, the film will be destroyed, basically no problem 2 times)
--> Not suitable for crimping technology, line binding.
--> Visual inspection and electrical measurement are not convenient.
--> N2 gas protection is required for SMT.
--> SMT rework is not suitable.
--> Storage conditions are high.
Chemical silver is a better surface treatment process.
Advantages of chemical silver:
--> Simple process, suitable for lead-free soldering, SMT.
-->The surface is very flat
--> Suitable for very fine lines.
--> Low cost.
The weakness of chemical silver:
--> Storage conditions are high and easy to pollute.
--> Welding strength is prone to problems (micro-cavity problems).
--> Easily occur electromigration and the occurrence of Giovanni bite in the copper under the solder mask.
-->Electric measurement is also a problem
Chemical tin is the most copper-tin replacement reaction.
Chemical tin advantages:
--> Suitable for horizontal production.
--> Suitable for fine line processing, suitable for lead-free soldering, especially suitable for crimping technology.
--> Very good flatness, suitable for SMT.
--> Need good storage conditions, preferably not more than 6 months to control tin whisker growth.
-->Not suitable for contact switch design
--> The process requirements for the solder mask process are relatively high, otherwise the solder mask will fall off.
--> When welding multiple times, it is best to protect N2 gas.
--> Electrical test is also a problem.
Chemical Nickel Gold (ENIG)
Nickel gold is a relatively large surface treatment process. Remember: the nickel layer is a nickel-phosphorus alloy layer. It is divided into high-phosphorus nickel and medium-phosphorus nickel according to the phosphorus content. The application is different. The difference is not introduced here.
Nickel gold advantages:
--> Suitable for lead-free soldering.
--> The surface is very flat and suitable for SMT.
--> Through hole can also be nickelized.
--> Long storage time, storage conditions are not harsh.
--> Suitable for electrical testing.
--> Suitable for switch contact design.
--> Suitable for aluminum wire binding, suitable for thick plates, strong against environmental attacks.
Electroplated nickel gold
Electroplated nickel gold is divided into "hard gold" and "soft gold", hard gold (such as: gold-cobalt alloy) is commonly used on gold fingers (contact connection design), soft gold is pure gold. Electroplated nickel gold is applied on IC carrier boards (such as PBGA). It is mainly suitable for gold wire and copper wire bonding. However, it is suitable for IC carrier plating. The bonded gold finger area needs extra conductive wire to be plated.
Remarks: Pure gold thickness 0.1-0.5 is suitable for welding, pure gold 0.5-1.0, suitable for gold and copper wire bonding; hard gold thickness > 1 micron.
Advantages of electroplating nickel gold:
--> Longer storage time > 12 months.
--> Suitable for contact switch design and gold wire binding.
--> Suitable for electrical testing
--> Higher cost, gold is thicker.
--> Additional design lines are required for electroplating gold fingers.
--> Because the thickness of gold is not always applied, when applied to welding, the solder joint may be embrittled due to too thick gold, which affects the strength.
--> Plating surface uniformity problem.
--> Electroplated nickel gold does not wrap the side of the line.
--> Not suitable for aluminum wire binding.
Nickel Palladium Gold (ENEPIG)
Nickel-palladium gold is now beginning to be applied in the field of PCBs, and has been used in semiconductors before. Suitable for gold and aluminum wire binding.
--> Application on IC carrier board, suitable for gold wire binding, aluminum wire binding. Suitable for lead-free soldering.
--> Compared with ENIG, there is no nickel corrosion (black plate) problem; the cost is cheaper than ENIG and electric nickel gold.
--> Long storage time.
--> Suitable for a variety of surface treatment processes and on the board.
--> Process is complicated. Control is difficult.
--> Short application history in the PCB field.