Comparison of lead free surface treatment of PCB
Pb-free HASL
Advantages
Disadvantages
Good weldability: good preservation
The amount of solder paste used is relatively small
Easily appears tin beads.
OSP
Low cost, smooth
Suitable for larger factories
It is easy to oxidize and the PC board can not be measured directly;
The use of solder paste is relatively increased.
ENIG
It has flatness and good conductivity
It is easy to oxidize and the use of solder paste is relatively increased.
Electrolytic Ni/Au
It has flatness;Low electrical resistance
High cost; If it is too thin, the gold is easy to catalyze and oxidize, and the use of solder paste is relatively increased.
I-Ag
It has flatness;Low electrical resistance
It is easy to oxidize and dilute without Tin, and the strength of solder is poor,and the use of solder paste is relatively increased.
I-Sn
It has flatness
The solder-ability decreases and the solder strength is poor,and the use of solder paste is relatively increased.
1) Tin spraying processing temperature is low (235-255 ℃), there will be no impact on the plate
2) The solder paste and solder rod (tin, silver and copper) produced by spraying tin material and PCBA are the same series of lead-free solder, so it has the best comprehensive performance, and there will be no brittle joint due to different melting points, resulting in dissolution stripping phenomenon.
3) The surface of solder joint is bright
4) The solder joint is not easy to oxidize and has a long storage life.
5) The thickness of tin spray is 40-100um, which can save the amount of solder paste used in the later stage. (the price of solder paste is 4 times that of tin bar)
6) It is easy to accept the processing habits of the engineers in the later stage of the process by continuing the process technology with lead tin spray.
7) The total cost (tin spray + reflow + wave soldering) is the lowest
8) Before the back welding + wave welding process, the inferior PC board can be screened in advance.
9) Mature technology
Types and analysis of lead free tin spray
Solder composition
Sn-Cu
Sn-Cu-Ni-X
Sn-Au-Cu
Sn-Au-Cu-Ni
Processing price
1(250)
1.4(300)
2(350)
2(350)
Temperature (℃)
265-275
265-275
260-270
245-260
Back processing (wave soldering)
Not easy
Not easy
Easy
Easy
Back processing (back peak welding)
Not easy
Not easy
Easy
Easy
Oxidation (preservation)
3(fast)
1
2
1(slow)
Brightness comparison
4(foggy)
2
3
1(bright)
Heat resistance
Bad（high temperature）
Bad（high temperature）
Good
Good
Counter explosion plate& ink heat resistance
4（high）
3
2
1(low)
Recommended
High temperature processing
Low temperature processing
Introduction to horizontal tin spraying
As one of the most common surface coating forms of PCB surface treatment, tin spraying smobc & Hal is widely used in the production of circuit boards. The quality of tin spraying directly affects the quality and solderability of soldering in subsequent production; Therefore, the quality of tin spraying has become a key point in the quality control of PCB manufacturers; there are two kinds of tin spraying: vertical tin spraying and horizontal tin spraying. The main functions of tin spraying are as follows
(1) prevent the oxidation of bare copper surface
2) keep the solder-ability;
Other methods of surface treatment are: hot melt, organic protective film OSP, chemical tin, chemical silver, chemical nickel gold, nickel plating gold, etc;
The main disadvantages of vertical tin spraying are as follows:
(1) uneven heating up and down of the board, last in first out, easy to appear the defect of plate bending and warping.
The thickness of the solder pad is not easy to stick due to the effect of hot air on the surface of the solder pad.
(3) the contact time between the solder pad on bare copper and the hole wall and solder is longer, generally more than 6 seconds. The copper solubility increases rapidly in the soldering furnace. The increase of copper content will directly affect the solder-ability of the pad, because the thickness of IMC alloy layer is too thick, which greatly shortens the shelf life of the board.
Compared with vertical tin spraying, horizontal tin spraying has the following advantages
(1) the contact time between molten tin and bare copper is short, about 2 seconds, the thickness of IMC is thin and the storage life is longer
(2) the time of tin dipping was short, wet time, about 1 second
(3) the board is heated evenly, the mechanical properties are good, and the board warping is less
The technological process of horizontal tin spraying is as follows
Pre cleaning treatment -- preheating --- flux coating --- horizontal tin spraying --- hot air knife tin scraping --- cooling --- post cleaning treatment