Finishing coating HASL
Wishing to keep the technical characteristics of the contact areas, the manufacturers use protective coatings. HASL (POS-63 and POS-61) is the most widespread finishing coating. The alloy is a specialized eutectic lead-tin solder. The percentage of lead is 61% or 63%.
HASL or HAL
Some manufacturers use HASL - HAL instead of the usual name for the hot tinning method. The secret is in the abbreviation of Hot Air Solder Leveling (HASL) to the following combination - Hot Air Leveling (HAL). Despite this, the methods and the nuances of applying the technological processes of finishing coatings HASL and HAL are identical.
Finishing coating HASL is applied by hot-dip plating at the final stage of dielectric plate production. Solder bath temperature: 230-250˚.
The stages of applying the protective coating
- Preparatory stage: washing off possible contamination, heating and fluxing of printed circuit boards.
- Hot tinning: horizontal (conveyor) or vertical. In the first case, the printed circuit boards, moving along the conveyor at an angle of 45˚, are immersed for a few seconds in the bath with POS-63. In the second one, the dielectric is immersed in a recirculating HASL bath in a vertical position for a few seconds, then it is rapidly removed.
- Elimination of layer surplus and its smoothing. After tinning, the protective layer is smoothed by blowing with air knives (high-temperature air flow under pressure).
Advantages of application
- Good strength indexes of solder joints.
- Durability of protective properties.
- Inexpensive and technologically advanced method of applying finishing coatings.
- Ability to use multiple solder cycles.
Feature of application
HASL is not applied in flexible printed circuit boards, as well as in plates with a high terminal density (flatness with a pitch less than 0.5 mm). It is not used when splicing crystals (COB), the presence of lamellas - knife connectors. With thermal shock, the possibility of waving of the printed circuit board increases in the process of applying a finishing coating. The coating thickness ranges within 1-40 um.
Technological parameters of HAL Pb/Sn
|
Technology opportunities |
Index |
|
Period of storage, months |
12 |
|
Solderability quality |
Excellent |
|
Multiple solderability |
Available |
|
Thermal profile state when flowing solder |
Not critical, but it requires the application of methods that ensure the preservation of gold thickness within 0.3 mm |
|
Availability of gold welding process compatibility |
No |
|
Availability of aluminum welding process compatibility |
No |
|
Finishing coating thickness, μm |
1-40 |
|
Optimum pitch of landing areas, mm |
≥ 0,5 |
|
Process temperature characteristics, ˚С |
250 |
|
Torsion, bending and dimensional stability problems |
Available |
|
Complanarity state of landing areas |
Bad |
|
Surface cleanness state |
Medium |
|
Corrosion resistance index |
Medium |
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Visual defects |
No |
|
Qualitative risks |
The possibility to reduce the sizes of the holes |
|
Solder reliability index |
Good |
|
Compatibility with lead free methods |
No |
|
Index of the environmental damage, safety for the human body |
High |
|
Cost characteristics |
Low |
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