TWI Frequently asked question
By Michèle Routley
There are a number of different surface finishes available; the best one to choose will depend on a product's specific application. Some suggestions are included in the table below.
| Material | Thickness
(µm) | Solderability | Reflow Survivability | Shelf Life | Cost | Advantages | Disadvantages |
|---|
| Immersion Ag | 0.05-0.20 | Good | 5 reflows | 12 months | Medium | Consistent flat surface for SMT
Good electrical probe surface
Good solderability | Must be recycled quickly if mis-screened
Not available from all suppliers
Multiple board finishes difficult |
SnAgCu
HASL | 1-25 | Good | 5 reflows | 12 months | Medium | Good electrical probe surface
Withstands multiple process steps
Good solderability | Higher thermal stress process
No planar surface for SMT |
Immersion
Sn | 0.75-1.25 | Good | 5 reflows | 12 months | Medium | Good solderability
Consistent flat surface for SMT
Good electrical probe surface | Must be recycled quickly if mis-screened
Multiple board finishes difficult
Tin whiskers risk |
Electroless Ni
Immersion Au | 5 Ni
0.05-0.2 Au | V. Good | 4 reflows | 6 months | High | Excellent solderability
Consistent flat surface for SMT
Minimal handling issues | Expensive
'Black Pad'
Multiple board finishes difficult |
SnCu
HASL | 1-25 | Good | 5 reflows | 12 months | Medium | Good solderability
Good shelf life
Withstands multiple process steps | Wider thickness variation than Sn-Pb HASL
Intermetallics formed before assembly |
| Electroless Pd | 0.15-0.40 | Good | 5 reflows | 12 months | High | Minimal handling issues
Good solderability | Thick Pd causes brittle joints
Cost
Availability |
Organic Solderable Preservative
(OSP) | 0.15-0.50 | Good | 3+ reflows | 12 months | Low | Good planar surface
Low cost
No intermetallics before assembly
Strongest solder joints | Handling concerns; probing issues
Difficult obtaining adequate hole fill
Narrow process window for multiple passes |
A consideration when changing to lead-free PCBs is that the board itself will need to withstand the higher processing temperatures for lead-free solder applications; this, like many of the changes required to comply with the RoHSDirective, may have cost implications. When considering laminate properties, some organisations are proposing to use other thermal parameters such as the coefficient of thermal expansion (CTE), time to delamination, and decompositiontemperature (T d ), as better predictive indicators than T g (glass transition temperature). Therefore, not only are increases in T g necessary, but also decreases in CTE and increases in the T d of the material.
For further assistance from TWI on RoHS and WEEE matters, contact Damien Kirkpatrick
What is the WEEE Directive?
