Types of Surface Treatment
In the PCB market, there are 4 primary lead-free surface treatment for PCB which are suitable for assembly of fine pitch QFP / BGA devices:
Immersion Tin
Organic Solder Preservatives (OSP)
Electroless Nickel Immersion Gold (ENIG)
The table below shows a comparison between these four final finishes and ENEPIG. None of the four primary final finish options is perfect for lead-free assembly, due to a variety of different concerns including multiple reflow cycle capability, shelf life before assembly and wire bond capability. In contrast, ENEPIG shows substantial advantages combining excellent shelf life, solder joint reliability, gold wire bondability and usage as a contact surface.
The protection of the electroless nickel interface with the formation of a thin electroless palladium layer, prior to immersion gold, eliminates the potential for excessive gold attack on the electroless nickel surface.
Comparison of Final Finish Performance
|
Characteristics |
OSP |
ENIG |
ENEPIG |
Immersion Silver |
Immersion Tin |
|
Shelf Life (controlled conditions) |
< 12 Months |
> 12 Months |
> 12 Months |
< 12 Months |
3 – 6 Months |
|
Handling / Contact with Soldering Surfaces |
Must be avoided |
Should be avoided |
Should be avoided |
Must be avoided |
Must be avoided |
|
SMT Land Surface Planarity |
Flat |
Flat |
Flat |
Flat |
Flat |
|
Multiple Soldering Cycles |
Fair to good |
Good |
Good |
Fair to good |
Fair to good |
|
No Clean Flux Usage |
PTH/via fill concerns |
No concerns |
No concerns |
No concerns |
No concerns |
|
Solder Joint Reliability |
Good |
Good process control required to avoid "black pad" |
Good |
Interfacial microvoid concerns |
Good |
|
Gold Wire bonding |
No |
No |
Yes |
No |
No |
|
Electrical Test Probing |
Poor, unless solder applied during assembly |
Good |
Good |
Good |
Good |
|
Corrosion Risk after Assembly |
Yes |
No |
No |
No |
Yes |
|
Contact Surface Applications |
No |
Yes |
Yes |
No |
No |
|
Total Coating Thickness (micron) |
> 0.15 |
u 0.08 – 0.13 |
Soldering: |
0.05 – 0.5 typical |
1.0 – 1.1 |
When we consider the final finish performance in a variety of different assembly methods, it can be seen that ENEPIG is suitable for a wide range of assembly requirements.