How to Choose Surface Finish for Your PCB Design
Ⅱ Evaluation and Comparasion
Posted: Nov 16, 2022
Categories: Blogs
Tags: pcb, pcba, pcb assembly, pcb manufacturing, pcb surface finish
There are many tips about surface finish, such as lead-free HASL has problem to have a consistent flatness. Electrolytic Ni/Au is really expensive and if too much gold is deposited on pad, can lead to brittle solder joints. Immersion tin has solderability degradation after exposure to multiple heat cycles, as in a top and bottom side PCBA reflow process, etc.. The differences of the above surface finishes needed to be clearly aware. The below table shows a rough evaluation for the often-applied surface finishes of printed circuit boards.
Table1 Briefly description of manufacturing process, significant pros and cons, and typical applications of popular lead-free surface finishes of PCB
|
PCB Surface Finish |
Process |
Thickness |
Advantages |
Disadvantages |
Typical Applications |
|
Lead-free HASL |
PCB boards are immersed in a molten tin bath and then was blow by hot air knives for flat pats and excess solder removing. |
30µin(1µm) -1500µin(40µm) |
Good Solderability; Widely available; Can be repaired/reworked; Long shelf long |
Uneven surfaces; Thermal shock; Poor wetting; Solder bridge; Plugged PTHs. |
Widely applicable; Suitable for larger pads and spacing; Not suitable for HDI with <20 mil (0.5mm) fine pitch and BGA; Not good for PTH; Not suit for thick copper PCB; Typically, application: Circuit boards for electrical testing, hand soldering, some high-performance electronics such as aerospace and military devices. |
|
OSP |
Chemically applying an organic compound to boards surface forming an organic metallic layer to protect exposed copper from rust. |
46µin (1.15µm)-52µin(1.3µm) |
Low cost; Pads are uniform and flat; Good solderability; Can be unit with other surface finishes; Process is simple; Can be reworked (inside the workshop). |
Sensitive to handling; Short shelf life. Very limited solder spreading; Solderability degradation with elevated temp & cycles; Nonconductive; Difficult to inspect, ICT probe, ionic & press-fit concerns |
Widely applicable; Well suited for SMT/fine pitches/BGA/small components; Serve boards; Not good for PTHs; Not suitable for crimping technology |
|
ENIG |
A Chemical process which plates the exposed copper with Nickel and Gold, so it consists of a double layer of metallic coating. |
2µin (0.05µm)– 5µin (0.125µm) of Gold over 120µin (3µm)– 240µin (6µm) of Nickel |
Excellent solderability; Pads are flat and uniform; Al wire bendability; Low contact resistance; Long shelf life; Good corrosion resistance and durability |
“Black Pad” concern; Signal loss for signal integrity applications; unable to rework |
Excellent for Assembly of fine pitch and complex surface mount placement (BGA, QFP…); Excellent for multiple Soldering types; Preferable for PTH, press fit; Wire Bondable; Recommend for PCB with high reliability application such as aerospace, military, medical and high-end consumers, etc.; Not recommended for Touch Contact Pads. |
|
Electrolytic Ni/Au (Soft gold) |
99.99% pure – 24 carat Gold applied over nickel layer through an electrolytic process before soldermask. |
99.99% Pure gold, 24 Karat 30µin (0.8µm) -50µin (1.3µm) over 100µin (2.5µm) -200µin (5µm) of Nickel |
Hard, durable surface; Great conductivity; Flatness; Al wire bendability; Low contact resistance; Long shelf life |
Expensive; Au embrittlement if too thick; Layout constrains; Extra processing/labor intense; Not suit for soldering; Coating is not uniform |
Mainly used in wire (Al & Au) bonding in chip package such as COB (Chip on Board) |
|
Electrolytic Ni/Au (Hard gold) |
98% pure – 23 carat Gold with hardeners added to the plating bath applied over nickel layer through an electrolytic process. |
98% Pure gold, 23 Karat30µin(0.8µm) -50µin(1.3µm) over 100µin(2.5µm) -150µin(4µm) of Nickel |
Excellent solderability; Pads are flat and uniform; Al wire bendability; Low contact resistance; Reworkable |
Tarnish (handling & storage) corrosion in high sulfur environment; Reduced supply chain options to support this finish; Short operating window between assembly stages. |
Mainly used for electrical interconnection such as edge connectors (gold finger), IC carrier boards (PBGA/FCBGA/FCCSP...) , Keyboards, battery contacts and some test pads, etc.. |
|
Immersion Ag |
a Silver layer is deposited on copper surface through an electroless plating process after etch but before soldermask |
5µin(0.12µm) -20µin(0.5µm) |
Excellent solderability; Pads are flat and uniform; Al wire bendability; Low contact resistance; Reworkable |
Tarnish (handling & storage) corrosion in high sulfur environment; Reduced supply chain options to support this finish; Short operating window between assembly stages. |
Economical alternative to ENIG for Fine Traces and BGA; Ideal for high speed signals application; Good for membrane switches, EMI shielding, and aluminum wire bonding; Suitable for press fit. |
|
Immersion Sn |
In an electroless chemical bath, a white thin layer of Tin deposits directly on copper of circuit boards as a barrier for avoiding oxidation. |
25µin (0.7µm)-60µin(1.5µm) |
Best for press fit technology; Cost-effective; Planar; Excellent solderability (when fresh) and reliability; Flatness |
Solderability degradation with elevated temps & cycles; Exposed tin on final assembly can corrode; Handling issues; Tin Wiskering; Not suitable for PTH; Containing Thiourea, a known Carcinogen. |
Recommend for large amount productions; Good for SMD placement, BGA; Best for press fit and backplanes; Not recommended for PTH, contact switches, and usage with peelable masks |
Table2 An evaluation of typical properties of modern PCB Surface Finishes on production and application
|
Production of most common used surface finishes |
|||||||||
|
Properties |
ENIG |
ENEPIG |
Soft Gold |
Hard gold |
IAg |
ISn |
HASL |
HASL- LF |
OSP |
|
Popularity |
High |
Low |
Low |
Low |
Medium |
Low |
Low |
High |
Medium |
|
Process Cost |
High (1.3x) |
High (2.5x) |
Highest (3.5x) |
Highest (3.5x) |
Medium (1.1x) |
Medium (1.1x) |
Low (1.0x) |
Low (1.0x) |
Lowest (0.8x) |
|
Deposit |
Immersion |
Immersion |
Electrolytic |
Electrolytic |
Immersion |
Immersion |
Immersion |
Immersion |
Immersion |
|
Shelf Life |
Long |
Long |
Long |
Long |
Medium |
Medium |
Long |
Long |
Short |
|
RoHS Compliant |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
No |
Yes |
Yes |
|
Surface Co-planarity for SMT |
Excellent |
Excellent |
Excellent |
Excellent |
Excellent |
Excellent |
Poor |
Good |
Excellent |
|
Exposed Copper |
No |
No |
No |
Yes |
No |
No |
No |
No |
Yes |
|
Handling |
Normal |
Normal |
Normal |
Normal |
Critical |
Critical |
Normal |
Normal |
Critical |
|
Process Effort |
Medium |
Medium |
High |
High |
Medium |
Medium |
Medium |
Medium |
Low |
|
Rework Capacity |
No |
No |
No |
No |
Yes |
Not suggested |
Yes |
Yes |
Yes |
|
Required Thermal Cycles |
multiple |
multiple |
multiple |
multiple |
multiple |
2-3 |
multiple |
multiple |
2 |
|
Whisker issue |
No |
No |
No |
No |
No |
Yes |
No |
No |
No |
|
Thermal Shock (PCB MFG) |
Low |
Low |
Low |
Low |
Very Low |
Very Low |
High |
High |
Very Low |
|
Low Resistance / High Speed |
No |
No |
No |
No |
Yes |
No |
No |
No |
N/A |
|
Applications of most common used surface finishes |
|||||||||
|
Applications |
ENIG |
ENEPIG |
Soft Gold |
Hard Gold |
IAg |
ISn |
HASL |
LF-HASL |
OSP |
|
Rigid |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
|
Flex |
Restricted |
Restricted |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
|
Flex-Rigid |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Not Preferred |
|
Fine Pitch |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Not Preferred |
Not Preferred |
Yes |
|
BGA & μBGA |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Not Preferred |
Not Preferred |
Yes |
|
Multiple Solderability |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
Restricted |
|
Flip Chip |
Yes |
Yes |
Yes |
Yes |
Yes |
Yes |
No |
No |
Yes |
|
Press Fit |
Restricted |
Restricted |
Restricted |
Restricted |
Yes |
Excellent |
Yes |
Yes |
Restricted |
|
Through-Hole |
Yes |
Yes |
Yes |
Yes |
Yes |
No |
No |
No |
No |
|
Wire Bonding |
Yes (Al) |
Yes (Al, Au) |
Yes (Al, Au) |
Yes (Al) |
Variable (Al) |
No |
No |
No |
Yes (Al) |
|
Solder Wettability |
Good |
Good |
Good |
Good |
Very good |
Good |
Poor |
Poor |
Good |
|
Solder Joint Integrity |
Good |
Good |
Poor |
Poor |
Excellent |
Good |
Good |
Good |
Good |
The shelf life is a critical element you need to consider when making your manufacturing schedules. Shelf Life is the operative window which grants the finishing to have a complete PCB weldability. It is vital to make sure all your PCBs are assembled within the shelf life. In addition to material and process which make surface finishes, shelf life of finish is strongly influenced by PCBs packaging and storage. Strictly applicant of the right storage methodology suggested by IPC-1601 guidelines will preserve finishes’ weldability and reliability.
Table3 Shelf life Comparison among Popular Surface Finishes of PCB
|
|
Typical SHEL LIFE |
Suggested Shelf Life |
Rework Chance |
|
HASL-LF |
12 Months |
12 Months |
YES |
|
OSP |
3 Months |
1 Months |
YES |
|
ENIG |
12 Months |
6 Months |
NO* |
|
ENEPIG |
6 Months |
6 Months |
NO* |
|
Electrolytic Ni/Au |
12 Months |
12 Months |
NO |
|
IAg |
6 Months |
3 Months |
YES |
|
ISn |
6 Months |
3 Months |
YES** |
* For ENIG and ENEPIG finishing a reactivation cycle to improve surface wettability and shelf life is available.
** Chemical Tin rework not suggested.
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Post time: Nov-16-2022
