What are the different finishes for PCBs and their impact on Assembly?

Thursday, 27 October 2016

PCB Power

It is very important that an ideal surface finish be selected for a PCB. The high assembly density and Ball Grid Array (BGA) have led to the higher usage of electronic packages for commercial applications. Besides the Hot Air Solder Level (HASL / HAL), there are many other surface finishes too to fulfill the advanced assembly needs. With each finish having its individual benefits and limitations, the ideal choice on a product’s application.

Major decision drivers for Ideal Surface finish:

Compatible with the assembly process
Co-planarity – Flat and uniform
Good solder ability
An extended shelf life
Lowest overall cost
Easily rework able
Highest yield in production
Complying with all ROHS requirement
Testability

Surface finishes available with PCB Power:

PCB Power offers following surface finishes:

HAL
Lead free HAL
Immersion Tin
ENIG (Electroless Nickel and Immersion Gold)

HAL (Hot Air Leveling):

Most commonly used surface finish in automotive and industrial applications
“Nothing can solder like solder”
Low cost
Easily applied and reworked
Long shelf life
Good bond strength
Easy visual inspection
Considered the standard surface finish as related to assembly needs.

Huge co-planarity difference
In consistent surface finish thickness
Not suited for fine pitch SMTs and BGA package
Fabrication process itself introduces thermal excursion
Exhibit issues for high aspect ratio holes
Dimensional stability issues
Contain Lead

Lead free HAL:

“Nothing can solder like solder”
Low cost
Easily applied and reworked
Long shelf life
Good bond strength
Easy visual inspection
Complying with all ROHS requirement

Huge co-planarity difference
In consistent surface finish thickness
Not suited for fine pitch SMTs and BGA package
Fabrication process itself introduces thermal excursion
Exhibit issues for high aspect ratio holes
Dimensional stability issues

Immersion Tin:

Flat / Planar surface
Suitable for fine pitch applications
Suitable for horizontal and vertical application
Can be a substitute for reflowed solder
Good solder joint reliability
Fairly good shelf life
Relatively low-cost deposit
Suitable for compliant pin (press fit) connection
Complying with all ROHS requirement

Handling concerns
Horizontal process needs Nitrogen blanket
Not suitable for contact switching applications
Ionic contamination for automotive applications

ENIG (Electroless Nickel and Immersion Gold):

The second most commonly requested finish
Flat / Planar surface
Easy to solder
Long shelf life
Multi reflow approved
Can be used for wire bonding
Can be used for contact pads
Complying with all ROHS requirement
Aesthetically very pleasing

The process is very difficult to control and is laboratory intensive
Higher cost
Black pad
Risk of brittle solder joint
Not optimal for higher speed signals
Manufacturing process uses cyanide and other unpleasant chemicals

OSP (Organic Solderability Preservaive):

Flat / Planar surface
Easily rework able
Doesn’t affect finished hole size
Low cost
Easy process
Good solder mask integrity

Difficult to inspect
Limited thermal cycles
Limited self life

Comparison:

	HAL (SNPB)	LEAD FREE HAL	IMMERSION TIN	ENIG	OSP
ROHS compliant	No	Yes	Yes	Yes	Yes
Cost	Low	Low	Medium	High	Low
Shelf life	1 Year	1 Year	Up to 3 Months	1 Year	Up to 3 Months
Fine pitch quality	Poor	Poor	Best	Best	Best
Assembly cycle capacity	Multiple	Multiple	Multiple	Multiple	Multiple
Multiple rework capacity	Limited	Limited	No	Limited	No
Solder Wattebility	Excellent	Good	Good	Good	Good
Co-planarity	Poor	Good	Excellent	Excellent	Excellent
Solder joint integrity	Excellent	Good	Good	Good	Good
Low resistance / High speed	No	No	No	No	N/A
Wave soldering	Excellent	Excellent	Good	Good	Good
Reflow soldering	Excellent	Excellent	Good	Good	Good

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