Surface Finish Options
All Printed Circuit Boards(PCB's) have copper finishes on
their surface. If they are left unprotected then the copper will oxidise and
deteriorate, there are various protective finishes available, namely:
Hot air solder levelling (HASL)
- Hot air solder levelling (HASL)
- Organic Solder Preservative (OSP)
- Electroless-Nickel Immersion Gold (ENIG)
- Immersion Silver ImAg and Immersion Tin ImSn
HASL is the predominant surface finish used in the industry. The process
consists of immersing circuit boards in a tin/lead alloy and the excess solder
is then removed by 'air knives', which blow hot air across the surface of the
board. For the circuit board manufacturer, the HASL process is expensive and
maintenance intensive. Not to mention the increased health and safety hazards.
For the printed circuit assembly (PCA) process, HASL has many advantages. It is
the cheapest PCB available and the surface finish remains solder able through
multiple reflow, wash and storage cycles.
HASL has been working well for many years but with the
advent of 'greener', more environmentally friendly processes, it's days are
numbered. In addition to the lead free movement, the increased complexity of
boards and finer pitches has exposed many limitations with the HASL finish.
Pro's: Lowest cost PCB's, remains solderable through
whole manufacturing process, no negative effect at ICT
Con's: Uses lead process which is currently
restricted and eventually eliminated by 2007, for fine lead pitches (<0.64mm)
can lead to solder bridging and thickness issues, unevenness of finish causes co
planarity problems in assembly process.
What does it look like? PCA's test points and via
holes will have solder covering.
Organic Solder Preservative (OSP)
OSP is designed to produce a thin, uniform, protective layer on the copper
surface of the PCB's. This coating protects the circuitry from oxidisation
during storage and assembly operations. It has been around for quite a while but
is only recently gaining popularity with the search for lead free techniques and
fine pitch solutions. For PCB manufacturers it's very simple and easy to
control. For PCA assembly, it has superior capabilities over traditional HASL
with regard to coplanarity, solderability but requires significant process
changes with the type of flux and number of heat cycles. Careful handling is
needed as acidic fingerprints degrade the OSP and leave the copper susceptible
to oxidisation. Assemblers prefer to work with metal finishes that are more
flexible and endure more heat cycles. It also causes major problems at ICT with
the bed of nails fixture contact. More aggressive probing is required to break
through the OSP layer which could potentially even lead to damage and piercing
of the PCA test via's or testpoints.
Pro's: Comparable per unit cost to HASL, excellent
coplanarity, lead free process, improved solderability, no 'black pad' issues.
Con's: Significant changes needed to assembly
process, not ICT friendly, aggressive ICT fixture probes can potentially damage
the PCB, manual handling precautions needed, ICT limits and repeatability
What does it look like? There will be no solder on
testpoints, operators handle OSP boards with gloves, OSP thickness and presence
is difficult to ascertain.
Electroless-Nickel Immersion Gold (ENIG)
These coatings have been used with great success on many boards despite the high
per unit cost. It has a flat surface and excellent solderability. The main
drawback is that the electroless nickel layer is brittle and has been found to
break up during mechanical stress. This effect is known in the industry as
'black pad' or 'mudflat cracking'. This has lead to bad press for ENIG, if these
problems could be overcome this finish would be the ideal coating - apart from
the incremental cost.
Pro's: Excellent solderability, coplanar - flat
surface, excellent shelf life, withstands multiple reflows
Con's: Higher cost (approx 5x HASL), 'black pad'
issue, manufacture process uses cyanide and other unpleasant chemicals.
What does it look like? Gold plated finish on pcb
Immersion silver is a relatively recent addition to the PCB finish. It's main
use has been in Asia and is gaining popularity in North America. Europe now has
emerging market. During the soldering process the silver layer gets dissolved
into the solder joint leaving a tin/lead/silver alloy on the copper which
provides very reliable solder joints for BGA packages. The contrasting colour
makes it easy to inspect, as opposed to OSP. It's also a drop in replacement for
HASL for soldering operations. There is although an underlying question mark
over silver 'migration', but newer finishes have anti-migration agents added to
minimise the effect. Immersion Silver is a very promising finish but end users
are very conservative. Many customers place this finish as 'under
investigation', but it may well emerge as the best finish. There have been some
initial studies into reliability at ICT and it has emerged that etch time
(surface rough/shiny) and thickness of finish are important considerations
regarding repeatability. There are no suggestions that there will be any
negative effects seen at in circuit test stage.
Pro's: Good solderability, coplanar - flat surface,
'drop in' replacement for HASL
Con's: Slow uptake of technology.
What does it look like? Silver finish appears 'white'
on PCB surface.
Immersion tin is a newer alternative surface finish, with many similar
characteristics to it's silver counterpart. However, there are major health and
safety issues to consider. It is mainly used in Europe and Asia whilst it's use
in the US is restricted due to the concern over the thiourea used in tin
solution (a suspected carcinogen).
Pro's: Good solderability, flat surface, relatively
Con's: Heath and Safety concerns, limited number of
What does it look like? Tin finish appears 'white' on
PCB Surface Finish Summary
These are the main players in the PCB finish arena. HASL continues to be the
most widely used PCB finish and in this case the test engineer will not see any
differences. In some countries, HASL is already outlawed and alternatives are in
place. With PCA manufacturing spread across a more diverse and global arena the
possibilities of seeing non-HASL finishes at In-Circuit test are increasing. OSP
is not the natural replacement for HASL but in despite of that it has been one
of the first alternate finishes that PCA manufacturers try out which is leading
to real test reliability issues at ICT.
The conclusion is that there is no 'holy grail' of PCB
finishes, each have there own set of issues that need consideration and some are
worse than others. The facts are that these different PCB finishes will need
adaptation at the ICT stage.
PCB summary table
Helmut Kroener - Multek Europe
Paul Owens - 3Com Ireland
Steve Changelon (US), Steve Doherty (UK), John Kirby(UK), Jun Balangue
(Singapore) - Agilent Technologies, Inc.