<back  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)
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 compromised.
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 joints


Immersion Silver
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
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 low cost
Con's: Heath and Safety concerns, limited number of heat cycles
What does it look like? Tin finish appears 'white' on PCB surface.

SUMMARY

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

Assy Technology PCB Finish
  HASL OSP Silver Gold
Fine Pitch > 0.5mm <<0.5mm <<0.5mm <<0.5mm
Coplanarity Poor Excellent Excellent Excellent
Wave Soldering Excellent Good Good Good
Reflow Soldering Excellent Good Good Good
Press Fits Good Good Excellent Good
ICT Probing Good Poor Good Good


Thanks to:
Helmut Kroener - Multek Europe
Paul Owens - 3Com Ireland
Steve Changelon (US), Steve Doherty (UK), John Kirby(UK), Jun Balangue (Singapore) - Agilent Technologies, Inc.