As we have mentioned before: All Printed Circuit Boards (PCB’s) have copper finishes on their surface. If they are left unprotected then the copper will oxidize and deteriorate. In this chapter, we will take a look at OSP, which means: Organic Solder Preservative.
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 PCB 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 PCB test via or test points.
Each surface finish method has its advantages and disadvantages. The following are the Pros and Cons of OSP.
Pro’s: Comparable per unit cost to HASL, excellent coplanarity, lead free process, improved solderability, no ‘black pad’ issues. Low cost, flat surface, good solderability and can be wire bonded.
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. Significant changes to assembly process make it unpopular with most assemblers. Special handling required since finger prints can easily erode the OSP finish exposing the copper to oxidation.
It is easy to judge if by ENIG or HASL surface. What does it look like by OSP finish? As there will be no solder on test points, operators handle OSP boards with gloves, OSP thickness and presence is difficult to ascertain.