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A PCB surface finish is a coating between a component and a bare board PCB., PCB surface treatment is the most basic process in order to to ensure solderability, and to protect exposed copper circuitry.  Due to the nature of the copper in the air tend to exist in the form of oxide, it is not possible to remain as the original copper, so we must do other process with Copper, HASL and OSP are very important PCB surface treatment. The following is the introduction about PCB surface treatment of HAS and OSP:

HASL and lead-free HASL

HASL/HASL(Lead free) is one of the major surface treatments in PCB fabrication industry. The process consist of emerging bare printed circuit board into tin lead alloy melting pot, and then use the “air knife” to blow hot air across the PCB board surface to remove excess solder.

For decades HASL was one of the most popular surface finish choices. Yet, in recent years, PCB manufacturers have realized its limitations. While a surface finish may be low cost and robust, fundamental changes in the PCB industry—namely, the rise complex surface mount technology—have exposed its shortcomings. HASL leaves uneven surfaces and is not suitable for fine pitch components. Although it does come in lead-free, there are other lead-free options which will likely make more sense for a high-reliability product.

Basically the advantages and disadvantages for HASL as below,

Advantages Of HASL include:
• Low cost,
• Can be widely used
• Rework-able
• long shelf life

Disadvantages Of HASL include:
• Uneven surface
• Not suitable for PCBS with fine pitch
• Contain Lead (HASL)
• Thermal shock
• Solder Bridge
• Plugged or Reduced plated through holes

OSP (Organic Solderability Preservative)

OSP, the full name is Organic Solderability Preservative, it’s also called preflux. OSP is a kind of surface finish on bare printed circuit board that preserve the copper surface from oxidation by applying a very thin protective layer of material over the exposed copper usually using a conveyorized process. It’s a kind of surface technology in PCB fabrication that meet RoHS requirements, and it could be alternative solution for the surface finish of HAL(Lead free).

OSP uses a water-based organic compound that selectively bonds to copper and provides an organometallic layer that protects the copper prior to soldering. It’s also extremely green environmentally in comparison with the other common lead-free finish, which suffer from either being more toxic or substantially higher energy consumption.

Basically the advantages of using OSP surface treatment for custom PCB include: Cost effective, re-workable, simple process, No Pb and flat surface on electronic boards.

The disadvantages of using OSP surface treatment include: Handling sensitive, short shelf life, Not good for plated through holes(PTH), can cause ICT Issues, exposed Cu on final PCB assembly, no way to measure thickness.

OSP is also one of surface finishes technology that FASTBOM are currently using, however, a large proportion of this surface technology is used for Single-sided PCB and Double-sided PCB.

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