Linked Electronics is frequently asked by customers about the option of surface treatment.

You may read below introduction. Here we introduce the common surface treatment with pros and cons.

PCB surface finish is a coating between a component and a bare board PCB. It is applied for two basic reasons: ensuring solderability, and protecting exposed copper circuitry. As there are many kinds of surface treatment, selecting the right one is no easy job, especially as surface mounts have become more complex and regulations such as RoHS and WEEE have changed industry standards.

Criteria

You should consider the following to make a decision:

• Lead vs. Lead free
• Component Type
• Reworkability
• Cost
• Productivity
• Environment

HASL and lead-free HASL

For decades, HASL was one of the most popular surface finish choices. In recent years, manufacturers find 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 disadvantages. HASL leaves uneven surfaces and is not suitable for fine pitch components. Although it does come in lead-free to meet Rohs standards, there are other lead-free options which will likely make more sense for a high-reliability product.

Pros:

• Low-cost
• Repairable

Cons:

• Uneven Surfaces
• Not good for fine pitch components
• Thermal Shock
• Not good for plated through-hole (PTH)
• Poor wetting

Electroless Nickel Immersion (ENIG)

ENIG is quickly becoming the most popular surface finish choice in the industry. It's a double layer metallic coating, with nickel acting as both a barrier to the copper and a surface to which components are soldered. A layer of gold protects the nickel during storage. ENIG is an answer to major industry trends such as lead-free requirements and rise of complex surface components (especially BGAs and flip chips), which require flat surfaces. However immersion gold can be expensive, and at times can result in what is commonly known as “black pad syndrome,” a buildup of phosphorous between the gold and nickel layers that can result in fractured surfaces and faulty connections.

Pros:

• Flat surfaces
• Solid
• Rohs Compliant
• Good for PTH

Cons:

• Black pad syndrome
• Expensive
• Not good for rework

OSP 

OSP is a water-based, organic surface finish that is typically used for copper pads. It selectively bonds to copper and protects the copper pad before soldering. OSP is environmentally friendly, provides a coplanar surface, is lead-free, and requires low equipment maintenance. However, it's not as robust as HASL and can be sensitive while handling.

Pros:

• Lead-free
• Flat surface
• Simple process
• Repairable
• Low Cost

Cons:

• Not good for PTH
• Sensitive
• Short Shelf life

Immersion Tin

Immersion Tin is a good surface finish for planar, fine pitch products and is popular with press fit and backplanes. But for the most part, ISn has more pros than cons. While it works well to protect underlying copper from oxidation, it's intermetallic relationship with copper can be problematic. The diffusion of one metal into the other can impact shelf life and the performance of the finish.

nPros:

• Lead-free
• High Reliability
• Planar
• Cost Effective
• Can subsitute for reflowed solder

Cons:

• Not good for PTH
• Contains Thiourea, a known Carcingen
• Limited rework
• Tin wiskering
• Could damage soldermask
• Handling concerns

Immersion Silver

Immersion Silver benefits far outweight its costs. It gained widespread popularity since the RoHS and WEEE directive took effect, and can be a good alternative option to ENIG for fine pitch and flat pack coating. It's a stable finish with a moderate shelf-life (roughly 12 months) and relatively simple process control. Immersion silver contains OSP, which works to prevent tarnishing. But it can be sensitive to contaminants, both in the air and on the board, and should be packaged immediately. Immersion Silver is commonly used for membrane switches, EMI shielding, and aluminum wire bonding. But it is not frequently used recently, more customer choose ENIG. Immersion Silver will take longer days to complete.

Pros:

• RoHS complaint
• Planar
• Fine pitch
• Cost effective
• Good alternative to ENIG
• High stability

Cons:

• Tarnishes
• Silver Whiskering
• Some systems cannot throw into microvias aspect ratios of > 1:1
• High friction coefficient/not suited for compliant-pin intertion (Ni-Au Pins) 

According to Linked Electronics' manufacturing record, 95% of boards are ENIG or HASL in our factory.To put it simply, for fine pitch components, we advise ENIG. Others, you can choose HASL , the cost of HASL is much lower cost than ENIG.

Thank you for your time with Linked Electronics, if you need further assitance, contact our sales team.

Look foward to working with you.