Most common used PCB surface finishing comparison

A printed circuit board surface finishing is a coating between a component and a bare board PCB.It’s applied for two basic reasons:
To protect exposed copper circuitry and to make sure the solderability.As there are many types of surface finishing,Selecting the right one would be very important,Especially when the surface mounts have become complex and regulations.Below factors need to be considered before making a choose.
1. Lead vs Lead free
2. Component Type
3. Cost
4. Reworkability
5. Productivity
6. Environment

There are listed below most common used of PCB surface finishing for your reference.
1. HASL and Lead-free HASL
2. OSP(Organic Solderability Preservative)
3. Immersion Tin
4. Immersion silver
5. Electroless Nickel Immersion (ENIG)

PCB Surface finishes comparison
1.HASL and lead-free HASL
--Description:For decades HASL was one of the most popular surface finish choices. Yet, in recent years, 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.

--Advantage:

Low-cost
Available
Repairable

--Disadvantage

Poor Wetting
Not good for plated through-hole (PTH)
Thermal Shock
Not good for fine pitch components
Uneven surfaces

2.OSP (Organic Solderability Preservative)
--Description: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.

--Advantage:

Leed-Free
Flat surface
Simple process
Repairable

--Disadvantage

Not good for PTH
Sensitive
Short shelf life

3.Immersion Tin
--Description:Immersion Tin (ISn) 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.

--Advantage:

Leed-Free
High reliability
Planar
Cost Effective
Can subsitute for reflowed solder

--Disadvantage

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

4.Immersion Silver
--Description:This is a surface finish whose benefits far outweight its costs. It gained widespread popularity since the RoHS and WEEE directive took effect, and can be a good alternative 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 as soon as possible. It is commonly used for membrane switches, EMI shielding,and aluminum wire bonding.

--Advantage:

ROHS Complaint
Planar
Fine pitch
Cost effective
Good alternative to ENIG
High stability

--Disadvantage

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)

5.ENIG(Electrole ss Nickel Immersion)

--Description:ENIG is quickly becoming the most popular surface finish 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. But ENIG 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.

--Advantage: