About PCB Surface Finishes

The main objective of a final PCB surface finish is to create thermal as well as electrical continuity with the circuit and components. 

Final finishes give a surface for the PCB assembly provider to either solder, cable bond, or conductively attach a part pad or result in a pad, opening, or location of a PCB. 

One more use for final finishes is to supply a known call resistance as well as a life cycle for secrets, buttons, or connectors.

There are a number of final finishes being used in the industry today, consisting of:

• ENIG (electroless nickel, immersion gold).

• ENIPIG (electroless nickel, immersion palladium, immersion gold).

• ENEPIG (electroless nickel, electroless palladium, immersion gold).

• ImmAg (immersion silver).

• ImmSn (immersion tin).

• Sulfamate nickel/hard or soft gold (electrolytic nickel/gold).

• HASL (hot air solder progressing).

• SnPb (63/37 tin/lead).

• LF HASL (lead-free).

• OSP (organic solderability preservative).

• Lead (Pb)- Free Finishes.

Lead-free finishes are taken into consideration RoHS compliant (< 0.1% BW of finish, for Pb, cd, or hg) with the solitary exemption of tin/lead HASL. 

The RoHS-compliant finishes consist of the following:

• ENIG.

• ENIPIG.

• ENEPIG.

• ImmAg.

• ImmSn.

• Electrolytic nickel/gold.

• LF HASL.

• OSP.

Lead-free PCBs need that the basic HASL surface finish can not be used. There is still a considerable quantity of discussion on what the lasting surface finishes will be. 

Currently, the immersion silver and OSP surface finishes are the most prevalently defined surface finishes for solderable PCBs. Immersion tin is the widespread surface finish for press-fit backplanes.

PCB Surface Finishes Comparision

Hasl Lead-free

The structure of hasl treatment typically is Tin (63%) as well as Lead (37%), normal densities vary from 30-1500 μ", of which the thaws point has to do with 183 degrees C.

For hasl lead-free, the usual alloys describe Sn/Ag/Cu (SAC), Sn/Cu/Co, Sn/Cu/Ni/ Ge, and so on, of which the densities range from 300-1000 μ". 

The hasl PCBs (lead-free) also can bear a high temperature of 260 degrees C. Hasl lead-free is the predominant PCB surface finish for circuit boards for it is RoHS & WEEE compliant.

Pros:

Low cost.

Commonly appropriate.

Easy handling.

Reworkability.

Superb service life (12+ months).

Visual assessment & testing.

Cons:

Unequal surface (solder bead).

Not good for PTH (high thermal anxiety).

Not good for thick copper boards.

HASL is unfriendly to environment.

ENIG (Immersion Gold)

Two-layer metal layer - ENIG cover the copper utilizing thick, well-electrically nickel-gold, commonly 2-8 μ" Au over 120-240 μ" Ni, in which nickel is deposited on palladium-catalyzed copper surfaces, gold sticks to the nickel-plated location by molecular exchange, which protects the nickel (real solder surface) up until the soldering process, likewise Au provides low get in touch with resistance and great wetting.

ENIG technology responded to the request for the flat surface used in fine pitch assembly and lead-free, despite the fact that it is a little bit more expensive than others, yet no doubt ENIG has currently come to be one of the most commonly-used PCB surface finish for lots of groups, such as cellular phone or power supply.

Pros:

Flat surface outstanding for fine-pitch (BGA, QFP ...).

Lead-free.

Good for PTH.

Lengthy life span (concerning 12 months).

Great call resistance.

Outstanding for thick copper PCBs.

Cons:

Costly.

Black phosphorus pad.

Signal loss at high-frequency.

Electro-magnetic interference.

Not re-workable.

OSP (Organic Solderability Preservative)

On the tidy bare copper surface, a layer of the natural movie is chemically used. 

This film is a water-based natural compound that selectively incorporates copper and offers a natural steel layer to protect the copper surface from corrosion in the typical atmosphere. 

In the subsequent soldering, it will certainly be promptly gotten rid of.

Pros:

Lead-free.

Flat surface, helpful for fine-pitch pads (BGA, QFP ...).

Extremely thin coating.

Can be used together with various other finishes (e.g. OSP+ENIG).

Affordable.

Reworkability.

Cons:

Not good for PTH.

Taking care of Sensitive.

Brief Shelf life (< 6 months).

Not appropriate for kinking modern technology.

Not Good for numerous reflow.

Copper will get revealed at assembly, and requires relatively aggressive flux.

Hard to inspect, and could cause concerns in ICT testing.

ImmSn (Immersion tin)

Immersion tin is a single-layer metallic finish of 15.7 μin to 23.7 μin [0.4 to 0.6 μm] of immersion transferred Sn. * This process layers a slim layer of tin straight in addition to the copper surface. 

The tin creates an incredibly flat surface for mounting of surface mount components with ultra-fine-pitch gadgets. This additionally provides a thicker, uniform surface that supplies lubrication for press-fit pins. 

ImmSn is low-cost and commonly used for the flat surfaces. ImmSn is OK for fine-pitch gadgets with solder mask dams greater than 5 mils [127 μm]

ImmSn is the leading option for press fit pin insertion, for the following reasons:

- Careful handling is required.

- Poor ecological resistance of intermetallic.

- Not terrific for numerous reflow cycles.

* Per IPC-4554 Nominal pad size of 0.060" x 0.060" (1.5 mm x 1.5 mm).

PCB Surface Finish Is the Application Driven

Final finishes are primarily application driven, so there are a number of factors to consider that must belong to any decision to choose a final finish:

- Lead (Pb) tolerant or lead-free (LF) process.

- Shelf life.

- Flatness.

- Lead or ball pitch.

- Wire bondability.

- Lead insertion.

- Solder joint stability.

- Corrosion resistance.

- Potential troubles.

- Cost.

Recognizing the price vehicle drivers in custom PCB manufacturing as well as very early engagement between the producer as well as the developer are essential components that lead to affordable layout success.