Anyone involved within the printed circuit board (PCB) industry understand that PCBs have copper finishes on their surface. If they are left unprotected then the copper will oxidize and

deteriorate, making the circuit board unusable. The surface finish forms a critical interface between the component and the PCB. The finish has two essential functions, to protect the exposed copper circuitry and to provide a solderable surface when assembling (soldering) the components to the printed circuit board.

Hot Air Solder Leveling (HASL) was once the tried and true method of deliver consistent assembly results. However, the ever-increasing circuit complexity and component density has stretched the capabilities of even horizontal solder levelling systems to their limits.

As component pitches became finer and a need for a thin coating became greater, HASL represented a process limitation for PCB manufacturers. As an alternative to HASL, alternative coatings have been around for several years now, both electrolytic and immersion processes.

Listed below are some more common surface finishes used in PCB manufacturing.

HASL / Lead Free HASL

HASL is the predominant surface finish used in the industry. The process consists of immersing circuit boards in a molten pot of a tin/lead alloy and then removing the excess solder by using ‘air knives’, which blow hot air across the surface of the board.

One of the unintended benefits of the HASL process is that it will expose the PCB to temperatures up to 265°C which will identify any potential delamination issues well before any expensive components are attached to the board.

Advantages:

Low Cost

Widely Available

Re-workable

Excellent Shelf Life

Disadvantages:

Uneven Surfaces

Not Good for Fine Pitch

Contains Lead (HASL)

Thermal Shock

Solder Bridging

Plugged or Reduced PTH’s (Plated Through Holes)

Immersion Tin

According to IPC, the Association Connecting Electronics Industry, Immersion Tin (ISn) is a metallicfinish deposited by a chemical displacement reaction that is applied directly over the basis metal of the circuit board, that is, copper. The ISn protects the underlying copper from oxidation over its intended shelf life.

Copper and tin however have a strong affinity for one another. The diffusion of one metal into the other will occur inevitably, directly impacting the shelf life of the deposit and the performance of the finish. The negative effects of tin whiskers growth are well described in industry related literature and topics of several published papers.

Advantages:

Flat Surface

No Pb

Re-workable

Top Choice for Press Fit Pin Insertion

Disadvantages:

Easy to Cause Handling Damage

Process Uses a Carcinogen (Thiourea)

Exposed Tin on Final Assembly can Corrode

Tin Whiskers

Not Good for Multiple Reflow/Assembly Processes

Difficult to Measure Thickness

OSP / Entek

OSP (Organic Solderability Preservative) or anti-tarnish preserves the copper surface from oxidation by applying a very thin protective layer of material over the exposed copper usually using a conveyorized process.

It 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 finishes, which suffer from either being more toxic or substantially higher energy consumption.

Advantages:

Flat Surface

No Pb

Simple Process

Re-workable

Cost Effective

Disadvantages:

No Way to Measure Thickness

Not Good for PTH (Plated Through Holes)

Short Shelf Life

Can Cause ICT Issues

Exposed Cu on Final Assembly

Handling Sensitive