Time：2016-01-20  Back

Anyone involved in the PCB industry understands that PCBs have copper finishes on their surface and, if left unprotected, 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 PCB. Hot air solder leveling (HASL) was once the tried and true method of deliver consistent assembly results. However, 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 the need for a thin coating became more critical

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. Here are some of the more common surface finishes used in PCB manufacturing, along with key advantages and disadvantages for each.

Figure 1: Printed circuit board with HASL/lead-free HASLs surface finish.

HASL/Lead-free HASL

HASL is the predominant surface finish used in the PCB 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’ that blow hot air across the surface of the board. One of the unintended benefits of the HASL process is that it exposes 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 PTHs

(Plated through holes)

Figure 2: PCB with immersion tin surface finish.

Immersion Tin

According to IPC, immersion tin (ISn) is a metallic ?nish 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 ?nish. The negative effects of tin whiskers growth are well described in industry related literature and the focus 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

Figure 3: PCB with OSP/Entek surface finish.

OSP/Entek

Organic solderability preservative (OSP) 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

organo-metallic 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 consuming substantially more energy.

Advantages:

? Flat surface

? No Pb

? Simple process

? Re-workable

? Cost effective

Disadvantages:

? No way to measure thickness

? Not good for PTH

? Short shelf life

? Can cause ICT Issues

? Exposed Cu on final assembly

? Handling sensitive

Figure 4: PCB with ENIG surface finish.

Electroless Nickel Immersion Gold (ENIG)

ENIG is a two-layer metallic coating of 2–8μin Au over 120–240 μin Ni. The nickel is the barrier to the copper and is the surface to which the components are actually soldered to. The gold protects the nickel during storage and also provides the low contact resistance required for the thin gold deposits. ENIG is now arguably the most used finish in the PCB industry due the growth and implementation of the RoHs regulation.

Advantages:

? Flat surface

? No Pb

? Good for PTH

? Long shelf life

Disadvantages:

? Expensive

? Not re-workable

? Black pad/black nickel

? Damage from ET

? Signal loss (RF)

? Complicated process

Figure 5: PCB with gold/hard gold surface finish.

Gold/Hard Gold

Hard electrolytic gold consists of a layer of gold plated over a barrier coat of nickel. Hard gold is extremely durable, and is most commonly applied to high-wear areas such as edge connector fingers and keypads. Unlike ENIG, gold’

s thickness can vary by controlling the duration of the plating cycle, although the typical minimum values for fingers are 30 μin gold over 100 μin nickel for Class 1 and Class 2, and 50 μin gold over 100 μin nickel for Class 3. Hard gold is not generally applied to solderable areas, because of its high cost and its relatively poor solderability. The maximum thickness that IPC considers to be solderable is 17.8 μin, so if this type of gold must be used on surfaces to be soldered, the recommended nominal thickness should be about 5–10 μin.

Advantages:

? Hard, durable surface

? No Pb

? Long shelf life

Disadvantages:

? Very expensive

? Extra Processing/labor intensive

? Use of resist/tape

? Plating/bus bars required

? Demarcation

? Difficulty with other surface finishes

? Etching undercut can lead to slivering/

flaking

? Not solderable above 17 μin

? Finish does not fully encapsulate trace

sidewalls, except in finger areas

Conclusion

It is important to select the appropriate surface finish for your project by considering the various options while factoring in performance requirements and material costs. For example, if you are looking for the lowest cost, then tin-lead HASL might seem like a good choice, but it is not suitable for RoHS-compliant products. If your product does require RoHS, you might consider lead-free HASL. But that is only if there are no fine pitch components, since LFHASL cannot be applied perfectly flat. If your design needs to be RoHS-compliant and uses fine-pitch components, then you’ll need to select a flat, lead-free finish, such as immersion silver or ENIG. Bear in mind that doing so will necessitate the use of more costly high-temperature laminate. Unsure of what you will need? Consult with a PCB fabricator prior to making a selection. This will ensure that the combination of the surface finish and material will result in a highyielding, cost-effective design that will perform as expected.

Post by: pcb007