Lead-Free Printed Circuit Board Surface Finishes

Printed Circuit Board finishes used to be simple consisting of either HASL (hot air solder leveling) or electrolytic nickel / gold. For years lead based finishes dominated the industry but technology and environmental concerns have altered the landscape for manufacturing printed circuit boards and more importantly the use of lead-free finishes used on circuit boards.

RoHS compliance , which took effect July 1, 2006, has established a new set of regulations requiring circuit board manufacturers to use lead-free finishes. There are a variety of lead-free finishes available today that meet RoHS compliance but it is important to understand the pros and cons of each finishing option and which to choose for your application.

Please select from the list below to learn more about lead free circuit board finishes:

• Organic Solderability Preservative – OSP
• Immersion Silver
• Electroless Nickel Immersion Gold – ENIG
• Hard Gold
• White Tin
• Lead-Free Hot Air Solder Leveling – Lead-Free HASL
• Lead Free Finishes - Comparison Chart

Organic Solderability Preservative (OSP's) lead-free finishes are available in different thicknesses, are inexpensive, provide good surface oxidation, coplanarity and are easy to apply. OSPs are limited in the number of heat cycles and some require a nitrogen environment at time of assembly. OSPs have a poor shelf life, degrade with high temperatures, have poor wettability and are not suited for wire bonding or as a contacting surface.

Immersion Silver lead-free finishes offer good wettability, low costs, and are re-workable. Tarnishing is handled with an anti-tarnishing ingredient applied during the silver bath step. Immersion Silver finishes have a good shelf life and must be packed using sulphur-free paper.


Electroless Nickel Immersion Gold - ENIG This finish is by far one of the best lead-free finishes available, but does require proficiency, knowledge and experience to properly fabricate printed circuit boards. Because this finish has special requirements Triangle Circuits has created a separate page for more details on ENIG.

Hard Gold This lead-free finish offers a variety of good properties including wettability, coplanarity, shelf-life as well as offering a good process window and process control. Unfortunately, Gold is a very expensive metal and is often too high priced to be used for some applications, in addition it can crack or break and cannot be reworked.

White Tin This finish is by far one of the best lead-free finishes available, but does require proficiency, knowledge and experience to properly fabricate printed circuit boards. Beeches this finish has special requirements Triangle Circuits has created a separate page for more details

Lead-Free Hot Air Solder Leveling This finish is by far one of the best lead-free finishes available, but does require proficiency, knowledge and experience to properly fabricate printed circuit boards. Because this finish has special requirements Triangle Circuits has created a separate page for more details

The Printed Circuit Board Manufacturing Process:

The process starts once your files and specifications have been sent to Triangle Circuits. Our engineers review each order to ensure design for manufacturability and proper selection of materials and technologies before manufacturing begins.

Patterning | Etching

The majority of printed circuit boards are manufactured by applying a layer of copper over the entire surface of the circuit board substrate either on one side or both sides. This creates what is referred to as a blank printed circuit board, meaning the copper is everywhere on the surface. From here the unwanted areas are removed, this is called a subtractive method, the most common subtractive method is known as photoengraving.

Photoengraving

The photoengraving process uses a mask or photomask combined with chemical etching to subtract the copper areas from the circuit board substrate The photomask is created with a photoplotter which takes the design from a CAD PCB software program. Lower resolution photomasks are sometimes created with the use of a laser printer using a transparency.

Lamination

Many printed circuit boards are made up of multiple layers, these are referred to as multi-layer printed circuit boards. They consist of several thin etched boards or trace layers and are bonded together through the process of lamination.

Drilling

Each layer of the printed circuit board requires the ability of one layer to connect to another, this is achieved through drilling small holes called "VIAS". These drilled holes require precision placement and are most commonly done with the use of an automated drilling machine. Theses machines are driven by computer programs and files called numerically controlled drill or (NCD) files also referred to as excellon files. These files determine the position and size of each file in the design.

Some files require very small vias to be drilled which results in heavy wear and tear of the drill bit itself. Drilling through different substrates may require the drill bit to be made of tungsten carbide and are costlier than other materials but required to provide a proper hole.

Controlled depth drilling can be used to drill just one layer of the circuit board rather than drilling through all the layers. This can be accomplished by drilling the individual sheets or layers of the PCB prior to lamination.

• Blind Vias : Is when the holes connect a layer to the outside surface
• Buried Vias : Is when the holes only connect interior layers and not to the outside surface.

The walls of each hole (for multi-layer boards) are copper plated to form plated-through holes that connect the conductive layers of the printed circuit board.

Solder Plating | Solder Resist

Pads and lands which will require components to be mounted on are plated to allow solderability of the components. Bare copper is not readily solderable and requires the surface to be plated with a material that facilitates soldering. In the past a lead based tin was used to plate the surfaces, but with RoHS compliance enacted newer materials are being used such as nickel and gold to both offer solderability and comply with RoHS standards.

Areas that should not be solderable are covered with a material to resist soldering. Solder resist refers the a polymer coating that acts as a mask and prevents solder from bridging traces and possibly creating short circuits to nearby component leads.

Silk Screen

When visible information needs to be applied to the board such as company logos, part numbers or instructions, silk screening is used to apply the text to the outer surface of the circuit board. Where spacing allows, screened text can indicate component designators, switch setting requirements and additional features to assist in the assembly process.

NOTE: "Red Print" refers to the silk screening of a one sided printed circuit board.

Testing

Unpopulated circuit boards are subjected to a bare board test where each circuit connection (as defined in a netlist) is verified as correct on the finished circuit board. In high volume circuit board production, a bed of nails tester or fixture is used to make contact with the copper lands or holes on one or both sides of the board to facilitate testing. Computers are used to control the electrical testing unit to send a small current through each contact point on the bed of nails and verify that such current can be detected on the appropriate contact points.

For small to medium volume production runs, a flying probe tester is used to check electrical contacts. These flying probes employ moving heads to make contact with the copper lands and holes to validate the electrical connectivity of the board being tested.