Printed Wiring Boards
Do the printed wiring boards have to be lead-free?
Estimates are that 60-70% of printed wiring boards are tin-lead solder coated, usually by hot air solder leveling (HASL). The solder coatings are applied to the copper surfaces of the printed wiring board to preserve solderability and protect the copper conductors from environmental corrosion. To comply with European Restriction of Hazardous Substances (RoHS) Directive 2002/95/EC, tin-lead solder cannot be used.
What lead-free finishes are alternatives for tin-lead finishes?
There are many proposed alternatives for tin-lead finishes, none performing equal to tin-lead. Changing to lead-free is not going to be as easy as just changing to another coating. Some alternatives are: •hot air solder leveling (HASL) with lead-free solder, such as tin-silver-copper, tin-silver, or tin-copper
• organic solderability preservatives (OSP) placed on the copper
• immersion tin or bismuth, applied in thin coatings of 1-2 microns, or silver coatings of less than 0.1 micron
• palladium applied electrolessly directly on the copper, or coating the copper with nickel and then palladium
• electroless nickel coated with immersion gold (ENIG)
What problems might be anticipated with lead-free printed wiring boards?
The least amount of problems would be putting no coating on the copper and trying to keep the copper surface clean and active during storage and assembly. The choice of coatings presents different problems. •Hot air solder leveling (HASL) coatings present a problem common also with tin-lead solder, i.e., non-uniform pads causing problems with placement of surface mount components. Also, the lead-free alternatives do not have the same appearance as tin-lead, generally being dull or grainy.
• Organic Solderability Preservatives (OSP) are not very heat-stable. Though barely being able to withstand one heat of soldering, the OSP coating makes soldering very difficult for a second reflow.
• Immersion tin coatings less than 1-micron directly on copper can result in decreased solderability as the copper-tin intermetallics form. This can lead to the formation of tin whiskers erupting out of the tin coating. A nickel coating under the tin can improve solderability and minimize the tin whisker formation.
• Palladium is deposited on nickel over the copper. This is an expensive coating that is used on components but not often on boards.
• Electroless nickel under immersion gold (ENIG) provides a solderable coating, but is expensive. The gold (about 0.1 micron thick) dissolves instantly in the melted solder, and soldering is done to the underlying nickel. The amount of phosphorous deposited with the nickel determines whether the soldering is going to be reliable or not.
Source: Kester Solder co.