What is lead-free soldering?

Why has lead-free solder become mainstream?

In the 1990's, a problem was revealed that lead (Pb) contained in soldering materials (such as Sn-Pb) of waste electronic components was melted away by acid rain and contaminated groundwater. Since then, the study of lead-free solder has been pursued. Since the year 2000, under the leadership of major electrical machinery manufacturers, lead-free solder has begun to be put into practical use. The use of lead-free solder has become mainstream due to the increasing global trend in environmental issues such as the RoHS directive (the restriction on the use of hazardous substances which will be enforced by the European Union starting July 1, 2006).
Green ProjectGreen Project - HAKKO's initiatives in environmental issues

What is difficult about using lead-free solder?

Let us consider the characteristics of lead-free solder.

1. Poor solderability

Lack of lead (Pb) decreases the flowability of solder, resulting in poor solderability. This is problematic not only for targeting P.W.B. or components, but also for the tips of soldering irons.

Satisfactory Good solderability Dissatisfactory Poor solderability
Good solderability
Poor solderability

2. Higher melting point

In general, the melting point of lead-free solder is 20℃ to 45℃ higher than conventional eutectic solder. (A popular type contains approximately 40% lead.)

Assume, for example, that a soldering iron is set to a temperature of 340℃ to use eutectic solder (Sn-Pb). If the eutectic solder is replaced with lead-free solder (Sn-0.7Cu), the soldering iron must be set to around 380℃, which is close to the maximum temperature for soldering. Such a high temperature also causes shorter tip life (due to oxidation or erosion), carbonization of the flux in the solder, and splashing of the flux and solder. (It is said that using lead-free solder decreases the tip life by 4 to 5 times compared with eutectic solder.)

As well, many recently-developed devices are heat-sensitive. There are cases in which the high melting point affects not only the iron tips but also devices and P.W.B.

Eroded iron tip Flux and solder spatter
Eroded iron tip Flux and solder spatter

Then, in actual use, what problems are occurring at the site of soldering?
Soldering problems reported after the change to lead-free solder are:
・Feeding of improper amount of solder
・Overheating and cracking
・Solder coming off
・Resin joint or protrusion icicle

Moreover, additional cost is required because iron tips must be replaced more frequently.

Satisfactory Good soldering
Satisfactory Good soldering
insufficient solder too much solder overheating  cracking
× Insufficient solder × Too much solder × Overheating × Cracking
Solder coming off Bridges Resin joint Protrusion icicle
× Solder coming off × Bridges × Resin joint × Protrusion icicle

 We also show the example of failure soldering here.

What is the solution for successful soldering using lead-free solder?