Contrast on Soldering Technologies Used in Lead and Lead-Free Wave Soldering

Wave soldering plays a decisive role in fixing components on Printed Circuit Board during PCB Assembly process. As manufacturing technology gradually goes upgrading and people's environmental protection awareness takes off, wave soldering is further classified into lead wave soldering and lead-free wave soldering. Content difference will definitely bring about differences in terms of manufacturing technologies just in order to ensure optimal quality. As a result, it's of much significance to get aware of difference between soldering technologies used in lead and lead-free wave soldering.

Solder Content Contrast

• Commonly-used solder in lead wave soldering is

a. Eutectic solder: Sn37Pb. It performs as a single homogeneous phase whose melting temperature is 183°C, mixed by two phases (Sn and Pb) with different melting points.

b. Solder: Sn36Pb2Ag. A little Ag is mixed in SnPb solder for the following two aims:

Aim#1: It can decrease solder's melting temperature such as Sn36Pb2Ag whose melting temperature is 179°C, improve diffusivity and soldering strength and make solder joints beautifully shining. This type of solder works suitably for quartz crystal units, ceramic devices, thermistor, thick-film devices, Integrated Circuits (ICs) and components with Ag plated on.

Aim#2: To stop solder and Ag from mutual diffusion on base metal, Ag needs to be added to solder in advance. As a result, Ag can be stopped from diffusion on ceramic and mica and Ag layer can't be peeled off, which is the leading application feature of this type of solder.

• Commonly-used solder in lead-free wave soldering is

a. Alloy Sn3.0Ag0.5Cu (briefed as SAC305). It is right now an element that is most widely applied in industries with its melting temperature range from 217°C to 220°C. Sn3.8Ag0.7Cu (briefed as SAC387) is a monocrystal element of SnAgCu alloy with a melting temperature of 217°C.

b. Alloy Sn0.7Cu. As a monocrystal element of SnCu-serial alloy, Sn0.7Cu features its melting temperature of 227°C which is 9°C higher than that of SAC305. Therefore, when soldering temperature exceeds 250°C, it won't be suitable for reflow soldering any more.

Process Window for Wave Soldering

• Lead Wave Soldering

a. About eutectic solder of Sn37Pb:

1). Wave soldering temperature. When it comes to the soldering temperature of Sn37Pb eutectic alloy, it should be 37°C higher than melting temperature. Thus, theoretical soldering temperature is 220°C (183°C plus 37°C).

Soldering temperature is not equivalent to that of solder bath in soldering process. During the overall process ofwave soldering, melting temperature is inter-temperature between solder bath temperature and soldered working temperature. To ensure excellent wettability of solder, after it reaches the lowest wettability temperature, solder bath temperature has to be further improved to approximate 250°C to compensate for other thermal loss so that thermal balance can be achieved in wave soldering.

2). Wave soldering time . To get optimal wave soldering effect, a solder joint should be soaked in wave solder for 2 to 4 seconds.

b. When it comes to solder of Sn36Pb2Ag, its wave soldering process window can be set with Sn37Pb solder as a reference.

• Lead-Free Wave Soldering

Temperature selection of lead-free wave soldering is a significant method to overcome insufficient wettability of lead-free solder. In accordance with the best wettability temperature range during wave soldering, it's a usual case that temperature should be selected which is 50°C higher than highest melting temperature. Accordingly, the usually-used recommended lead-free solder processing window is depicted as below for best wettability capturing.

a. About solder of SAC305:

1). Wave soldering temperature: 250°C to 260°C

2). Wave soldering time: recommended time period is 3 to 5 seconds

b. About alloy Sn0.7Cu:

1). Wave soldering temperature: 260°C to 270°C

2). Wave soldering time: equivalent to SAC305