Cored Solder Wire

17 March 2008

Cored solder wire has been used in the electronics industry for many years and is very familiar to everyone from production staff in a factory to the home hobbyist. Cored solder wire is simply a thin wire with flux running through the centre like a stick of rock.

Cored solder wire is used for manual soldering, second stage assembly and, of course, touch-up and repair of solder joints. The core refers to the flux that is provided as one or multiple cores positioned inside the wire during manufacture.

The solder alloy is produced as ingots that are extruded and then passed through a series of dies to reduce the diameter to the required size. The size can range from 0.3 – 2mm. During this process the flux is introduced into the wire, the type of flux will depend on the solder alloy, the quantity will generally depend on the diameter of the wire and the temperatures used during soldering. Cored solder wires are available in a number of solder alloys and the alloy can have an effect on the minimum wire diameter.

Traditionally the most popular tin/lead alloys have consisted of

Sn60/Pb40
Sn63/Pb37
Sn62/Pb36Ag2

[x-head] Lead-free alloys
Most of the better known lead-free solders, as well as those which have been recently developed, fall into the mid-temperature range. Most of these compositions are combinations of the base metal tin with copper, silver, bismuth and antimony. Examples of the most popular of these alloys are shown below.

Sn-Ag Sn-3.5Ag 221°C
Sn-2Ag 221-226°C
Sn-Cu Sn-0.7Cu 227°C
Sn-Cu-Ni Sn-0.7Cu<0.1Ni 227°C
Sn-Ag-Bi Sn-3.5Ag-3Bi 206-213°C
Sn-7.5Bi-2Ag 207-212°C
Sn-Ag-Cu Sn-3.8Ag-0.7Cu 217°C
Sn-Ag-Cu-Sb Sn-2Ag-0.8Cu-0.5Sb 216-222°C

Although their melting temperatures are slightly higher than that of the tin-lead alloy, and therefore they cannot be described as absolute drop-in replacements, it is alloys from this melting temperature range that will be used as general purpose lead-free solders.

Generally there is a choice of five alloy groups from which to select a lead-free replacement. As with tin-lead, each lead-free alloy has particular properties. In the majority of situations two alloy types are being used for the assembly and hand soldering of lead-free products and fall into the following two families: Sn/Cu and Sn/Ag/Cu

Regardless of alloy type the hand soldering operation is conducted as follows. Cored wire is used to wet the soldering iron tip which is term is the bridge between the two surface being joined. When the soldering iron tip is placed in close proximity to the joint surfaces it is the solder that transfers the heat to the joint. When the solder on the tip is seen to start wetting the joint the cored wire is fed into the opposite side of the joint from the soldering iron tip. Prior to the solder wire melting the flux in the core is already flowing out of the wire to help clean the surfaces to be joined and protect the surfaces and solder for oxide formation.

The cost of solder wire is affected by the alloy used, the gauge of the wire, flux type and of course the popularity of the product. If you use a specialised lead-free solder wire you will pay more. Changes to lead-free alloys have affected not only the price but also the manufacturing process steps due to the changes in the hardness of the metal compared to tin/lead. The following are typical wire prices quoted in the industry but will again vary based on the amount purchased and the price variations in the metals markets.

60/40 (Sn/Pb) 183-187°C @ £18.00 per kg
62S (62/2/36) 179°C @ £25.00 per kg
99C (Sn/Cu) 227°C @ £22.00 per kg
SAC305 217-219°C @ £32.00 per kg


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