Choosing Solder Paste
Last updated
Was this helpful?
Last updated
Was this helpful?
Alloys, Flux, T4, T5, Wetting - Not sure what this all means? Here's a crash course on solder paste.
We currently offer three different pastes in our store:
When looking at any solder paste there are 3 main properties you want to consider. The alloy of the solder paste, the flux used, and the particle size of the metal in the solder.
Solder powder can be made up of a variety of alloys, but the two most common are composed of a tin-lead combination or a lead-free combination. Both of these alloys contain a large amount of tin, which has a low melting point and high tensile and shear strength. The mechanical properties of the alloy can be further modified through the addition of other metals.
With our solder pastes, we offer both of the alloys mentioned above. Our tin-lead combination is listed as Sn63Pb37. Breaking this label down it means that the alloy is 63% Tin and 37% Lead. The best use case for this alloy is printing solder pads onto traditionally manufactured PCBs for the purpose of reflow. This alloy provides a strong bond and is capable of handling higher heats than our lead-free alloy. This alloy is not recommended for PCBs printed on the V-One however, the lead does not play nicely with the printed silver traces. One other thing to consider for this alloy is the need for proper ventilation since it includes lead.
Our lead-free alloy is listed as Sn42Bi57.6Ag0.4. Breaking this label down it means that the alloy is 42% Tin, 57% Bismuth, and 0.4% Silver. We went with this alloy because the inclusion Bismuth provides a much lower reflow temperature than other alloys. This provides the ideal working temperature to play nicely with our printed silver traces. The downside of this alloy is that it can be a bit tricky to rework. The lower working temperature takes some getting used to, so a precise soldering iron is needed and a very gentle touch.
Flux serves a variety of purposes in solder paste, but the two big functions it serves is as a flowing agent and a stabilizer. We won’t cover too much on this topic because all of our solder pastes use a rosin based flux. Rosin flux has excellent soldering performance, a long working time before reflow is needed, and it’s also less aggressive than water soluble based fluxes causing less corrosion.
Solder paste can be made with a variety of solder powder sizes. Solder powder sizes are classified by Type in the IPC standard J-STD-005 Requirements for Soldering Pastes and range from Type 1 as the largest powder size to Type 8 as the smallest powder size. This table is an overview of the size ranges:
IPC Category
Powder Size (microns)
Type 1
150-75μ
Type 2
75-45μ
Type 3
45-25μ
Type 4
38-20μ
Type 5
25-15μ
Type 6
15-5μ
Type 7
11-2μ
Type 8
8-2μ
With our Pastes you’ll notice that we have selected Type 4 and Type 5 pastes. For most use cases with printed circuits produced on the V-One, Type 4 (T4 Solder Paste Sn42Bi57.6Ag0.4) is the perfect size for solder. It works well for the size of components we typically use and reflows well on printed pads.
The Type 5 pastes would be for more specialized situations. This could be that a smaller nozzle size is needed for higher precision or smaller components are being used. There are some issues that may arise when using Type 5 pastes, this is due to the increased surface area exposed on the smaller sized powder. The main problem you may encounter is they can generate more random solder balling and graping.
For boards printed on the V-One the typical solder choice is the T4 Solder Paste Sn42Bi57.6Ag0.4 unless you have very small components, then you may want to consider the T5 Solder Paste Sn42Bi57.6Ag0.4.
For traditionally manufactured PCBs with a HASL finish you may want to consider the T5 Solder Paste Sn63Pb37, especially if you may need to do any rework on the board.