|Ask the Experts|
May 18, 2020
Solder Paste Viscosity
What key factors affect solder paste viscosity? Assuming we only focus on the powder, will more fine powders increase the paste viscosity or the reverse?
|Expert Panel Responses|
Paste viscosity can be dependent on many factors including:
Viscosity, also referred to often as Thixotropy, or slump, is primarily a function of the amount, size and shape of the solid fillers suspended in the medium. As you increase the filler material, (what you are calling Powders), the viscosity will increase, and slump will decrease.
Round Rock Consulting
Solder paste viscosity is impacted by a number of factors. If all other variables are held constant, the viscosity will increase as you decrease the powder size (for example moving from Type 3 to Type 4). This viscosity increase is due to greater packing density of the finer powder (smaller particles can pack more tightly together).
Solder suppliers account for this by adjusting the metal percent. If you are buying a Type 3 solder paste, the metal percent may be something like 89%. When moving to Type 4, the metal percent would probably be 88.5%which would result in approximately the same viscosity as the Type 3.
Product Manager, Advanced Assembly Materials
If you decrease the particle size of the powder while keeping the metal percentage the same, the viscosity will increase. The increase will probably be dramatic. Normally, you specify the alloy, particle size distribution (e.g. Type 4) and viscosity range, and the manufacturer sets the metal percentage to achieve the range.
In paste manufacturing nearly everything affects the final viscosity. As the powder gets finer, it gets more and more difficult to control viscosity. The particle size distribution of the specific lot of powder, the particle shapes, surface condition (texture, oxides...) will all have large effects. Even the manner in which viscosity is tested makes a huge difference.In order for a customer to obtain data that correlates well to manufacturer's data, the testing parameters must be duplicated precisely.
Unless you have a specific need to test and control viscosity and have the capability to correlate your data to the manufacturer's data, attempting to measure and control it may be more effort than it's worth. Functional tests maybe a better avenue.
Increasing the metal loading will drive the viscosity up even with the current PSD, and with the typical metal loading of solder paste relatively small amounts of extra metal (of the order of 1.0%) can significantly increase viscosity.
Reducing the particle size distribution with the same metal load will achieve the same but this approach is probably less sensitive than increasing metal load. There is also more potential for an adverse effect to the reflow performance of the paste by over stressing the flux activation system.
Senior Applications Chemist
In using solder paste for circuit assemblies, one needs to test and understand the various rheological properties of a solder paste.
The degree to which the material resists the tendency to flow. In this case, varying viscosities of solder paste are desired at different levels of shearing force. Such a material is called thixotropic. When solder paste is moved by the squeegee on the stencil, the physical stress applied to the paste causes the viscosity to break down, thinning the paste and helping it flow easily through the apertures on the stencil.
When the stress on the paste is removed, it regains its shape, preventing it from flowing on the circuit board.The viscosity for a particular paste is available from the manufacturer's catalog; in-house testing is sometimes needed to judge the remaining usability of solder paste after a period of use.
The characteristic of a material's tendency to spread after application. Theoretically, the paste's sidewalls are perfectly straight after the paste is deposited on the circuit board, and it will remain like that until the part placement.
If the paste has a high slump value, it might deviate from the expected behavior, as now the paste's sidewalls are not perfectly straight. A paste's slump should be minimized, as slump creates the risk of forming solder bridges between two adjacent lands, creating a short circuit.
The amount of time solder paste can stay on a stencil without affecting its printing properties. The paste manufacturer provides this value.
Electronic Technology Corporation
The factors that affect solder paste viscosity are paste flux, metal load and powder size. Finer powder size will increase both viscosity and tackiness of the solder paste.
Director New Product Development
Metallic Resources, Inc
A simple method to distinguish Leaded and Lead-free solder paste will be a simplified solder ball test (IPC-TM-650). Use stainless steel of plastic stencil (76 mm x 25 mm x 0.1 mm) provided with round hole of about 3 -4 mm diameter.
Print the paste on a frosted glass microscope slide, alumina substrate or glass/epoxy printed circuit board with a thickness of 0.60 to 0.80 mm and a minimum length and width dimension of 76 mm and 25 mm, respectively. Set the temperature of the solder bath or hot plate at a temperature of 208 °C. A tin/lead solder paste will reflow and ball up in a few seconds. Lead free solder paste will not reflow under this condition.
Director New Product Development
Metallic Resources, Inc
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