Ask the Experts
June 26, 2023 - Updated
May 22, 2013 - Originally Posted

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:
  • Formulated (initial viscosity) plus the metal content, of course. The more powder per unit volume, the thicker the paste will be. Fine powders have more surface area and will present as a more viscous paste.
  • Shear-thinning through squeegee action. Many pastes are thixotropic (e.g., subject to thinning through mechanical perturbation.)
  • Environmental conditions (temperature and humidity.)
  • If an automatic bottom-side wipe is employed, a wet-wipe may slowly contribute solvents to the paste and alter paste viscosity with time. Be careful of this. Dry wipe is generally a better option.
  • Natural dry-out o fthe paste with time will also change rheology.

Gary Freedman
Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.

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.

James DiBurro
Round Rock Consulting
Following a successful 20 year career within world class OEMs and EMS providers including Honeywell, Bull Electronics, IBM, Celestica and Plexus, James founded Round Rock Consulting a business consultancy focused on supporting OEMs and EMS providers with product realization strategies.

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.

Tim Jensen
Product Manager, Advanced Assembly Materials
Indium Corp.
Timothy Jensen is the Product Manager for Indium Corporation's PCB Assembly Materials and has spent over a decade troubleshooting and optimizing SMT process lines. Having worked directly on hundreds of surface mount lines, and building thousands of different products, Tim understands each customer's unique requirements, and tailors Indium's products and services to meet or exceed those needs.

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.

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

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.

Neil Poole
Senior Applications Chemist
Henkel Electronics
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.

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.

Working life
The amount of time solder paste can stay on a stencil without affecting its printing properties. The paste manufacturer provides this value.

Terry Jeglum
Electronic Technology Corporation
Mr. Jeglum has 35+ years experience and is the founder of Electronic Technology Corporation. He is responsible for 22 years of program management for the Company.

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.

David Bao
Director New Product Development
Metallic Resources, Inc
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.

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.

David Bao
Director New Product Development
Metallic Resources, Inc
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.
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