Viscosity of Solder Paste Before Printing

What is the general recommended viscosity of solder paste before printing.

I.M.

Expert Panel Responses

Short answer... the one that works (four that paste). Long answer we could write a textbook on. In essence, the number we call viscosity can have dramatically different values for the same material, under the same environmental conditions, depending on the test method.

There are several different types of viscometer used, and they all yield different numbers. Further, the paste viscosity is very shear rate dependent, so you need to know the specific shear rate (and equipment configuration) that the manufacturer used. Even worse, because solder pastes are "non-Newtonian" fluids, the viscosity is not the only parameter we need to deal with. Finally, the viscosity that works best for paste A may not be the one that works well for paste B.

In general though, what you should be able to do is to track the lot-to-lot variation in viscosity as reported by the manufacturer, and relate that to either your measured viscosity (which may be quite different) and/or your production performance.

If you see a lack of correlation for a lot, e.g. manufacturer reports a viscosity at mid-spec, you measure low,and your production process shows evidence of defects or process indicators traceable to low-viscosity paste, then you have a reason to question that lot.

Fritz Byle
Process Engineer
Astronautics

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.

Due to the different brands, flux chemistries, test methods and solder paste properties I do not believe there is a specific value for the viscosity of solder paste before printing.

I'd recommend you contact your solder paste vendor. Their technical group will be able to provide you with information based on your specific product.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman

Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

The optimal viscosity of solder paste is normally determined by the solder paste manufacturer. There are occasions where an end user might require a specific viscosity for the solder paste. This is determined through testing in the end user's process, and a viscosity is chosen which helps to optimize the process.

Every solder paste formula is different and most SMT processes can tolerate a range of viscosities. Ideally the viscosity is low enough so that the solder paste can print and release well at higher print speeds (>50 mm/sec), but does not slump excessively after print. Most no clean solder pastes have viscosities in the 500 to 800 Kcps range (Brookfield). Most water soluble solder pastes have viscosities in the 700 to 1000 Kcps range (Brookfield).

Unfortunately there is no hard specification range to apply to the viscosity of solder paste. Each solder paste manufacturer can recommend an optimal viscosity range for their products.

Tony Lentz
Field Applications
FCT Assembly

Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

Solder paste viscosity varies from manufacturer to manufacturer and from product to product. This makes it impossible to make a general recommendation.

Best practice is to follow the paste handling and preparation instructions provided by the manufacturer.

Tim O'Neill
Director of Product Management
AIM

Timothy O'Neill is the Director of Product Management for AIM Solder. AIM Solder is a leading global manufacturer of assembly materials for the electronics industry. Mr. O’Neill has 25 years of industry experience is a Certified IPC Specialist.

Mr. O’Neill’s responsibilities include developing product and technical information; he is a technical writer and presenter for industry trade publications and events and has been recognized as a Speaker of Distinction by the SMTA.

Whatever the viscosity range listed on the Technical Data Sheet for the particular solder paste you are using should be your target. Printing paste while it is outside of the recommended viscosity range can have a huge negative impact on your solder paste performance, and thus your DPMO.

But there is no magic viscosity target range that can be applied to any or all solder pastes; the recommended viscosity range for different pastes varies widely.

It depends on the Type (as in Type 2, 3, 4, or 5), the flux classification, and the particular paste's rheology and formulation and performance characteristics for electronic-grade solder pastes (refer to J-STD-004).

The Type number refers to the particle size, not the flux.

For example, Kester R562 Water Soluble, alloy Sn63, Type 3, can have a different recommended viscosity range than Kester R562 Water Soluble, alloy Sn63, Type 4. This is because there are theoretically more solder fines (and less flux) in the same amount of Type 4 paste as there are in Type 3 paste. Thus the paste manufacturer may have a different optimum viscosity range for the Type 3 particle size than the same exact paste formulation in a Type 4 particle size.

One manufacturer's recommended water soluble paste viscosity can be quite different from another manufacturer's equivalent water soluble paste viscosity. It varies due to the different formulations of the flux. For example, Alpha WS619 water soluble Type 3 solder paste has a completely different chemical thixotropic binder and working rheology than Henkel Loctite XP136 (GC 10). They are radically different pastes.

I am not saying one is better than the other, I am just trying to explain their recommended viscosities are probably very different, and as such their printing properties can vary differently according to how they react to humidity, temperature, number of times sheared back and forth on the stencil, amount of time left on the stencil before printing before replenishment is recommended, etc. There are many variables that affect the paste performance, and thus the manufacturer's recommended viscosity ranges are different for each one.

Solder pastes with different flux formulations can also vary widely. For example, an RMAflux classified as ROL0 can have a radically different recommended viscosity range than a No-Clean flux formulation from the same manufacturer, even though both are Sn63/Pb37 with the same particle size. Likewise, a SAC305 alloy No-Clean flux formulation may have a different recommended viscosity than a SN100C No-Clean from the same manufacturer. While both flux types may be classified as no-clean, their rheological properties could be very different.

So as you can see, the "magic viscosity range" you should be mixing to achieve is whatever is recommended on that particular solder paste's Technical Data Sheet.These are available on-line at every reputable solder paste manufacturer's website.

And finally, every respectable electronics manufacturer (EMS) should have a document outlining all of their solder paste handling procedures, from the point of Purchasing (P.O.requirements for minimum shelf life remaining, how it is to be packaged, how it is to be shipped), Receiving and Receiving inspection (how it is to be immediately refrigerated, steps for testing viscosity, inspection to ensure it is proper Type, proper flux, etc.) storage (refrigerated, FIFO, never returned to refrigeration once removed), and finally Usage (length of time it may be used from date of removal of refrigeration,length of time it should be left out to achieve room temperature before printing, or length of time it should be mixed to achieve both the optimum temperature and viscosity, the time limit a paste can sit out at room temperature before it should be used up or discarded, number of times it can be sheared back and forth on the stencil before it should be replenished with fresh paste, the number of hours it can sit on the stencil before it should be replenished, etc.).

And every person who purchases, receives, inspects, stores, and uses solder paste should be trained and certified to that Paste Handling document.If you don't have this, you don't have control over the paste, and if that is the case the viscosity is the least of your concerns.

And within that document, perhaps attached as an addendum or a table, there should be a list of the solder pastes that you have qualified for use at your company, their Type number, and their recommended viscosity for optimum printing and reflow characteristics. Do this and live by it, and your DPMO will be optimized by how that is controlled more than any other parameter in your assembly process.

Imperfect placement is forgiving, imperfect reflow profiles are somewhat forgiving, but imperfect paste performance due to improper handling can be very, very unforgiving. You don't want to go there!

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Viscosity is a complicated issue with regards to solder paste. Solder paste is usually a shear thinning material, meaning the viscosity changes as different rates of shear are applied. In the paste printing process, several different shear rates are applied to solder paste (squeegee to paste roll, paste to stencil aperture, stencil release, resistance to gravity in preventing slump).

Having said that,the range viscosities of pastes that are commercially viable, when measured at 10 RPM on a Malcolm viscometer are 1100 CPS to 2200 CPS. High speed printing applications tend to require pastes in the lower end of this spectrum. The most popular pastes are in the 1300 to 1900CPS range.

Having viscosity stability is more important than the initial viscosity, as long as it is within the range mentioned above.

Mitch Holtzer
Director of Reclaim Business
Alpha Assembly Solutions

I've been in the soldering materials/applications industry for 25 years. Since joining Alpha, Ive been the global product manager for preforms, wave soldering flux, solder paste and more recently the Director of the soldering materials reclaim business.

That is a really tough question.

Static viscosity tests like Malcom and Brookfield do not wholly predict how a solder paste will act on the stencil. It's a little bit like predicting how fast a car will go based upon the tire size.

Solder pastes shear thin differently depending on the squeegee blade speed and pressure. How the paste acts when shear thinned will ultimately determine how well the stencil apertures fill and the transfer efficiency. A paste that prints poorly at low speed may print great at high speed and vice versa.

Kay Parker
Technical Support Engineer
Indium Corporation

Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.

Solder paste material is a mixture of solder powder, flux and various additives that improve the rheology and other characteristics. Majority of defects in mount assemblies are caused due to the issues in printing process of due to defects in the solder paste. Characteristics of the paste, like viscosity and flux levels, need to be monitored periodically as it would depend on the temperature and humidity on the shop floor.

In order to get a good print, solder paste must flow easily and readily through the openings on the stencil and maintain its shape (slump) after printing. The slump (shape) should be tight to and not loose as it will result in the possibilities of solder bridging. The purpose of flux in the paste is to give the solder paste its cream-like texture which will enable the smooth flow and retain its shape (slump) after the deposit and for the formation of metal joints by ensuring that the metal surfaces are clean.

Viscosity of paste will depend on the type of paste that is being used which is dependent on the type of SMD that are being soldered. The finer pitch SMD will require Type 4 or Type 5 solder paste depending on how fine the pitch is between two pads of a single device.

It is always recommended to stir the paste (preferred method would be to spin the container in a centrifugal spinning unit) to ensure that the solder paste has been mixed well for the flux to have mixed in a homogeneous way.

It's also recommended not to mix old paste off a stencil with existing paste in a jar just removed from the fridge. It's recommended to use an empty container and put the old paste in this container and measure the viscosity prior to use of this paste the next time.

Mahesh V Draksharapu
Independent Consultant

Over 28 years experience in the area of PCB assembly with 10 years working in various EMS companies in the US (Avex, GSS Array Technology now Benchmark, Everex computer Systems), 10 years working in OEM companies in the US (Trillium Test Systems, Intel & Meru Networks) and with Aristos EMS company in Bangalore since 2008. Engineering Degree from BIT, Mesra and Masters in Egg Mgmt from Wichita State University.

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