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February 28, 2018

Viscosity of Solder Paste Before Printing

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


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 callviscosity can have dramatically different values for the same material, underthe same environmental conditions, depending on the test method.

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

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

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

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.

Due to the different brands,flux chemistries, test methods and solder paste properties I do not believethere is a specific value for the viscosity of solder paste before printing.I'd recommend you contact your solder paste vendor. Their technicalgroup will be able to provide you with information based on your specificproduct.

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.

Theoptimal viscosity of solder paste is normally determined by the solder pastemanufacturer. There are occasions where an end user might require aspecific viscosity for the solder paste. This is determined throughtesting in the end user's process, and a viscosity is chosen which helps tooptimize the process.

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

Unfortunatelythere is no hard specification range to apply to the viscosity of solderpaste. Each solder paste manufacturer can recommend an optimal viscosityrange 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 manufacturerand from product to product. This makes it impossible to make a generalrecommendation. Best practice is to follow the paste handling and preparationinstructions provided by the manufacturer.

Tim O'Neill
Technical Marketing Manager
Tim O'Neill is the Technical Marketing Manager for AIM Products. AIM is a global supplier of materials for the PCB assembly industry including solders, fluxes and thermal management materials. Tim has a B.A. from Assumption College and post-graduate studies in education. He has 20 years of experience in the electronics soldering industry, beginning his career in 1994 with EFD and was key in business development of their fine pitch solder paste dispensing technology. Tim joined AIM in 1997 and has since assisted many clients with assembly challenges, specializing in Pb-Free process development and material selection.

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

Butthere is no magic viscosity target range that can be applied to any orall solder pastes; the recommended viscosity range for different pastes varieswidely.

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

TheType number refers to the particle size, not the flux.

Forexample, Kester R562 Water Soluble, alloy Sn63, Type 3, can have adifferent recommended viscosity range than Kester R562 Water Soluble, alloySn63, 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 3paste. Thus the paste manufacturer may have a different optimum viscosity rangefor the Type 3 particle size than the same exact paste formulation in a Type 4particle size.

Onemanufacturer's recommended water soluble paste viscosity can be quite differentfrom another manufacturer's equivalent water soluble paste viscosity. It variesdue to the different formulations of the flux. For example, Alpha WS619 watersoluble Type 3 solder paste has a completely different chemical thixotropicbinder and working rheology than Henkel Loctite XP136 (GC 10). They areradically different pastes.

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

Solderpastes with different flux formulations can also vary widely. For example, an RMAflux classified as ROL0 can have a radically different recommended viscosityrange than a No-Clean flux formulation from the same manufacturer, even thoughboth are Sn63/Pb37 with the same particle size. Likewise, a SAC305 alloyNo-Clean flux formulation may have a different recommended viscosity than aSN100C No-Clean from the same manufacturer. While both flux types may beclassified as no-clean, their rheological properties could be very different.

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

Andfinally, every respectable electronics manufacturer (EMS) should have adocument outlining all of their solder paste handling procedures, from thepoint of Purchasing (P.O.requirements for minimum shelf life remaining, how itis to be packaged, how it is to be shipped), Receiving and Receivinginspection (how it is to be immediately refrigerated, steps for testingviscosity, inspection to ensure it is proper Type, proper flux, etc.) storage(refrigerated, FIFO, never returned to refrigeration once removed), and finallyUsage (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 beforeprinting, or length of time it should be mixed to achieve both the optimumtemperature and viscosity, the time limit a paste can sit out at room temperaturebefore it should be used up or discarded, number of times it can be shearedback 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 usessolder 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 isthe case the viscosity is the least of your concerns.

Andwithin that document, perhaps attached as an addendum or a table, there shouldbe 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 reflowcharacteristics. Do this and live by it, and your DPMO will be optimized by howthat is controlled more than any other parameter in your assembly process.

Imperfectplacement is forgiving, imperfect reflow profiles are somewhat forgiving, butimperfect paste performance due to improper handling can be very, veryunforgiving. 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.

Viscosityis a complicated issue with regards to solder paste. Solder paste isusually a shear thinning material, meaning the viscosity changes as differentrates of shear are applied. In the paste printing process, severaldifferent shear rates are applied to solder paste (squeegee to paste roll,paste to stencil aperture, stencil release, resistance to gravity in preventingslump).

Having said that,the range viscosities of pastes that are commercially viable, when measured at10 RPM on a Malcolm viscometer are 1100 CPS to 2200 CPS. High speedprinting applications tend to require pastes in the lower end of this spectrum. The most popular pastes are in the 1300 to 1900CPS range.

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

Mitch Holtzer
Global Director of Customer Technical Support
Alpha Assembly Solutions
As the Global Director of Customer Technical Service (CTS) for Alpha, Mitch sets direction and provides coordination for the Alpha CTS group in a global capacity. A major focus of this position is to provide strategic support to OEM, CEM and Automotive customers and target accounts. Mitch joined Alpha in 1998 and has progressed through positions of increasing responsibilities in Marketing, Product Management and R&D. He is a graduate of Purdue University with a degree in Chemistry and holds an MBA from Temple University.

That is a really tough question. Static viscosity tests like Malcom and Brookfield do not whollypredict how a solder paste will act on the stencil. It's a little bit likepredicting how fast a car will go based upon the tire size. Solder pastes shear thin differently depending on the squeegeeblade speed and pressure. How the paste acts when shear thinned will ultimatelydetermine how well the stencil apertures fill and the transfer efficiency. Apaste that prints poorly at low speed may print great at high speed and viceversa.

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

In order to get a good print, solder paste mustflow easily and readily through the openings on the stencil and maintain itsshape (slump) after printing. The slump (shape) should be tight to andnot loose as it will result in the possibilities of solder bridging. Thepurpose of flux in the paste is to give the solder paste its cream-like texturewhich will enable the smooth flow and retain its shape (slump) after thedeposit and for the formation of metal joints by ensuring that the metalsurfaces are clean.

Viscosity of paste willdepend on the type of paste that is being used which is dependent on the typeof SMD that are being soldered. The finer pitch SMD will require Type 4 or Type5 solder paste depending on how fine the pitch is between two pads of asingle device.

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

It's also recommended not to mix old paste off a stencil withexisting paste in a jar just removed from the fridge. It's recommended to usean empty container and put the old paste in this container and measure theviscosity 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.