Ask the Experts
February 6, 2019 - Updated
November 19, 2014 - Originally Posted

Solder Paste Prep Before Use

We use lead free solder paste and store it in refrigeration at 4-5 C. After removal from the refrigerator, we let the jar sit at room temperature for 1.5 - 2 hours. We then stir the paste with chemically resistive plastic spatula, for 1.5 minutes by hand. Is this a good practice?

How often should we clean the stencil printer, we now clean it every 2 hours?


Expert Panel Responses

S.S.for storage and preparation of the solder paste you should always refer to thebest practices defined by your solder paste manufacturer. Inregards to your question on frequency of cleaning the stencil printer I willattempt to answer. During the production cycle you should be performing anunder-stencil wipe. The cleaning frequency usually varies according to product,maybe ranging from 10 to 20 prints, averaging around 12 to 15. Many SMTprinters have this option to automatically clean under the stencil. Tocompletely remove and clean the stencil depends once again depends on thesolder paste used, product built and production cycle. Everytwo hours seems excessive but if it works for you and you have no miss-prints Iwould stay with the current routine. Manyfacilities schedule their cleaning by the recommended stencil life of thesolder paste, once again defined by the solder paste manufacturer. However keepin mind that the type of paste used and even environmental conditions will befactors to consider. Asan example here are the parameters of a common no-clean, halogen free, leadfree solder paste manufactured by Indium Corporation;Solder Paste Stencil Life: >8hrs. @30-60%RH& 22-28C Lastly,remember this, solder paste is like a can of paint. You can be very carefulwhere you apply it but you will find it in areas you had no intentions ofputting it.Thecleanliness and condition of your stencil printer (and SMT line) is a directreflection of your companies process quality and the attention and care youshow to your customers.

Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.

Solderpastes are all manufactured for their specific purpose and use. I recommend youfollow yoursolderpaste manufacturer's instructions for the storage, use and stencil life for theprecise paste type you are using.

Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.
Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.

Inorder to determine if your warm-up time after removal from refrigeration isadequate, insert a temperature probe into the center of the undisturbed pastein the jar after the 1.5-hour warm-up. The temperature should be very close tothe temperature of the surrounding room (within about two to three degrees F).If it is not, then you can:
  • Extend the warm-up time
  • Use forced-air circulation (don't use heated air) to increaseheat transfer and shorten the warm-up time
  • Use a circulating water bath that immerses just the bottom ofthe jar
Thelast method will result in the fastest warm-up, but is not something you wantto do if you don't have to. The forced-air warm-up can be very effective.Withregard to stirring the paste, this will be a benefit with some pastes, howeverit also inevitably incorporates air into the paste, which is not good. Yourpaste manufacturer can provide guidance on whether stirring is required for theparticular formulation. If it is not required, don't do it. In any case,minimize it. Finally,there are two cleaning frequencies that are important:
  1. Frequency ofunder-stencil wipe.
  2. Frequency of completecleaning of stencil, including removal of old paste, cleaning, and applyingfresh paste.
Therequired frequency for under-stencil wipe varies with the stencil geometry, PWBcharacteristics and specific paste used. The idea is to wipe often enough toprevent the bottom of the stencil from being contaminated by paste, whileavoiding wiping too often. For a well-designed process, often an under-stencilwipe every five to ten prints is sufficient. Occasionally, wiping every otherprint may be required. If frequent wiping is required, other processoptimizations may help to reduce the required frequency. Thefrequency with which the entire stencil is cleaned and the paste replaced is afunction of the specific paste. Most modern pastes have working lives of up to8 hours, and so it may not be necessary to completely clean the stencil morethan this often. I should caution you here, however, that your specificexperience has a lot to do with your plant floor environment (temperature,humidity). A paste that functions well for 8 hours in one environment may notsurvive 4 hours in another. Keep in mind that it's the environment within theprinter that really counts.

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.

I'drecommend more time out of the refrigerator, say closer to 4 hours minimum tomake sure the entire jar gets to room temperature. Stirring it is a goodpractice, the spatula you mention is most likely fine, but we used metal puttyknives and have had no problems. Be careful when stirring, you want toeliminate air bubbles, not create them. Asfor how often you clean your printer, I am assuming you mean the stencil.Best practice in my experience is about 1 time per 10 prints for normal(non-fine-pitch devices) stencils. Once you get to fine pitch apertures in yourstencil, you should clean much more often. Roughly around 1 time per 3-4prints, or even as often as every other print. It really is up to you, I'dwatch your prints and determine how they look to set up your cleaning frequencyrequirements. I'd definitely go by prints though, not time. If you are using acleaning solution to clean with, I'd recommend making sure you do several"dry-wipes" after each "wet-wipe" to ensure no chemistry is left over on yourstencil. Once you are done with that particular run, it's best practice tothoroughly clean the stencil. An ultrasonic cleaner is a really good way toensure you get the best cleaning.

T.J. Hughes
Manufacturing Engineer
Esterline Interface Technologies
Mr. Hughes has been in the electronics manufacturing field for 20 years. Operating the processes and as a manufacturing engineer for the last 14 years. He is also a CIT as well as an SMTA Certified Process Engineer.

Refrigerated storage is always preferable to room temperaturestorage as it ensures the paste is stored in a controlled environment andlengthens shelf life. The issue with refrigeration is that the pasteneeds to return to ambient temperature prior to use without additionalheat. This requires advance planning to ensure there is sufficientconditioned material to meet production requirements. Your stated warm uptime is insufficient based on AIM testing. The absolute minimum timeregardless of package type (500g jar/cartridge) is five (5) hours to ensure theentire contents of the paste are at ambient temperature, our TDS recommendseight (8) hours. Using cold paste will:
  1. Promotecondensation on the paste. As you can imagine, adding water to a mix ofmetal and acids will cause unintended print and reflow performancevariation.
  2. As the paste warms, the viscosity will drop and inputs willhave to be made to the printer to accommodate these changes.
Ourrecommendation is to simply remove paste the night or shift prior to use.

Tim O'Neill
Director of Product Management
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.

It may be a good idea tocheck the temperature of your paste with a good thermometer, before beginningyour stirring process (and in the future, know how long it takes to equal roomtemperature, before opening the jar). Since condensation will always formif there is a temperature difference between the paste and the outsidetemperature, you may wish to determine the time it takes for the paste to reachroom temperature before breaking the seal on the jar. Otherwise, watermay collect on your paste - and affect the reliability of your soldering.We all must remember how small the solder joints are and the effect ofeven the tiniest water droplet which would boil off as steam in the oven.

Mark J. Curtin
Transition Automation, Inc.
Mr. Curtin is the founder and president of Transition Automation, Inc. Their main product is Permalex Edge Metal Squeegees. They also sell the PrinTEK Ultra-Fine Pitch stencil printers. His background includes 25 years of designing squeegees and SMT printers. He is closely familiar with the SMT printing process, squeegee design, and all aspects of in-line printing machines, double-sided tooling, stencils and solder paste performance.

Youwill know if 1.5 to 2 hours is enough by checking the inside temperature of thejar by introducing a thermometer into the solder paste looking for a rangebetween 19-25C. Stirring the solder paste for 1.5 minutes after itreaches room temperature is a good practice to ensure paste homogeneity. Do you mean cleaning of the stencil printer or the stencil? For the stencil, itdepends on volume and aperture sizes. I would need a little moreinformation on your printer but as long as your PCB's are coming out clean withperfect solder deposition on all pads (especially the fine pitch ones) you maydecrease or increase your cleaning interval.

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.

Below are the general guidelines to beused for handling solder pastes. The product Technical Bulletin should alwaysbe referred to for verification of any specific recommendations or conditions.You should be able to get this from your solder paste manufacture. SHIPPING
  • Solder paste products are engineered tobe shipped between 0-29C (32-84F)
  • Long term storage of solder paste isbest achieved by refrigeration 0-10C (32-50F). The material should be placedin a storage area designed to maintain this temperature range (refrigerator orcold room) immediately upon receipt.
  • Typical no-clean solder paste shelf lifeis 6 months, water-soluble paste ranges from 3-6 months in a refrigeratedenvironment. Verify product Technical Bulletin for exceptions. Please noteshelf life is determined from the date of manufacturing.
  • Cartridges are best stored vertically,tip down. If stored horizontally, best practice is to turn cartridges 180 degrees onceevery week.
  • Solder paste should never bestored at room temperature 19-25C (66-77F) for prolonged periods of time.Room temperature stability is intended to provide manufacturing flexibilityafter storing the product. The typical room temperature shelf life of unopenedpaste is 2 weeks. Please refer to the specific TB for any exceptions to thisrecommendation.
  • Exposure to temperature above 29C(84F) will decrease the useful life of paste, meaning shorten its shelf life.
  • Paste should always be used on a FirstIn First Out (FIFO) basis. To maintain optimum performance paste should not bestored outside the refrigerator any longer than necessary (never more than four(4) days).
  • Solder paste should be allowed to reachroom temperature, 19-25C (66-77F), without forced heating or machine mixer.We recommend a typical period of 3-4 hours out of refrigeration, depending onpackaging size. The specified viscosity of each paste is based on measurementat 25C.
  • Best practices for solder past printingare to maintain the temperature inside the printer between 22C - 29C, 40% -60% RH. Please consult the product Technical Bulletin for pastes that mayoperate outside of this window.
  • For jar packaging, manually stir thesolder paste with spatula for 30 - 60 seconds to ensure paste homogeneity.Rotating/centrifugal force mixing equipment is not recommended. If arotating/centrifugal force mixing equipment is used, a maximum time of 1 minuteat 300 RPM is recommended.
  • Apply an even paste bead on the stencilover the length of the squeegee with a diameter of approximately 12 mm (1/2").Replenish when paste bead is <12 mm (1/2") and replace the material whenexceeded stencil life or has been exposed to high temperatures inside theprinter >29C (84F). If the solder paste is designed for printing attemperatures over 29C, it will be indicated on the technical bulletin.
  • Refer to product Technical Bulletin forroom temperature stability life.
  • Unopened paste jars that have beenexposed to 25C (77F) for 4 days may be returned to the refrigerator to stopfurther degradation of the product and can be expected to perform per theproduct Technical Bulletin.
  • Unopened paste jars are stable in roomtemperature for a maximum of 2 weeks. Room temperature stability is intended toprovide manufacturing flexibility after storing the product. Exceeding 2 weeksof room temperature storage will compromise the performance of the solder pasteduring printing. Please refer to the specific TB for any exceptions to thisrecommendation.
  • If the paste hasbeen at room temperature for two weeks, the paste should be used immediately ordispose of the paste.
  • This refers to paste on stencil.
  • Do not remove paste from stencil and mixwith unused paste in jar. This will alter the rheology and possibly themoisture content of the unused paste and, potentially, negatively affect pasteperformance.
  • Failure to follow these guidelines willresult in reduced shelf life and diminished product performance and may makethe product unsuitable for use.
  • Any used paste on stencil should be putin an empty and clean jar and stored at room temperature.
  • Once applied to the stencil, NC and WSpastes are to be consumed within the stencil life, typically 8 hours.
  • Opened jars may pick up moisture in thesolder paste and are subjected to condensation when refrigerated.
  • Do not return jars that were alreadyopened to refrigeration.
  • If a jar is opened and unused, returnthe lid to the jar, keep the jar at room temperature, and use the paste withinits two week room temperature shelf life.
  • Always dispose of any unused solderpaste in accordance with local environmental legislation.
  • Always reference the Technical Bulletinof the specific product before use. Information presented in a productTechnical Bulletin supersedes information contained in this Reference Bulletin.

Kishan Sarjoo
Process Engineering Manager - Electronics
Altech UEC, South Africa
Currently with Altech UEC and responsible for technology road map in PCBA electronic manufacturing and technical support for PCBA electronic manufacturing for Altech UEC and its JDM's. Over 7 years in SMT, Radial Insertion, Wave solder & Test Applications.

Youshould really allow 6 to 8 hours for the paste to acclimatise naturally and nottry to force heat the paste for instance putting the pot on top of he reflowoven. Once warmed, open and stir gently for around 30 - 60 seconds untilmixed, don't mix too long or too vigorously or you could get paste shear. Only clean your paste off when needed there is no fixed time. However, if youhave over pressurized the paste during print then remove the paste and put backinto a clean pot and mix gently to distribute the flux again and return tostencil for further printing. Some paste will last many days now and are not effectedby higher ambient temperatures or even require refrigerator storage atall. Hopeit helps.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

It is a good ideato let the solder paste come to room temp before opening the container toprevent water from condensing on the paste and to allow proper paste rheology It is also a goodidea to stir the paste gently with a spatula before placing it on the stencil.This is because the solder paste behaves as a non-Newtonian fluid,meaning the paste viscosity reduces when the material is sheared. Shearing the paste in the jar helps reduce the viscosity to nearer the properprinting viscosity. This lessens the chance that the first few printswould be incomplete due to "stiff" paste. The only caution here is trynot to introduce air pockets into the paste as you mix it. Cleaning thestenciling equipment and stencil every so often is a good practice. Thiskeeps partially dried paste from clogging up the works. The recommendedfrequency vary depending on the solder paste formula, the ambient temperatureand the print frequency. 2 hours is a good rule of thumb.

Steve Stach
Austin American Technology
Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.

Hello,S.S. Theseare very good questions; it is really good that you understand the importanceand have an interest in controlling your company's solder paste handlingmethods. Doing this properly pays off immensely over time in averting all kindsof paste printing issues that lead to solder defects, line-down time, andrework, and quite possibly even recalled product, depending on your industry(automotive, medical, military and other high-reliability) or high-volumecommercial markets. Solderpaste control (management) needs to begin through Purchase Order boilerplatelanguage that spells out:
  1. The shipping packaging(insulated container with frozen gel packs inside)
  2. The type of container(sealed 100 gram syringes or 500 gram Semco tubes are better, jars not so goodbut allow these as non-preferred alternate)
  3. Limited shipping time(preferably couriered from a local distributor within a few hours from receiptof P.O., or overnight delivery, but not shipped 5-7 day ground in dark brown truckin July). Absolutely do not allow purchasing from someplace in Malaysia to yourlocation in Minnesota in order to save 50 cents per cartridge.
  4. Notification of delivery(distributor notifies buyer of pending delivery date and time to ensure it willbe promptly received and immediately refrigerated, even during receivinginspection if applicable. Paste shall not sit on your shipping dock over theweekend)
  5. Minimum remaining shelflife before expiration (you do not want to buy a calculated year's supply offifty 500 gram cartridges only to find they are 6 months into their 12-monthshelf life from date of manufacture upon receipt) Thatshould cover the first half of the supply line, now for your part:
  6. Ensure that you have adocumented solder paste storage and handling Procedure, and the MaterialHandling people and the line operators are trained and certified in pastehandling.
  7. The procedure shallcover method for removal from refrigeration and validating remaining shelf lifewill not expire before it is printed and reflowed.
  8. Method for allowingmaterial to reach room temperature (example: 3 or 4 hours dwell at RT, or 1hour @ RT with benchtop fan, or precisely 2.8 minutes with centrifuge mixer setat xx RPM (best method because it is fast and very repeatable, you can achieveperfect viscosity per the Technical Data Sheet every single time)
  9. Upon completion of roomtemperature settling, perform a small-sample viscosity check with Brookfield orMalcom viscosimeter, and have the allowable range defined in the HandlingProcedure (a small chart listing the different paste part numbers and theirminimum, preferred, and maximum viscosity). This check seldom takes more than 5minutes and may only need to be performed once or twice per shift, but it goesa long way in ensuring a good print, nice brick formation with minimal slumpingand no pullback upon stencil separation from the PWB.
  10. Immediate resealingafter dispensing shall be required for jars (if used). Jars may never bere-refrigerated once opened.
  11. Open time life must alsobe controlled. Once a jar is opened, allow a 5 to 8 day limit before it is usedup, after that it shall be discarded (note the removal time from refrigerationon a label). A sealed tube or cartridge may be left out for 10 to 15 days (myown suggested guidelines, depends on paste brand and type. Some can stay outlonger, others may not be any good after only a couple of days at RT).Shocker!: If only a small amount of paste is needed to completea run and you have no immediate need for the rest of a nearly full tube, I feelthat it is perfectly OK to remove a tube (but not a jar) from refrigeration,uncap and immediately dispense onto the stencil, recap the tube andre-refrigerate within two minutes. Then simply mix the small amount you dispensedright on the stencil, and finish the run. If done quickly there is no danger ofcondensation forming inside the sealed tube, and you will not notice anydeviation in print quality, and you do not have an entire tube sitting out atRT for a long time before being depleted.
  12. The suggested(guideline) open stencil life for each solder paste part number should bedefined in your handling procedure. Some are good for 3-4 hours, while otherscontinue to print well up to 8 hours on the stencil.
  13. Be sure your Paste PrintingProcedure references your Paste Handling Procedure (perfectly OK to have bothin a single document)
  14. The frequency ofreplenishment onto the stencil shall be defined. The best practice is tocontinually add smaller amounts of paste to that which is already on thestencil, to preserve or maintain the viscosity. The frequency of replenishmentshould be determined by the number of shears (this should be a guideline, not ahard requirement for you to trip over during a process audit, so after every 7-10prints, add paste, mix with the spatula, plow it into a nice line that willroll properly across the stencil, do not allow sliding in a pile, etc.)
Whenqualifying a new paste, be sure to set up the new paste part number so that itincludes the boilerplate language in the Purchase Order automatically. Theremay not be a method in your Purchased Material Requirements section of your MRPsystem to do this automatically by Material Type, so you may need to just keepthat in mind when setting up new solder paste part numbers. Nothing worse thanfinding a shipment of 50 warm tubes of solder paste in a brown paper bag onyour receiving dock on a Monday morning after the Fourth of July 4-day weekend,and you have to scrap it out and eat the cost because you did not have theboilerplate rules on the new material purchase order. Comparethe condition of your solder paste with the jar of peanut butter you have athome. If the peanut butter is badly separated with about an inch of oil on top,you don't eat it do you? And if what remains in the jar is dry and has thecracked-earth appearance at the bottom of the jar, don't you throw it out andstart with a fresh jar? Well, do the same with your solder paste. Ihope you have found this helpful, and if you like you can send a cold 6-pack ofbeer packaged per item #1 above to:RichardStadem,14644Hayes RoadAppleValley, MN 55124 Iam going to take a break from all this now and have a peanut butter sandwichand a cold beer (Go Gophers!). Goodluck,

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.

First - the time out of refrigerator before use is a little short at 2 hrs. Try to increase that time frame in order to get the solder paste slowly to the room temperature.Condensation can give you headaches in the reflow process. Paste can be stirred by hand, however the time and consistency is variable as the operators can do it different ways. There is equipment out there that can offer you more control for this operation. You can use one of those and do a capability study in order to determine the standards. Stencil cleaning frequency is another item that depends on multiple factors: temperature and humidity in the manufacturing facility, board population/stencil design (nano-coated stencils are already used widely in the industry and this reduces drastically the stencil cleaning frequency), machine performances, solder paste manufacturer's recommendations, solder paste type, etc. As usual, capability studies are necessary if you want to make a very informed decision.

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at

Although thismethod of preparing your solder paste is definitely acceptable, there arechances of running into a few issues. The main issues stem from uservariance and optimal efficiency. Relying on a person to hand mix thepaste can create variances in paste temperature, viscosity, and uniformity. Using an automatic paste mixer, there are a couple of good ones on themarket, takes user error and variance out of the equation. The paste ismixed to a uniform consistency, as well as optimal viscosity & temperature... all within 10~15 minutes of taking the paste out of the refrigerator. Although hand-mixing your paste is acceptable, it is not the optimal ormost efficient way to prepare your paste for production.

Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

1.5 - 2 hours may not be enough to let the solder paste equalize with ambient temperature. My recommendation is at least 4 hours out from refrigerator before opening the container to prevent moisture condensation into the paste.

Mixing the paste by hand for 1.5 minutes is a good practice. Solder pastes are designed to have thixotropic property (time-dependent shear thinning property). A gentle hand mixing will pre-shear thin the solder paste and make it roll properly during printing process.

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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