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July 13, 2017

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?


Experts Comments

S.S. for storage and preparation of the solder paste you should always refer to the best practices defined by your solder paste manufacturer.

In regards to your question on frequency of cleaning the stencil printer I will attempt to answer. During the production cycle you should be performing an under-stencil wipe. The cleaning frequency usually varies according to product, maybe ranging from 10 to 20 prints, averaging around 12 to 15. Many SMT printers have this option to automatically clean under the stencil.

To completely remove and clean the stencil depends once again depends on the solder paste used, product built and production cycle. Every two hours seems excessive but if it works for you and you have no miss-prints I would stay with the current routine.

Many facilities schedule their cleaning by the recommended stencil life of the solder paste, once again defined by the solder paste manufacturer. However keep in mind that the type of paste used and even environmental conditions will be factors to consider.

As an example here are the parameters of a common no-clean, halogen free, lead free 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 careful where you apply it but you will find it in areas you had no intentions of putting it. The cleanliness and condition of your stencil printer (and SMT line) is a direct reflection of your companies process quality and the attention and care you show 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.
Solder pastes are all manufactured for their specific purpose and use. I recommend you follow your solder paste manufacturer's instructions for the storage, use and stencil life for the precise 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.
In order to determine if your warm-up time after removal from refrigeration is adequate, insert a temperature probe into the center of the undisturbed paste in the jar after the 1.5-hour warm-up.

The temperature should be very close to the 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 increase heat transfer and shorten the warm-up time
  • Use a circulating water bath that immerses just the bottom of the jar
The last method will result in the fastest warm-up, but is not something you want to do if you don't have to. The forced-air warm-up can be very effective. With regard to stirring the paste, this will be a benefit with some pastes, however it also inevitably incorporates air into the paste, which is not good. Your paste manufacturer can provide guidance on whether stirring is required for the particular 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 of under-stencil wipe.
  2. Frequency of complete cleaning of stencil, including removal of old paste, cleaning, and applying fresh paste.
The required frequency for under-stencil wipe varies with the stencil geometry, PWB characteristics and specific paste used. The idea is to wipe often enough to prevent the bottom of the stencil from being contaminated by paste, while avoiding wiping too often.

For a well-designed process, often an under-stencil wipe every five to ten prints is sufficient. Occasionally, wiping every other print may be required. If frequent wiping is required, other process optimizations may help to reduce the required frequency.

The frequency with which the entire stencil is cleaned and the paste replaced is a function of the specific paste. Most modern pastes have working lives of up to 8 hours, and so it may not be necessary to completely clean the stencil more than this often.

I should caution you here, however, that your specific experience has a lot to do with your plant floor environment (temperature, humidity). A paste that functions well for 8 hours in one environment may not survive 4 hours in another. Keep in mind that it's the environment within the printer 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'd recommend more time out of the refrigerator, say closer to 4 hours minimum to make sure the entire jar gets to room temperature. Stirring it is a good practice, the spatula you mention is most likely fine, but we used metal putty knives and have had no problems. Be careful when stirring, you want to eliminate air bubbles, not create them.  

As for 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 your stencil, you should clean much more often.

Roughly around 1 time per 3-4 prints, or even as often as every other print. It really is up to you, I'd watch your prints and determine how they look to set up your cleaning frequency requirements. I'd definitely go by prints though, not time. If you are using a cleaning 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 your stencil.

Once you are done with that particular run, it's best practice to thoroughly clean the stencil. An ultrasonic cleaner is a really good way to ensure 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 temperature storage as it ensures the paste is stored in a controlled environment and lengthens shelf life. The issue with refrigeration is that the paste needs to return to ambient temperature prior to use without additional heat. This requires advance planning to ensure there is sufficient conditioned material to meet production requirements.

Your stated warm up time is insufficient based on AIM testing. The absolute minimum time regardless of package type (500g jar/cartridge) is five (5) hours to ensure the entire contents of the paste are at ambient temperature, our TDS recommends eight (8) hours. 

Using cold paste will:
  1. Promote condensation on the paste.  As you can imagine, adding water to a mix of metal and acids will cause unintended print and reflow performance variation.
  2. As the paste warms, the viscosity will drop and inputs will have to be made to the printer to accommodate these changes.
Our recommendation is to simply remove paste the night or shift prior to use.
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.
It may be a good idea to check the temperature of your paste with a good thermometer, before beginning your stirring process (and in the future, know how long it takes to equal room temperature, before opening the jar).

Since condensation will always form if there is a temperature difference between the paste and the outside temperature, you may wish to determine the time it takes for the paste to reach room temperature before breaking the seal on the jar. 

Otherwise, water may collect on your paste - and affect the reliability of your soldering. We all must remember how small the solder joints are and the effect of even 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.
You will know if 1.5 to 2 hours is enough by checking the inside temperature of the jar by introducing a thermometer into the solder paste looking for a range between 19-25C. 

Stirring the solder paste for 1.5 minutes after it reaches room temperature is a good practice to ensure paste homogeneity.

Do you mean cleaning of the stencil printer or the stencil? For the stencil, it depends on volume and aperture sizes. I would need a little more information on your printer but as long as your PCB's are coming out clean with perfect solder deposition on all pads (especially the fine pitch ones) you may decrease 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 be used for handling solder pastes. The product Technical Bulletin should always be referred to for verification of any specific recommendations or conditions. You should be able to get this from your solder paste manufacture.

  • Solder paste products are engineered to be shipped between 0-29C (32-84F)
  • Long term storage of solder paste is best achieved by refrigeration 0-10C (32-50F). The material should be placed in a storage area designed to maintain this temperature range (refrigerator or cold room) immediately upon receipt.
  • Typical no-clean solder paste shelf life is 6 months, water-soluble paste ranges from 3-6 months in a refrigerated environment. Verify product Technical Bulletin for exceptions. Please note shelf 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 once every week.
  • Solder paste should never be stored at room temperature 19-25C (66-77F) for prolonged periods of time. Room temperature stability is intended to provide manufacturing flexibility after storing the product. The typical room temperature shelf life of unopened paste is 2 weeks. Please refer to the specific TB for any exceptions to this recommendation.
  • 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 First In First Out (FIFO) basis. To maintain optimum performance paste should not be stored outside the refrigerator any longer than necessary (never more than four (4) days).
  • Solder paste should be allowed to reach room temperature, 19-25C (66-77F), without forced heating or machine mixer. We recommend a typical period of 3-4 hours out of refrigeration, depending on packaging size. The specified viscosity of each paste is based on measurement at 25C.
  • Best practices for solder past printing are to maintain the temperature inside the printer between 22C - 29C, 40% - 60% RH. Please consult the product Technical Bulletin for pastes that may operate outside of this window.
  • For jar packaging, manually stir the solder paste with spatula for 30 - 60 seconds to ensure paste homogeneity. Rotating/centrifugal force mixing equipment is not recommended. If a rotating/centrifugal force mixing equipment is used, a maximum time of 1 minute at 300 RPM is recommended.
  • Apply an even paste bead on the stencil over 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 when exceeded stencil life or has been exposed to high temperatures inside the printer >29C (84F). If the solder paste is designed for printing at temperatures over 29C, it will be indicated on the technical bulletin.
  • Refer to product Technical Bulletin for room temperature stability life.
  • Unopened paste jars that have been exposed to 25C (77F) for 4 days may be returned to the refrigerator to stop further degradation of the product and can be expected to perform per the product Technical Bulletin.
  • Unopened paste jars are stable in room temperature for a maximum of 2 weeks. Room temperature stability is intended to provide manufacturing flexibility after storing the product. Exceeding 2 weeks of room temperature storage will compromise the performance of the solder paste during printing. Please refer to the specific TB for any exceptions to this recommendation.
  • If the paste has been at room temperature for two weeks, the paste should be used immediately or dispose of the paste.
  • This refers to paste on stencil.
  • Do not remove paste from stencil and mix with unused paste in jar. This will alter the rheology and possibly the moisture content of the unused paste and, potentially, negatively affect paste performance.
  • Failure to follow these guidelines will result in reduced shelf life and diminished product performance and may make the product unsuitable for use.
  • Any used paste on stencil should be put in an empty and clean jar and stored at room temperature.
  • Once applied to the stencil, NC and WS pastes are to be consumed within the stencil life, typically 8 hours.
  • Opened jars may pick up moisture in the solder paste and are subjected to condensation when refrigerated.
  • Do not return jars that were already opened to refrigeration.
  • If a jar is opened and unused, return the lid to the jar, keep the jar at room temperature, and use the paste within its two week room temperature shelf life.
  • Always dispose of any unused solder paste in accordance with local environmental legislation.
  • Always reference the Technical Bulletin of the specific product before use. Information presented in a product Technical 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.
You should really allow 6 to 8 hours for the paste to acclimatise naturally and not try to force heat the paste for instance putting the pot on top of he reflow oven.  Once warmed, open and stir gently for around 30 - 60 seconds until mixed, 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 you have over pressurized the paste during print then remove the paste and put back into a clean pot and mix gently to distribute the flux again and return to stencil for further printing. Some paste will last many days now and are not effected by higher ambient temperatures or even require refrigerator storage at all.

Hope it helps.
Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 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 idea to let the solder paste come to room temp before opening the container to prevent water from condensing on the paste and to allow proper paste rheology  

It is also a good idea 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 proper printing viscosity. This lessens the chance that the first few prints would be incomplete due to "stiff" paste. The only caution here is try not to introduce air pockets into the paste as you mix it.   

Cleaning the stenciling equipment and stencil every so often is a good practice. This keeps partially dried paste from clogging up the works. The recommended frequency vary depending on the solder paste formula, the ambient temperature and 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.

These are very good questions; it is really good that you understand the importance and have an interest in controlling your company's solder paste handling methods. Doing this properly pays off immensely over time in averting all kinds of paste printing issues that lead to solder defects, line-down time, and rework, and quite possibly even recalled product, depending on your industry (automotive, medical, military and other high-reliability) or high-volume commercial markets.

Solder paste control (management) needs to begin through Purchase Order boilerplate language 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 good but allow these as non-preferred alternate)
  3. Limited shipping time (preferably couriered from a local distributor within a few hours from receipt of P.O., or overnight delivery, but not shipped 5-7 day ground in dark brown truck in July). Absolutely do not allow purchasing from someplace in Malaysia to your location 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 will be promptly received and immediately refrigerated, even during receiving inspection if applicable. Paste shall not sit on your shipping dock over the weekend)
  5. Minimum remaining shelf life before expiration (you do not want to buy a calculated year's supply of fifty 500 gram cartridges only to find they are 6 months into their 12-month shelf life from date of manufacture upon receipt) That should cover the first half of the supply line, now for your part:
  6. Ensure that you have a documented solder paste storage and handling Procedure, and the Material Handling people and the line operators are trained and certified in paste handling.
  7. The procedure shall cover method for removal from refrigeration and validating remaining shelf life will not expire before it is printed and reflowed.
  8. Method for allowing material to reach room temperature (example: 3 or 4 hours dwell at RT, or 1 hour @ RT with benchtop fan, or precisely 2.8 minutes with centrifuge mixer set at xx RPM (best method because it is fast and very repeatable, you can achieve perfect viscosity per the Technical Data Sheet every single time)
  9. Upon completion of room temperature settling, perform a small-sample viscosity check with Brookfield or Malcom viscosimeter, and have the allowable range defined in the Handling Procedure (a small chart listing the different paste part numbers and their minimum, preferred, and maximum viscosity). This check seldom takes more than 5 minutes and may only need to be performed once or twice per shift, but it goes a long way in ensuring a good print, nice brick formation with minimal slumping and no pullback upon stencil separation from the PWB.
  10. Immediate resealing after dispensing shall be required for jars (if used). Jars may never be re-refrigerated once opened.
  11. Open time life must also be controlled. Once a jar is opened, allow a 5 to 8 day limit before it is used up, after that it shall be discarded (note the removal time from refrigeration on a label). A sealed tube or cartridge may be left out for 10 to 15 days (my own suggested guidelines, depends on paste brand and type. Some can stay out longer, 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 complete a run and you have no immediate need for the rest of a nearly full tube, I feel that it is perfectly OK to remove a tube (but not a jar) from refrigeration, uncap and immediately dispense onto the stencil, recap the tube and re-refrigerate within two minutes. Then simply mix the small amount you dispensed right on the stencil, and finish the run. If done quickly there is no danger of condensation forming inside the sealed tube, and you will not notice any deviation in print quality, and you do not have an entire tube sitting out at RT for a long time before being depleted.
  12. The suggested (guideline) open stencil life for each solder paste part number should be defined in your handling procedure. Some are good for 3-4 hours, while others continue to print well up to 8 hours on the stencil.
  13. Be sure your Paste Printing Procedure references your Paste Handling Procedure (perfectly OK to have both in a single document)
  14. The frequency of replenishment onto the stencil shall be defined. The best practice is to continually add smaller amounts of paste to that which is already on the stencil, to preserve or maintain the viscosity. The frequency of replenishment should be determined by the number of shears (this should be a guideline, not a hard requirement for you to trip over during a process audit, so after every 7-10 prints, add paste, mix with the spatula, plow it into a nice line that will roll properly across the stencil, do not allow sliding in a pile, etc.)
When qualifying a new paste, be sure to set up the new paste part number so that it includes the boilerplate language in the Purchase Order automatically. There may not be a method in your Purchased Material Requirements section of your MRP system to do this automatically by Material Type, so you may need to just keep that in mind when setting up new solder paste part numbers. Nothing worse than finding a shipment of 50 warm tubes of solder paste in a brown paper bag on your 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 the boilerplate rules on the new material purchase order.

Compare the condition of your solder paste with the jar of peanut butter you have at home. 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 the cracked-earth appearance at the bottom of the jar, don't you throw it out and start with a fresh jar? Well, do the same with your solder paste.

I hope you have found this helpful, and if you like you can send a cold 6-pack of beer packaged per item #1 above to: Richard Stadem, 14644 Hayes Road Apple Valley, MN 55124

I am going to take a break from all this now and have a peanut butter sandwich and a cold beer (Go Gophers!).

Good luck,
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 georgiansimion@yahoo.com.
Although this method of preparing your solder paste is definitely acceptable, there are chances of running into a few issues. The main issues stem from user variance and optimal efficiency.  Relying on a person to hand mix the paste can create variances in paste temperature, viscosity, and uniformity.

Using an automatic paste mixer, there are a couple of good ones on the market, takes user error and variance out of the equation. The paste is mixed 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 or most 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 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
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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