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December 19, 2017

Solder Paste Mixing After Storage

We purchase our lead-free solder paste in 500-gm jars and store them in a refrigerator.

After removal from cold storage, we let the jars sit at room temperature for 4 hours. We then place the jars in a centrifuge machine horizontally for 5 minutes prior to use.

Is this a good practice? Would you suggest any alternative mixing practice?

E. R.

Experts Comments

I think this is a good practice, except for one small detail. The vendor of solder paste centrifuges recommends going directly from the refrigerator into the centrifuge, then mixing (using the centrifuge) for about 15 minutes.

For those who do not know, spinning a jar of paste in a centrifuge made for that purpose causes the paste to repeatedly fold over onto itself. Hence a centrifuge is an ideal method for mixing the paste exactly the same way every time before it is loaded onto the printer or stencil.

I suspect waiting for 4 hours and then spinning in the centrifuge can actually mix the paste down to the lower end of the desired viscosity range. You can check this out using a Malcom or Brookfield viscosimeter.

You should always make sure the viscosity is as close to the middle of the recommended viscosity range on your paste's TDS (Technical Data Sheet). You may wish to record the length of time mixed, the room temp, humidity, and viscosity for awhile, at least two or three times for each jar until it is used up.

This data will give you a good idea of the how those factors affect the rheology of the paste, and you can optimize the mix time accordingly. You may also wish to track the paste viscosity with the data from your solder paste inspection system and/or the DPMO, and see if there is a target viscosity that gives you best printing results, and thus a lower DPMO.

Remember some of these points for handling paste:

  1. Don't put opened jars back into the refrigerator (this can cause condensation to form).
  2. If using paste in jars, keep the inner plastic seal pressed down, and keep the jar covered tight when not dispensing the paste.
  3. Don't put used (sheared) paste back into the jar with unused paste; use it up and discard any small amounts left over after being sheared on the stencil.
  4. Humidity and room temperature should be controlled as much as possible in the printing area. Avoid printing paste in very high humidity or very dry conditions. 25% to 70% RH is typically considered to be acceptable.
  5. Remember, just because your paste may have expired does not necessarily mean it is no longer usable. See the re-certification steps listed in J-STD-005. Be sure you qualify any re-certified paste and make sure you have your customer's approval before doing so.
  6. Paste in tubes or syringes may work better than jars, as very little is ever exposed to air and moisture, and there is less settling (separation of flux and fines).
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.

If solder paste has no flux separation, is should be ok to use without any mixing. If solder paste has flux separation, mixing with a spatula is the most practical way to remix a full jar of solder paste.

A true centrifuge will tend to separate solder paste, forcing alloy to settle to the bottom of a jar and flux rising to the top. Some machines rotate a jar while spinning.

This action is of limited effectiveness because such machines work best when a jar is half full or less, so the paste can recirculate within the jar. In a full jar, recirculation is poor.

John Vivari
Application Engineering Supervisor
Nordson EFD
Mr. Vivari has more than 15 years of electronic engineering design and assembly experience. His expertise in fluid dispensing and solder paste technology assists others in identifying the most cost effective method for assembling products.

Centrifuge mixing is one of the most consistent methods of preparing solder paste for initial use and is widely used by manufacturers throughout Asia. However, you really don't need to let the solder paste sit for 4 hours at room temperature.

The friction produced during a centrifuge mix will warm the solder paste to room temperature.

The only caution I would raise is the five minute mixing time. Typically centrifuge mixing time is one or two minutes. A five minute mix can excessively shear thin the solder paste and lower the viscosity too much.

I would recommend a two minute mix test with your current solder paste.

Robert Dervaes
V.P. Technology & Engineering
Fine Line Stencil, Inc.
Robert Dervaes has worked in the electronics industry since 1992 in both design and manufacturing. Over the past 11 years he has established the technical foundation of Fine Line Stencil, Inc. - a premier stencil supplier to the electronics industry.

This is actually the preferred and recommended solution for all paste we sell.

Chris Ellis
Sales Manager
Chris Ellis is a Sales Manager/Engineer for Manncorp with 19+ years experience in the PCB assembly equipment industry. Chris worked as a Manufacturing Engineer for 5 Years. Prior to going into sales, he was with PCB assembly equipment suppliers. Before joining Manncorp, Chris was an independent rep in the Carolinas for several years.

I would think that a centrifuge would cause the metals to migrate to the bottom of the jar and the fluxes up to the top. This would cause all kinds of process problems.

Why don't you try hand mixing after it warms up (4 hours).

Edward Zamborsky
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

You are doing nothing wrong with your current practice. As long as you use the same method each time it is considered good practice. In fact, we have recently done a study using centrifuge type systems.

As a result of this, you could shorten the room temperature time to 1 hour followed by the centrifuge. But if you are happy with the 4 hours, I would not change anything.

Doug Dixon
Global Marketing Director
Henkel Electronics
Mr. Dixon has been in the electronics field for over twenty years and is the Global Marketing Director with the electronics group of Henkel. Prior to joining Henkel, he worked for Raytheon, Camalot Systems, and Universal Instruments.
I have found, for conditioning the solder paste, the time-proven (pun intended) method of sitting at ambient works fine. Determine, through use of a thermocouple, the amount of time required for the volume cartridge (preferred) or jar in use and use that interval. With a little discipline, a FIFO system works great. And don't put the container on top of the reflow oven to "speed things up" - doesn't work!  

I have seen and used the centrifuge system in various facilities and, as Tom F. depicts, it does, indeed, work. But with reference to Steven Adamson's comment, how do you fill out a purchase req for a "THINKY" and keep a straight face?
Phil Zarrow
Principal Consultant
ITM Consulting
Mr. Zarrow has been involved with PCB assembly for more than thirty years. He is recognized for his expertise in troubleshooting SMT manufacturing and lead-free implementation. He has extensive hands-on experience with set-up and troubleshooting through-hole and SMT processes throughout the world.
Reader Comment
Solder paste mixing is very important to provide a uniform viscosity and temperature. The centrifuge motion (low speed 400-500 RPM) with 45 degree angle gives a smooth mixing motion to the solder paste in/out, top/bottom and controls the solder paste viscosity and temperature with in 5-10 minutes of mixing.
There are different methods that you can use to mix the solder paste and get it to the room temperature. The thing is how are you going to know if the parameters of the solder paste after your process are where are they supposed to be?

For mixing there is equipment out there that measures the temperature also (they have a thermo-couple in the bottom of the fixture.

How much mixing is enough and how much is too much?

Again, you will need a device to measure that a viscometer will fix this problem. Perform a capability study and find out what is the magic number for how long are you going to mix the paste for. Remember to do that for each product as the mixing time will vary based on paste type (lead, lead-free), solder type (III, IV, V, etc.), flux composition (water soluble, no-clean, RMA)
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.
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