|Ask the Experts|
March 7, 2018
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?
|Expert Panel Responses|
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:
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.
Application Engineering Supervisor
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.
V.P. Technology & Engineering
Fine Line Stencil, Inc.
This is actually the preferred and recommended solution for all paste we sell.
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).
Regional Sales Manager
OK International Inc.
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.
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?
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.Tom Fujikawa, MALCOMTECH INTERNATIONAL
There are different methods that you can use to mix thesolder paste and get it to the room temperature. The thing is how are you goingto know if the parameters of the solder paste after your process are where arethey supposed to be? For mixing there is equipment out there that measures thetemperature 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 – aviscometer will fix this problem. Perform a capability study and find out whatis the magic number for how long are you going to mix the paste for. Rememberto 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 (watersoluble, no-clean, RMA)
Engineering and Operations Management
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