Shear Thinning

Solder pastes are a very complex “fluid” of high viscosity. Their behavior, when experiencing shear stresses, is “non-Newtonian,” meaning that their viscosity is not constant as the shear stress varies. The viscosity of solder pastes is high when there is little or no shear stress and low when shear stresses are high such as when the paste is forced through a stencil aperture. This property is called thixotropy. The solder paste being thixotropic is ideal, as it enables the stencil printed “brick” of solder paste to retain its shape after it is printed, yet the low viscosity, when stressed, allows good filling of the stencil aperture.

Many might assume that this relatively complex phenomenon is the end of the story. However, there is at least one other well-known property of solder pastes during printing that is important: response to pause. A solder paste with a poor response to pause will stiffen when permitted to idle for as few as 15 minutes. When this occurs, the first print likely will have insufficient solder paste for effective assembly. Hence, response to pause is a critical variable to measure when evaluating a potential solder paste.

Another important solder paste property has only recently become well known: shear thinning. My Indium Corp. colleague Tim Jensen was one of the first to point this out. Shear thinning is a property of some solder pastes in which the viscosity becomes lower and lower as the paste is repeatedly printed (see figure below). The x axis is number of prints, the y axis is the viscosity. It is normal for the viscosity to go down during the print, but the viscosity should recover as the “good paste” does, not have a downward trend as the “bad paste.” The resulting drop in viscosity that the bad paste exhibits will often result in too much paste being printed and potentially lead to defects such as shorts or solder balls. Unfortunately, if not tested for, shear thinning might first be observed after a paste has been implemented on the line.

If you are interested in a method to test a paste for its resistance to shear thinning, send me a note and I will send a test protocol.

shear-thinning

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About Dr. Ron

Materials expert Dr. Ron Lasky is a professor of engineering and senior lecturer at Dartmouth, and senior technologist at Indium Corp. He has a Ph.D. in materials science from Cornell University, and is a prolific author and lecturer, having published more than 40 papers. He received the SMTA Founders Award in 2003.

4 thoughts on “Shear Thinning

  1. Dr Ron,
    I enjoyed this article, full of good detail, my question is as follows:

    Currently some paste suppliers have changed the directions/guidance on cartridge paste storage, they now recommend storage on the nose (cartridge tip), my experience has been that paste separation occurs, mainly after two thirds of the cartridge has been used,
    We have verified this by first replacing the faulty cartridge (defects disappear) and storing the cartridge on there sides with rotation; the faults have not re appeared on this batch of paste nor any future batches.

    Paste separation can look very much like sheer thinning in its effect but as you are aware they have different visual wet properties.

    My question is this I believe there is another fault which may introduce a third similar in appearance and effect, and that is high blade pressure, particularly in low humidity environments, where I am certain the blade gets quite hot and can affect the paste (similar to sheer thinning).

    Can you discuss these issues?

    Mike Cummings

  2. I would be very interested in learning the test method for evaluating the “shear thinning” property of the paste we are currently using. I would appreciate it very much if you would send more information to the email listed above.

    Thanks,

    David

  3. Mike,

    Thank you for your kind and thoughtful comments!

    I discussed your questions with a team of my Indium Corporation colleagues and here is our joint response:

    Properly refrigerated paste takes a good while to separate in our experience. It is not clear if your experiment took place at room temperature or under refrigeration and the amount of time is not mentioned either. So this is hard to comment on. Our position is to store cartridges and syringes tip down under refrigeration until the time of use. Any separation that does occur will be limited to the last bits of paste in the cartridge/syringe. We feel that a good paste should not have significant separation.

    We have always preached using the minimum blade pressure to produce a clean swipe of the stencil. If you are finding that you have to use “high blade pressure” our question would be “Why?”

    Granted, with friction between the squeegee and stencil there is probably some temperature rise. Although, we would consider it negligible. The paste “lubricates” the stencil and squeegee. And the paste is always in motion, “rolling”. So the same part of the paste bead is not in contact with the “hot” squeegee for very long before being rolled down on to the “cool” stencil. It would be easy to measure the temperature of the bead with one of those IR guns. For all of these reasons, we doubt that the temperature is significantly different than the ambient temperature inside the printer. We also don’t think the humidity would have a significant effect.

    All the best,

    Dr. Ron

  4. Thanks for your response answers (I have been on travel sorry for the late response), I have seen IR test images from a trial (I beleive similar to what you described) which actually showed considerable heat increase, NPL (UK) have shown that temperature changes (Fast changes) to the paste of as little as 4ºC can alter/impact paste performance, however the paste mass and its roll and the heat sink effect of the stencil will as you say disperse this very quickly (hopefully).

    The paste seperation issue was not your advisors paste, but I have seen this issue with several companies over the years, and agree that I have not witnessed paste issues like this with Indium. I believe most of these issues start at packaging (catridge loading) and dispatch/packaging etc, and agrevated by the storage process on the nose, Good paste does not suffer from these issues basically because it is loaded/packaged/stored etc in the correct transport conditions.

    Trials conducted on metal on metal (rubbing) show a direct link between friction and RH levels, aproaching 30%RH increases the friction (and stiction therefor metal wear) by as much as 10X,
    We are gathering stencil wear information based on shop floor environment, a slow data gathering operation, but worth the time?

    Mike C

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