Workability of Electrically Conductive Adhesives
Electrically conductive adhesives are widely used in semiconductor packaging, LED packaging and photovoltaic applications. In their product technical datasheets, conductive adhesive manufacturers often report these two rheological properties, viscosity at 5rpm and thixotropic index which is the ratio of viscosities at 0.5rpm and 5rpm.
These two parameters are related to the workability of electrically conductive adhesives which are non-Newtonian liquids. At higher shear rates or higher shear stresses, they exhibit lower viscosities, a phenomenon called shear thinning. Shear thinning is higher desired for the workability of these adhesives. At lower shear rate when the adhesive is idling, higher viscosity can prevent it from dripping. At higher shear rate when dispensing needle is pulled away from the adhesive, lower viscosity can prevent it from tailing.
When needle or nozzle dispensing is used to apply adhesives, it was found that these two rheological parameters showed good correlation with adhesive’s workability. An adhesive with 10,000cps viscosity at 5rpm and a thixotropic index greater than 4.0 tends to show good workability. However, such a correlation becomes quite poor for other ways to apply conductive adhesives, such as printing, jetting and pin transfer. Two conductive adhesives with similar viscosity at 5pm and similar thixotropic index may show quite different behavior during printing, jetting or pin transfer. One of the reasons is that the shear rates in printing, jetting and pin transfer are quite different from dispensing.
After more than 10 years of developing electrically conductive adhesives, Yongoo has accumulated significant amount of understanding on the correlations between rheological properties and adhesive workability during dispensing, printing, jetting and pin transfer. Today Yongoo offers products covering a wide range of applications with proven workability.
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