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While it is easy to achieve high bonding strengths on plastics such as polyamide, it is by no means as easy with other plastics. In this second part of our Most Difficult Materials series, we are looking at LCP and what products and methods can be used to achieve a durable and reliable bond.
LCP is a family of high-performance plastics with very good mechanical properties. It is also inherently flame-resistant and remains dimensionally stable at temperatures of up to and beyond 300 °C. They are also extremely flowable, enabling them to fill injection molds with long, narrow, and complex flow paths with minimal warpage. That is why they are well suited for use in filigree mold components needed for microelectronics.
LCP as an abbreviation does not denote any specific chemical composition in the way that, for example, PC stands for polycarbonate. Instead, it stands for 'liquid crystal polymers'. These are polymers with liquid crystalline properties. Certain areas of the material exist in the form of crystalline structures surrounded by amorphous areas.
The mechanical properties of liquid crystal polymers can be adjusted across a relatively broad spectrum by changing the ratio of crystalline and amorphous phases and the level of interlinking. In addition, the shape of these crystalline structures affects the properties of the material. LCPs therefore comes in different guises. The adhesion level achieved by the same adhesive on different grades of LCP such as E130i, E463i, or E525T, or VL96AC can vary significantly.
With that said, LCP is a difficult material to bond. This is due to its chemical and physical structure that makes this material class very robust against environmental and temperature influences. However, a low coefficient of thermal expansion (CTE of ~12 ppm/K), achieved via high filling degrees with glass or mineral fibers, in many cases leads to mechanical stress within the joined connection.
As the initial compression shear strength figures in Table 1 illustrates, adhesion to LCP is low in the beginning. This is especially true after climatic storage at higher temperatures and high air humidity. The same diagram also shows that adhesion can be improved significantly through plasma pretreatment. As the results show, LCP is easy to bond after all.
This improved adhesion is achieved by the plasma pretreatment, which breaks up the surface of LCP and exposes the glass fibers more strongly, as illustrated by these scanning electron microscopy (SEM) micrographs. However, excessive pretreatment can lead to poorer adhesion because the glass fibers are then no longer embedded sufficiently in the LCP matrix. Generally, low pressure plasma is the LCP pretreatment method of choice, and the gas being used (air, oxygen, or nitrogen) does not play any decisive role.
The image illustrates the process of plasma pretreatment on Liquid Crystal Polymer (LCP) surfaces.
Over the last few years, DELO has developed a range of adhesives that demonstrate optimal performance on LCPs. Since most of them are designed for consumer electronics, they not only exhibit good adhesion, but also high strength in light fixation, very good drop and humidity resistance, and are suitable for low temperature curing at 80 °C 60 °C. Along with these, there are also products for die attach applications that demonstrate good reflow stability.
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DELO is a leading provider of high-tech adhesives. For over 25 years, we have offered solutions for the automotive, electronics, and semiconductor industries. Our innovative technologies set industry standards. Companies like Bosch, Huawei, and Siemens rely on DELO for superior adhesive technologies.
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