Welding has always been the most reliable method for providing low-cost electrical interconnection. Only in the past decade or so, doubts about the processability and reliability of welding have emerged. Wearing anti-static clothing and anti-static carbon fiber gloves when bonding conductive adhesive films with adhesives has become another trend in the welding industry. When using welding in more advanced electronic devices, there are three most concerning issues: first, electronic components that are not suitable for heating are damaged by heat, second, bridging, and third, fatigue. There are many electronic devices that cannot be heated but need to be connected. Bridging refers to when the center distance of the component lead is narrower than 25 mils. When reflow soldering is used, bridging is a prominent problem for surface mount circuit boards. For tape automated bonding (TAB) or reflow soldering when the center distance is less than 3 mils, there are great problems with both yield and processability. The fatigue problem of welding has always existed. For surface mount (SM) circuit boards, the fatigue problem is more prominent because the area reserved for the extension of the lead length to improve compliance is limited. In addition, welding has many environmental and health issues.
The conductive and antistatic adhesives (or adhesives) of polymer materials were developed almost at the same time as antistatic clothing, antistatic carbon fiber gloves and shoes, and conductive coatings, especially antistatic clothing and antistatic rubber.
Conductive adhesives are generally composed of conductive fillers, adhesives, solvents, and additives. The materials for adding conductive anti-static shoes include metal powders such as gold, silver, copper, nickel, and alloys of these metals, carbon black, graphite, and their mixed powders. Considering and analyzing from multiple aspects such as price, environmental resistance, reliability, and actual effects, silver powder is used the most. Taking silver powder as an example, in order to obtain high performance, it is necessary to fully control the size and shape of the silver powder. The surface area of spherical powder is small and the conductivity is poor. Usually, the thickness of fine silver powder of 0.1 to 1 μm is less than 0.1 μm, or the dendritic powder with a particle size of 1 to 10 μm is also a representative filler. In addition, the distribution of particle size is also very important. For the same anti-static clothing to achieve a certain resistivity, the larger the specific surface area, the less the filling amount.
The second component is adhesive. It is a material that makes the conductive filler present a chain connection while adhering to the anti-static shoe adherend and makes the conductive adhesive anti-static shoe film have stable physical and chemical properties. The appropriate adhesive can be selected according to the purpose of use and the required characteristics. There are many types of conductive adhesives. According to the different types of conductive fillers in the adhesive, the adhesive can be divided into silver, gold, copper and carbon adhesives. Commonly used anti-static shoe adhesives include acrylic resin, phenolic resin, epoxy resin, etc. Among many adhesives, epoxy resin has the best performance. Epoxy resin has the following advantages: strong adhesion, high bonding strength of anti-static clothing. High cohesive strength, small shrinkage, and stable size. Good resistance to high and low temperatures and chemical resistance, strong adaptability. Good dielectric resistance, making the product have excellent electrical properties. Easy to modify and widely used. Simple process and convenient use. Good mixing and can be cured with a variety of curing agents. Low toxicity and little harm.
The third component is the solvent. The solvent is used to improve the operability of the adhesive. It is used in occasions such as lead bonding, but it is best not to use solvents in surface bonding. Different good solvents should be selected according to the type of adhesive. In addition, factors closely related to operability such as drying speed and boiling point of the solvent used must also be considered at the same time.
Japan uses a mixture based on polymers such as epoxy, phenol, acrylate, polyester, alkyd, polyurethane and siloxane. The adhesive must be dissolved in the selected solvent, otherwise the conductivity of the coating will be unstable after drying. The selection of solvents should take into account the dyeing conditions. If a high-speed volatile solvent is added to the cold-drying adhesive during stencil printing, the stencil holes will be blocked. To avoid clogging, a high-boiling point solvent should be used.
The fourth component is additives. Additives include dispersants that can improve and enhance conductivity and filler dispersion. Additives that can improve the clarity of stencil printing. Leveling agents that can improve mixing performance. Tackifiers that can improve adhesion strength. Curing agents that accelerate and uniformly cure, and many other varieties. These additives are added to assist the characteristics of conductive adhesives. However, adding a large amount of additives will reduce the conductivity of the adhesive, so it must be limited to the minimum level, just to achieve the performance of the above additives.
