Factors of affecting the properties of conductive adhesives

In practical production and life, conductive adhesive has many applications. In the application process, there are many factors that can affect the performance of conductive adhesive, such as: humidity, temperature, external force and so on. These factors may affect the adhesive strength of conductive adhesive and the resistance coefficient of conductive adhesive. Accelerated test is usually used to test whether the conductive adhesive meets the practical needs, such as high temperature and humidity, loading / stress, thermal cycle, thermal shock, etc. It is generally stipulated internationally that the resistance change rate of commercial conductive adhesives is less than 20% before and after accelerated tests (high temperature and high humidity (85, 85% RH) aging for 500 hours), so conductive adhesives are considered to be stable and up to standard [14]. Next we will introduce the factors that affect the properties of conductive adhesive.



1. the influence of moisture on conductive adhesive. The water in conductive adhesive may play two roles of plasticizing and soaking. The plasticization of water can reduce the glass transition temperature, strength and modulus of the adhesive; the immersion of water can produce immersion stress, even irreversible physical and chemical changes. Water can corrode the interface between the adhesive and the matrix, thereby reducing the strength of the joint [13].



2. effect of high temperature on bonding strength



The effect of high temperature on bonding strength is great: if the melting point of conductive adhesives is low, such as some thermoplastic adhesives, good performance at room temperature, but when the temperature increases, close to the glass transition temperature, plastic flow will lead to joint deformation, strength becomes low. If some thermosetting materials do not soften the flow at high temperatures, the key problem is that the strength decreases due to thermal oxidation and high temperature decomposition [13]. Low temperature also has an effect on bonding strength: there is stress in the conductive adhesive. At room temperature, low modulus conductive adhesive can easily eliminate stress concentration by deformation, but at low temperature, the elastic modulus increases, conductive adhesive is difficult to eliminate stress concentration. There are stress concentration and stress gradient in the conductive adhesive, which may cause the expansion and contraction of the adhesive joint to change, and lead to the failure of the adhesive joint [13].



3. effect of electrochemical corrosion on contact resistance



Some scholars have studied that electrochemical corrosion is the dominant factor for the instability of contact resistance. Electrochemical corrosion usually requires the following conditions: (1) the presence of water and electrolytes; (2) metal contact with different electrochemical potentials; (3) two metals must have an electrochemical potential below 0.4V [8].



Therefore, in order to inhibit electrochemical corrosion, we should take the following measures:

(1) select conductive adhesive with less water absorption. The resin matrix and curing agent have great influence on the water absorbency of conductive adhesive. High water absorption and chemical bonding between aliphatic epoxy resin and bisphenol F epoxy resin CHEMISTRYANDADHESION

Bisphenol F epoxy resin is preferred, and the contact resistance of the curing agent is the most stable after high temperature and humidity aging due to the few polar functional groups in the cured hexahydrophthalic anhydride.

(2) add organic corrosion inhibitor. Organic corrosion inhibitors can be attached to metal surfaces, forming a protective film to isolate water and air from the outside world. The experimental results show that the stability of conductive adhesives at high temperature and high humidity has been greatly improved by using anti-corrosive agents.

(3) add deoxidizer. An important element of electrochemical corrosion is oxygen, which can be removed from water in contact with conductive adhesives by adding reductants. The addition of deoxidizer has little effect on the mechanical properties and bonding properties of conductive adhesive, but as the deoxidizer is constantly consumed, with the passage of time, the deoxidizer in conductive adhesive decreases, its inhibition on oxygen will be reduced, so this method only plays a slow role [8]. In addition, the joints of metals such as Cu coated surfaces are severely eroded by water, possibly because the metal Cu reaches the surface of the substrate and oxidizes. Elastic fillers can better compensate the mechanical stress produced by thermal cycling compared with hard fillers. So many researchers have focused on polymer composite conductive particles. It is used as conductive filler [14] for conductive adhesive. For copper conductive adhesives, silver plating can also be used to prevent oxidation. After high temperature exposure test (80 C, 1000 h), the electrical and mechanical properties of the adhesives are almost the same as those of silver conductive adhesives, but the contact resistance increases after 50-100 thermal cycles.



4. impact of external force on conductive adhesive

In the process of assembling PCB, it is inevitable that impact and vibration will occur, so conductive adhesives used here must have good impact resistance. The strength of conductive adhesives is slightly less than that of the existing tin-lead solder. NCMS requires that PLCC carriers can withstand six drops from 1.524m height, but most of the existing conductive adhesives can not meet this point [8]. Matrix adhesive is an important factor affecting the impact toughness of conductive adhesive. Epoxy resin is usually used in conductive adhesive with poor toughness. Therefore, chemical or physical toughening of conductive adhesive matrix can improve the impact toughness of conductive adhesive.