What is conductive adhesive?
Conductive adhesive is adhesive that has conductive performance in solid or dried state. Its main components are matrix resin and conductive fillers, also means conductive particles. The particles of the conductive component are in contact to each other and in this way make electric current possible.
What is PTH?
PTH is “Electroless Plating Copper”, also known as "Plated Through Hole", which is a self-catalytic redox reaction. Two or multiple layer PCB will start the PTH technology after finishing drilling process. The role of PTH is to deposit a thin layer of chemical copper on the hole wall of the non-conductive substrates that have been drilled by chemical method to serve as the base for electroplating copper behind.
In the manufacture of PCBs, the cost of conductive adhesive process is only one-sixth of the PTH process. In order to save costs some enterprises reduce the quality standards, they take advantage of conductive adhesive technology rather than PTH technology.
The Hidden Trouble of Conductive Adhesive Application
The application of conductive adhesive mainly focuses on two aspects: its conductivity and adhesion strength. In actual production, conductive adhesive is usually that of the adhesive with the right amount of initiator or too much initiator, but it cannot be solidified due to the lack of initiator, which will directly lead to component scrapping when heated.
(1) The initiator fails. The initiator is responsible for inducing the resin polymerization to form a network polymer. Failure of the initiator, such as partial decomposition caused by medium and high temperature before the initiator is contaminated or mixed with two components, will lead to insufficient bonding strength of the bare chip and the risk of scrapping the adhesive chip. At the same time, incomplete resin polymerization will also lead to large resistance and other phenomena, and in serious cases, even a break will be formed, resulting in components scrap.
(2) Resin matrix is contaminated. If the resin body is polluted when it is used in encapsulation or unsealing, the pollutant may block the polymerization reaction and interfere with the polymerization process. In the same way, the adhesive strength of the bare chip will be insufficient, and the circuit will be broken in serious cases and cannot be used.
(3) Mixing process is out of control. The mixing process of the two-component conductive adhesive will determine the distribution density of the initiator in the resin matrix. When the density is not uniform, some of the resin matrix will be short of initiator or even no initiator. Since most of the two-component conductive adhesive is pre-mixed with silver powder in the two-component, the appearance of this part when extruded will not be significantly different from that of the adhesive with the right amount of initiator or too much initiator, but it cannot be solidified due to the lack of initiator, which will directly lead to component scrapping when heated.
(4) Use beyond the shelf life. In the actual use process, once the conductive adhesive has been used beyond the shelf life, it will bring a lot of problems of conductivity and adhesion strength. As it belongs to a special process, once the conductive adhesive curing cannot return to the state before curing, it is very difficult to find the causes of the problems.
(5) Abnormal temperature of the refrigerator. Daily conductive adhesive used for storage of cryogenic refrigerator generally set to -40℃, the experimental data showed that the temperature will be back to -10℃ or so, if the power goes out for 2 hours. The door of the refrigerator, not be closed well because of icing, also can cause abnormal temperature rise. These phenomena will cause the actual shelf life of conductive adhesive to be shorter than the standard shelf life, and affect the resistance and bonding strength after curing.
(6) Illegal repeatedly rising temperature. This phenomenon belongs to management problems. Maybe due to the concept of saving materials, and at the same time, a very tight schedule and the lack of the conductive adhesive, which makes few researchers think wantonly it is ok to use the conductive adhesive that was unsealing once because they think it will not affect the performance of conductive adhesive in the following process. Actually, the resin matrix of conductive adhesive is usually epoxy resin or acrylic ester. This conductive adhesive will produce a small amount of water vapor after returning to the certain temperature from a low temperature. Most of this water vapor will be heated and volatilized during curing. Many manufacturers have considered the impact on the adhesive itself, so they have made some preventive measures in the formula. However, once the conductive adhesive is put back into the refrigerator after the rising temperature process, the moisture absorbed in a long time will cause a chronic poisoning phenomenon of the initiator, slowly leading to the failure of a large number of initiator molecules, which may also cause serious problems of electrical conductivity, adhesion strength, reliability and other aspects.
(7) Process failure. In the whole manufacturing process, the failure during the process may cause the failure of the conductive adhesive, such as the excessive use time, the curing temperature not reached, the insufficient curing time ad so on. In these states, the solidified conductive adhesive resistance cannot reach the nominal value and may have an order of magnitude difference. The shear force test value representing the adhesive strength is also far from the nominal value.
Conclusion:
This paper summarizes some problems that may occur when conductive adhesive is used in PCB manufacturing. It is hoped that more people can see and understand that although the cost of using conductive adhesive is low and the production efficiency is high, the quality of products of customers should also be considered. If the problem of production is always considered from the perspective of cost saving, the enterprise will eventually decline. Truly successful businesses are those that take into account the needs of their customers and combine them with their own development needs.
We PCBWay insisted on quality first all the time and never will reduce production standards. We take advantage of PTH technology since we established. For the healthy development of the industry, I hope everyone joins us to refuse conductive adhesive, please do not buy very cheap PCB if you consider the quality. And we will develop ourselves to offer better service, welcome to supervise us.
