The SMT process involves a variety of adhesive materials, such as patch adhesives for fixing chip components, sealants for positioning coils and some components, plug-in adhesives for temporarily bonding surface assembly components, etc. These adhesives The adhesive mainly plays the role of bonding, positioning or sealing. In addition, there are some adhesives with special properties, such as conductive adhesive, which can replace solder and play a welding role during the assembly process.

Among the above adhesives, the most important to the SMT process is the patch adhesive.

1. The use of patch adhesive

      SMT glue, also known as mounting glue and SMT red glue, is a red paste with curing agents, pigments, solvents, etc. evenly distributed in the adhesive. It is mainly used to fix components on printed boards. Generally distributed by dispensing or screen printing. After attaching the components, place them in an oven or reflow machine for heating and hardening. Once heated and hardened, they will not melt if reheated. In other words, the thermal curing process of the patch adhesive is irreversible. The use effect of SMT patch adhesive will vary depending on the heat curing conditions, the objects to be connected, the equipment used, and the operating environment. When using it, you should choose the patch adhesive according to the production process. In SMT production, its main functions are:

(1) When using wave soldering, in order to prevent the components from falling when the PCB passes through the solder groove, the components are fixed on the printed board.

(2) In the double-sided reflow soldering process, in order to prevent the large components on the soldered side from falling off due to the heat melting of the solder, patch adhesive must be used to fix them.

(3) Used in the reflow soldering process and pre-coating process to prevent displacement and standing pieces during mounting.

(4) In addition, when PCBs and components are changed in batches, use patch tape to mark them

2. Chemical composition of patch adhesive

     Surface assembly patch adhesive usually consists of matrix resin, curing agent and curing accelerator, toughening agent and filler.

(1) Matrix resin. It is the core of patch adhesive, generally using epoxy resin and acrylate polymer. In recent years, polyurethane, polyester, silicone polymers and epoxy resin-acrylate copolymers have also been used.

(2) Curing agent and curing accelerator. Commonly used curing agents and curing accelerators are dicyandiamide, boron trifluoride-amine complex, imidazole derivatives, amides, triazines and tribasic acid hydrazides.

(3) Toughening agent. Since the simple matrix resin is relatively brittle after curing, in order to make up for this defect, a toughening agent needs to be added to the formula. Commonly used tougheners include dibutyl phthalate, dioctyl phthalate, liquid nitrile rubber and polysulfide rubber.

(4) Filling. Adding fillers can improve the electrical insulation performance and high temperature resistance of the patch adhesive, and also enable the patch adhesive to obtain appropriate viscosity and bonding strength. Commonly used fillers include silica powder, calcium carbonate, bentonite, white carbon black, diatomite, titanium dioxide, iron red and carbon black.

3. Classification of SMD adhesive

  1. According to base material

    According to the base material, patch adhesive can be divided into two categories: epoxy resin and polypropylene.

Epoxy resin is the oldest and most widely used thermosetting, high-viscosity patch adhesive, and is commonly used as a two-component adhesive. Polypropylene patch adhesive is commonly used as a single component. It cannot be cured at room temperature. It is usually cured by short-term ultraviolet irradiation or infrared radiation. The curing temperature is about 150°C and the curing time is about tens of seconds to several minutes. It belongs to ultraviolet rays. Heated dual cure type.

  2. According to functions

    According to function, patch adhesives are divided into structural type, non-structural type and sealing type.

The structural type has high mechanical strength and is used to permanently bond two materials together and can firmly join them under a certain load. The non-structural type is used to temporarily fix objects with small loads, such as bonding SMD to PCB for wave soldering. The sealed type is used to bond two unloaded objects for gap filling, sealing or encapsulation purposes. The first two adhesives are hard in the cured state, while sealing adhesives are usually soft.

  3. According to chemical properties

According to chemical properties, patch adhesives include thermosetting, thermoplastic, elastic and synthetic types.

(1) The thermosetting adhesive will not soften if heated again after curing, and the bonding connection cannot be re-established. Thermosetting types can be divided into two categories: single-component and two-component. The so-called one-component means that the resin and curing agent have been mixed during packaging. It is easy to use and has stable quality, but it is required to be stored under refrigerated conditions to avoid solidification; the two-component resin and curing agent are packaged separately and mixed only when used, so the storage conditions are not harsh. However, the ratio during use is often inaccurate, which affects performance. Thermosetting types can be used to bond SMD to PCB, mainly including epoxy resin, nitrile acrylate, polypropylene and polyester.

(2) The thermoplastic type can be softened again after curing to form a new adhesive. It is a one-component system.

(3) Elastic adhesive is a material with a large elongation rate. It can be formulated from synthetic or natural polymers with solvents and is in a milky form, such as urethane, silicone resin and natural rubber.

(4) Synthetic adhesives are formulated from a combination of thermosetting, thermoplastic and elastic adhesives. It exploits the most useful properties of each material. Such as epoxy-nylon, epoxy polysulfide and vinyl-phenolic plastics.

  4. According to usage method

According to the usage method, it can be divided into patch adhesives suitable for needle transfer, pressure injection, screen/template printing and other process methods. The characteristics of typical patch adhesives are 

 

4. Surface assembly requirements for patch adhesive

In order to ensure the reliability of surface assembly, the chip adhesive should meet the following requirements.

(1) The service life at room temperature should be long.

(2) Suitable viscosity. The viscosity of the patch adhesive should be able to meet the needs of different gluing methods, different equipment, and different gluing temperatures. Glue should not be drawn when dripping; it should maintain a sufficient height after application without forming a large glue base; glue droplets should not flow overflowing from application to solidification to avoid flowing to the welding part and affecting the welding quality.

(3) Rapid curing. SMD adhesive should be cured at the fastest possible speed at the lowest possible temperature. This can avoid PCB warping and component damage, as well as pad oxidation.

(4) The bonding strength is appropriate. The patch adhesive should be able to effectively fix chip components before welding, and should facilitate the replacement of unqualified components during maintenance. The shear strength of patch tape is usually 6 to 10 MPa.

(5) Others. There should be no outgassing after curing and during welding; it should be compatible with chemical agents in subsequent processes without chemical reactions; it should not interfere with circuit functions; it is colored for easy inspection; the typical color of patch adhesive for SMT is red or orange.

(6) Packaging of patch tape. Currently, there are two main forms of packaging for patch adhesives on the market. One is injection needle type packaging, which can be used directly on the dispensing machine. Its packaging specifications mainly include 5 ml, 10 ml, 20 ml and 30 ml; in addition, there are also large packages of 300 ml syringe tubes, which are divided into small syringes before use and put on the dispensing machine. Note that special tools should be used when dispensing large packages into small injection syringes.