What is The Selection And Use of Red Glue in The SMT Process

With the rapid development of the electronics industry, SMT has become the mainstream of electronic processing factories. From the earliest hand patch to the current automated equipment patch, most of them use the SMT solder paste process. However, many companies’ products have unavoidable plug-in device production (such as computer industrial control, power supply and other products), so the earlier red glue process and the later through-hole wave soldering process appeared. However, the production of the red glue process has very strict requirements on the control of wave soldering and the manufacturability design of the PCB.

Selection and use of red glue:

1. Since it is not a dispensing process, but a printing process of red glue, there are certain requirements for the thixotropic index and viscosity of the red glue. The body of some ICs cannot be glued to the red glue and is prepared. (reference thixotropic index: 4-5; reference viscosity: (8-10) x 1000000).

2. The uncured glue sticking to the circuit board can be wiped off with acetone or propylene glycol ether or cleaned with a special cleaning agent for red glue.

Understanding of glue: Red glue, also known as SMT adhesive, SMT red glue, is a red paste with evenly distributed hardeners, pigments, solvents, etc. On the board, it is generally distributed by dispensing or stencil printing. After attaching the components, put it in an oven or reflow soldering machine to heat and harden. It is different from the so-called solder paste. Once heated and hardened, it will not be melted when heated again. That is to say, the thermal hardening process of the patch glue is irreversible. The use effect of SMT patch glue will vary due to thermal curing conditions, connected objects, equipment used, and operating environment. When using, the patch glue should be selected according to the production process.

Main components: base material (ie main polymer material), filler, curing agent, other additives, etc.

Since red glue is affected by temperature and has its own viscosity, fluidity, wetting and other characteristics, red glue must have certain conditions of use and standardized management.

1. The red glue must have a specific serial number, which is numbered according to the quantity, date and type of the feed.
2. Red glue should be stored in the refrigerator at 2~8℃ to prevent the characteristics from being affected by temperature changes.
3. The red glue is required to be warmed for 4 hours at room temperature, and it is used in the order of first-in, first-out.
4. For the dispensing operation, the red glue of the hose should be defoamed. For the red glue that is not used up at one time, it should be put back in the refrigerator for storage. The old glue and the new glue cannot be mixed.
5. To accurately fill in the return temperature record form, return temperature person and return temperature time, the user needs to confirm that the return temperature is OK before use.

Usually, the expired red glue cannot be used. It can be stored for 7 days at room temperature, more than 6 months when it is less than 5 °C, and more than 30 days at 5~25 °C.

In the production process, the equipment selection is determined according to the process requirements and the special requirements of the product:

Printing method: The stencil hole should be determined according to the type of the part, the performance of the substrate, its thickness and the size and shape of the hole. Its advantages are high speed and high efficiency.

Dispensing method: Dispensing is to use compressed air to dispense red glue onto the substrate through a special dispensing head. The size and number of glue dots are controlled by parameters such as time and pressure tube diameter. The dispensing machine has flexible functions. For different parts, we can use different dispensing heads, set parameters to change, and also change the shape and number of glue points to achieve the effect. The advantages are convenience, flexibility and stability. The disadvantage is that it is prone to drawing and air bubbles. We can adjust the operating parameters, speed, time, air pressure, and temperature to minimize these shortcomings.

Needle rotation method: The needle rotation method is to immerse a special needle film into the shallow glue tray. Each needle has a glue point. When the glue point touches the substrate, it will be separated from the needle head. The amount of glue can be determined by the shape and diameter of the needle. vary in size.