PCB Technical - How to cure SMT red glue?

Characteristics of red glue before curing:

1.Viscosity: reflects the viscosity of the colloid, affecting the ease of sizing, the size of the viscosity is related to the temperature.

2.Thixotropy is the destruction and recovery of structure (1rpm viscosity/10rpm viscosity).

3.Yield value is the force required to make a sample just start to flow, we can think of a liquid with a yield value as behaving like a solid at rest.By measuring the yield value, we can understand the stability, use and processing performance of the sample.

4.Wet Strength: The strength of a patch adhesive to hold a component before it cures.

Wet strength of adhesive x area > mass of component x acceleration Allowable displacement not more than 150 microns

5.Bond strength: thrust meter test

Adhesive strength three types: shear strength, pull-off strength, torque strength.

After the red glue is applied,the components are mounted and then they can be sent to the curing oven for curing. Curing is a key process in the red glue-wave soldering process. In many cases, the red glue is poorly cured or not fully cured (especially PCB It is most common when the upper component keys are unevenly distributed), during transportation and welding, the components will fall off. Therefore, curing should be done carefully. The types of glue used are different, and the curing methods are also different.Two methods are commonly used for curing, one is thermal curing and the other is light curing. The following are discussed in turn:

1.Heat curing

Epoxy red glue is cured by heat.The early heat curing was carried out in an oven, but now, it is mostly cured in an infrared reflow oven to achieve continuous production. Before the formal production, the furnace temperature should be adjusted first, and the furnace temperature curing curve of the corresponding product should be made.When making the curing curve,pay attention to the curing curve of red glue of different manufacturers and different batches will not be exactly the same; even if the same kind of red glue,when used in different products,the set temperature will be different due to the difference in board size and components.This is often overlooked. It is often the case that when soldering IC devices, after curing, all the pins still fall on the pads,but after wave soldering,the IC pins will shift or even leave the pads and cause soldering defects. Therefore, to ensure the quality of welding, we should insist on making a temperature profile for each product, and we must do it carefully.

(1) Two important parameters for epoxy adhesive curing

The thermal curing of epoxy resin red glue is essentially the curing agent catalyzing the epoxy gene at high temperature.A chemical reaction occurs when the ring is opened.Therefore,during the curing process,there are two important parameters that should be paid attention to one is the initial heating rate; the other is the peak temperature.The heating rate determines the surface quality after curing,and the peak temperature determines the bonding strength after curing.These two parameters should be provided by the red glue supplier, which is more meaningful than the supplier only providing the curing curve.It can make you understand the properties of the red glue used.

It can be seen from the figure that the effect of bonding temperature on bonding strength is more important than the effect of time on bonding strength.At a given curing temperature,as the curing time increases,the shear force increases slightly,but when the curing temperature rises When it is high, the shear strength increases significantly in the same curing time,but pinholes and bubbles sometimes appear when the heating rate is too fast.Therefore, in production, the PCB light board without components should be used to dispense glue and then put in an infrared oven to solidify.After cooling, use a magnifying glass to carefully observe whether there are bubbles and pinholes on the surface of the red glue. If pinholes are found, analyze carefully. Reasons and find out how to eliminate them. When making the furnace temperature curing curve, these two factors should be combined with repeated adjustments to ensure a satisfactory temperature curve.

(2) Test method of curing curve

The test method and instrument used for the curing curve of red glue in the infrared reflow oven are the same as the method of the temperature curve of the solder paste infrared reflow oven, so I will not introduce it here. The heating rate and curing furnace temperature curve can be designed according to the parameters provided by the supplier. In addition to negotiating with the supplier when there is a dispute, you can also go to the relevant testing department to perform differential scanning thermal analysis (DSC) to identify the performance of the adhesive.

2.Light curing

When light-curing adhesives are used, a reflow oven with UV light is used for curing,and the curing speed is fast and the quality is high.Usually, the power of the additional lamp tube attached to the reflow furnace is 2-3kW, and the height is about 10cm from the PCA. After 10-15s, the red glue exposed to the outside of the component is quickly cured, while the temperature in the furnace continues to be maintained at 150-140 degree Celsius for about 1min. The red glue under the component can be cured thoroughly.

SMT red glue curing effect is not good solution:

1.Check curing conditions

Temperature control:Ensure that the curing temperature is at the recommended value (usually 150°C with a curing time of 90-120 seconds).If the temperature is too low,the curing is incomplete; too high may lead to brittle red adhesive.

Compare the cure curves of different batches of raw material to check for differences.

Curing time:Check that the curing time is sufficient.If the time is too short,the adhesive may not be fully cured.It is recommended to extend the curing time,especially when using multiple laminated components.

2.Increase the amount of glue

Glue volume:Ensure that a sufficient amount of red glue is applied.If the amount of glue is insufficient,the components may not be firmly fixed on the PCB.The amount of glue used can be increased by optimising the dispensing parameters or adjusting the template design.

3.Dealing with Contamination

Surface Cleanliness:Ensure that the PCB and component surfaces are free of contaminants such as dirt,grease or release agents.If necessary,use solvents to clean the surface to improve the adhesion of red glue.

Remove Air Bubbles:Ensure that air bubbles are not introduced during the red glue application process, which can lead to uneven curing and lack of strength.Observe that the glue application process is smooth and air bubbles should be eliminated as much as possible.

4.Re-curing Process

Hot Air Gun or Reflow Soldering:For boards that have been glued but not fully cured,a hot air gun or reflow soldering can be used to re-cure the red glue.Ensure that the hot air is applied evenly to the glue surface for effective curing.Proper curing time and temperature settings will help improve curing results.

Adjust the curing equipment:Check the operation of the curing equipment to ensure that the temperature is stable and that there is good hot air circulation.If necessary, adjust the temperature profile of the curing oven to ensure optimum results are achieved.

5.Monitoring and Testing

Thrust Test:After curing is complete,perform a thrust test to verify that the quality of the cure is up to standard.The maximum thrust that each component can withstand varies by size,and generally ranges from 1kg to 2kg for 0603,0805, and 1206 packages.

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