Patch glue application failure and its application quality control countermeasures
1. SMD glue application and its main faults
SMD glue is used as a bonding agent and is often used in wave soldering as the glue for SMC/SMD to be placed on the substrate.
Adhesive coating can use dispenser dispensing (also known as syringe dispensing) technology, needle transfer technology and screen (or template) printing technology. Dispenser dispensing is to apply the adhesive drop by drop on the PCB mounting SMC/SMD part; the needle transfer technology generally transfers the adhesive to all parts of the PCB mounting SMC/SMD at the same time in groups ± o Dispenser dispensing technology is characterized by strong adaptability, which is especially suitable for the application of adhesives in a variety of product occasions; easy to control, convenient to change the adhesive dosage to meet the requirements of components of different sizes; and because the placement glue is in a sealed state State, its bonding performance and coating process are relatively stable. Screen printing technology is similar to solder paste printing technology. It uses a printing machine to print a certain amount of adhesive onto all parts of the PCB mounting SMC/SMD through a screen (or template). When the method of applying the adhesive is different, the performance requirements of the adhesive are also different.
After the patch glue is applied, the SMC/SMD is positioned by the curing shrinkage of the glue. The shrinkage stress is directly related to the amount of patch glue applied. If the amount of coating is large, the bond strength will increase, but if the bond strength is too large, it is likely to cause microcracks in the SMC/SMD matrix. In addition, factors such as the control of the amount of coating, the guarantee of the accuracy of the amount of coating, and the reasonable ratio of glue components will affect the quality of the joint between the SMC/SMD electrode terminal and the substrate welding area. If the coating is too much, the adhesive will overflow after the SMC/SMD is mounted, which will cause failures between the solder joints or the soldering area and the wiring; if the coating is too small, it will be difficult to observe from the outside, but it will decrease Welding strength. The main parameters that affect the coating quality of the adhesive are the thixotropy of the adhesive, etc. Therefore, when applying the adhesive, performance tests should be carried out according to the coating method and bonding requirements in order to correctly select the process parameters.
For example, in the rotary screw pump coating method, there are nearly 10 main parameters that affect the quality of spot coating. See Table 5.1. These parameters can all be controlled. Each dot coating pattern is the result of the joint action of these parameters, and the dot coating quality directly affects the soldering quality of the component pins. Therefore, for components with different package shapes and sizes to achieve good soldering quality, it is necessary to set the appropriate coating process parameters, otherwise it will often cause the dot coating pattern to be tailed, the pad is contaminated, the material is blocked, and the pattern is incomplete. Kind of spot coating defects. Figure 5.6 shows the smearing and incomplete graphics caused by the unreasonable setting of the dot coating height.
Table 5.1 The influence of dispensing parameters on the quality of dispensing
The influence of parameter name on the quality of spot coating
Needle inner diameter Dispensing amount, material clogging
Dispensing height Concavity, tailing, accuracy of dispensing position, contamination of the needle
Application time or speed Inappropriate amount of application, depression, tailing
Retention time of coating point Tailing and contamination of needle
The residence time at the beginning and end of the line. Improper amount of coating at the beginning and end, smearing at the end and contamination of the needle
Close the length of the pump ahead of time from the end of the line
Z-axis recovery height
a) The height is too high, resulting in smearing; (b) the height is too low, and the graphics are incomplete.
2. Countermeasures for quality control of patch adhesive coating
There are some specific requirements for adhesives in different coating processes. For example, when the adhesive is applied by dispenser dispensing and needle transfer technology, it is required that the adhesive can smoothly leave the needle or the end of the needle without forming “strings”, inaccurate or random application. To apply the phenomenon, this requires:
(1) The adhesive has a certain wetting power on the PCB surface and can wet the PCB surface;
(2) The wetting force (surface tension) of the adhesive on the PCB surface must be greater than the wetting force (surface tension) of the needle tube and needle, and greater than its own cohesive force;
(3) The wetting power and other properties of the adhesive are stable, with a wide application range, and its performance is not affected by changes in the PCB material to be bonded.
This is because when using dispenser dot coating and needle transfer process, if the adhesive has a small wetting force on the PCB surface, it will be difficult to apply; if it has strong cohesion, it will form “strings” Coating phenomenon: If there is no stable performance and a certain range of adaptation, the coating process will be poor.
No matter what coating process is used, when the adhesive is applied, it should be avoided that there are contaminants in the adhesive and on the PCB and SMC/SMD; the adhesive cannot interfere with good solder joints, that is, it cannot contaminate the pads and component terminals; poor coating The adhesive can be cleaned from the PCB in time; the selected packaging form should be compatible with the coating equipment and storage conditions. Table 5.2 lists the common faults and cause analysis in the application of patch glue, as well as reference solutions.
Table 5.2 Common Problems and Cause Analysis of Glue Dispensing
Problem Cause Analysis Solution
The tail is silky. The distance ND between the needle and the substrate is too large. Adjust the height of the dispensing head to reduce ND
The delay time after dispensing is too short. Reset to extend the waiting delay time
The viscosity of the patch adhesive becomes larger 1. The temperature recovery time is not enough, generally about 24h
2. The dispensing temperature is not enough, generally about 25P~30C
3. There is no refrigeration or expired to make the glue deteriorate, the general storage temperature is 2*C ~
8C, storage period is 6 months to 12 months
The return height of the Z axis is too low. Increase the return height of the Z axis
Time is too short/pressure is too small 1. Extend the dispensing time
2, Increase the dispensing pressure
The needle is inappropriate, the inner diameter is too large, replace with a small inner diameter dispensing needle
The substrate is distorted and deformed, replace with a good quality substrate
The glue dot is uneven. The distance between the needle and the substrate ND is too large. Adjust the height of the glue head to reduce ND
There are bubbles in the glue. The glue in a large tank must be centrifuged to remove the bubbles in the glue.
Poor glue uniformity 1. Poor glue quality, change glue
2. The glue has deteriorated, change the glue
Impurities in the glue 1. The glue is of poor quality, change the glue
2. Impurities mixed into the packaging process
3. The needle and syringe are not cleaned, if possible, they should be cleaned once per shift
Lack of glue at the front of the needle 1. New glue should be forced to wipe off after pressing the glue from the needle to ensure that the glue is full of the needle
2. If the timeout is too long, the pilot function should be used
The substrate is distorted and deformed, replace with a good quality substrate
(Continued)
Problem Cause Analysis Solution
Component shifting uneven glue dots 1. The glue quality is poor, change the glue
2. The glue has deteriorated, change the glue
The amount of glue is too small, increase the amount of glue or use more glue points
Mushroom-shaped glue point The distance between the needle and the substrate ND is too small Increase the distance between the needle and the substrate
Needle inner diameter NID is too small, change to a large inner diameter needle
In the patch glue coating process, the following matters should be paid attention to:
(1) Choose the appropriate patch adhesive. The characteristics of patch adhesives vary with different components, and should be reasonably selected according to factors such as the selected coating form, the required curing time, the characteristics of the bonding object, the used dispensing equipment conditions, and the process environmental conditions.
(2) Choose appropriate process parameters. Bonding different SMC/SMD has different process parameters. According to the type of SMC/SMD (shape, weight, etc.), choose a reasonable dispensing thickness, number of glue dots, glue dot diameter, as well as the nozzle diameter and coating of the dispenser. Process parameters such as application pressure and time, application temperature and so on.
(3) Correct process operation specifications. The use of patch glue and dispenser must have correct process and operation specifications, otherwise it is extremely prone to failure. In addition to the correct operation and use according to the operating procedures of the dispenser and the use conditions of the patch glue, it should also be noted that the patch glue should be loaded into the syringe in a completely degassed (no bubbles) state; no bubbles and debris can be mixed into the vacuum tube The patch glue should not be exposed to the air for more than half an hour; do not let the patch glue contact the skin; try to ensure the constant temperature use conditions of the patch glue.