Basic knowledge of SMT patch glue: Why do you want to use red glue and yellow glue?
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Basic knowledge of SMT patch glue: Why do you want to use red glue and yellow glue? |
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Basic knowledge of SMT patch glue |
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Sheet rubber is a non-essential process product of pure consumption. Now with the continuous improvement of PCA design and process, through-hole reflow soldering and double-sided reflow soldering have been realized, and the PCA mounting process using patch adhesive is becoming more and more The less the trend.
Patch adhesive, also known as SMT adhesive, SMT red plastic, usually red (also yellow or white) paste is evenly distributed with a hardener, pigment, solvent, etc., mainly used to The device is attached to a printed board and is typically dispensed by dispensing or stencil printing. After the components are attached, they are placed in an oven or reflow oven for heat hardening. It differs from solder paste in that it cures when heated, and its freezing point temperature is 150 ° C. It does not dissolve even when heated, that is, the thermal hardening process of the patch adhesive is irreversible. The use effect of SMT patch adhesive may vary depending on the heat curing conditions, the connected object, the equipment used, and the operating environment. When using, select the patch adhesive according to the printed circuit board assembly (PCBA, PCA) process. .
Characteristics, application and prospect of SMT patch adhesive
SMT patch red glue is a kind of poly-dilute compound, the main components are base material (ie, main body high molecular material), filler, curing agent, other additives. SMT patch red plastic has viscosity fluidity, temperature characteristics, wetting characteristics and the like. According to this characteristic of red plastic, in the production, the purpose of using red plastic is to make the parts firmly adhere to the surface of the PCB to prevent it from falling. Therefore, the patch adhesive is a non-essential process product of pure consumption. Now with the continuous improvement of PCA design and process, through-hole reflow soldering and double-sided reflow soldering have been realized, and the PCA mounting process using the patch adhesive is There are fewer and fewer trends.
The purpose of SMT patch glue
1 Prevent the components from falling off during wave soldering (wave soldering process). When wave soldering is used, the components are fixed to the printed board in order to prevent the components from falling when the printed board passes through the solder bath.
2 Prevent the other components from falling off during reflow soldering (double-sided reflow soldering process). In the double-sided reflow soldering process, in order to prevent the large device on the soldered side from being detached due to the melting of the solder, it is necessary to have the SMT patch adhesive.
3 Prevent component displacement and standing (reflow process, pre-coating process). Used in reflow soldering processes and pre-coating processes to prevent displacement and slabs during placement.
4 Marking (wave soldering, reflow soldering, pre-coating). In addition, when the printed boards and components are changed in batches, they are marked with a patch adhesive.
SMT patch glue is classified according to usage
a) Squeegee type: sizing by stencil printing. This method is the most widely used and can be used directly on solder paste printers. The stencil opening is determined by the type of the part and the performance of the substrate, its thickness and the size and shape of the hole. The advantages are speed, efficiency and low cost.
b) Dispensing type: sizing on a printed circuit board by means of a dispensing device. Special dispensing equipment is required and the cost is high. The dispensing device uses compressed air to point the red glue through the special dispensing head to the substrate. The size, number of the glue points, time, pressure tube diameter and other parameters are controlled. The dispenser 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 convenient, flexible and stable. The disadvantage is that it is easy to have drawing and air bubbles. We can minimize these shortcomings by adjusting the operating parameters, speed, time, pressure, and temperature.
SMT patch adhesive typical curing conditions:
Curing temperature curing time
100 ° C 5 minutes
120 ° C 150 seconds
150 ° C 60 seconds
be careful:
1. The higher the curing temperature and the longer the curing time, the stronger the bonding strength.
2. Since the temperature of the patch adhesive varies with the size of the substrate components and the placement position, we recommend finding the most suitable hardening conditions.
Storage of SMT patch glue:
It can be stored for 7 days at room temperature, more than 6 months at less than 5 ° C, and stored for more than 30 days at 5 to 25 ° C.
SMT patch glue management
Because SMT patch red glue is affected by temperature, it has its own characteristics of viscosity, fluidity, wetting, etc. Therefore, SMT patch red glue must have certain conditions of use and standard management.
1) Red glue should have a specific serial number, which is numbered according to the quantity, date and type of feed.
2) Red rubber should be stored in a refrigerator at 2 to 8 °C to prevent temperature characteristics and affect characteristics.
3) Red rubber back temperature is required to return to room temperature for 4 hours at room temperature, in the order of first in, first out.
4) For the dispensing operation, the hose red glue should be defoamed. For the disposable red glue, it should be put back into the refrigerator for storage. The old glue and the new glue cannot be mixed.
5) To accurately fill in the temperature recovery record, return the temperature and return temperature, the user needs to confirm that the temperature is completed before use. Usually, red glue is not available for expiration.
Process characteristics of SMT patch adhesive
Connection strength: SMT patch adhesive must have strong joint strength. After being hardened, it will not peel off even at the temperature at which the solder melts.
Spot coating: At present, the dispensing method of the printed board is mostly by the point coating method, so the glue is required to have the following properties:
1 Adapt to various placement processes
2 Easy to set the supply amount for each component
3 Simple adaptation to replacement components
4 point coating stability
Adapt to high-speed machine: The patch glue used now must meet the high speed of spot coating and high-speed placement machine. Specifically, it is high-speed point coating without drawing, and when it is high-speed placement, the printing board is in the process of transmission. The viscosity of the sheet glue should ensure that the components do not move.
Drawing and slumping: Once the patch adhesive is applied to the pad, the component cannot be electrically connected to the printed board. Therefore, the patch adhesive must be drawn without drawing after coating, and no slump after coating. To avoid contaminating the pads.
Low-temperature curability: When curing, the heat-resistant components that are soldered by wave soldering must pass through the reflow soldering furnace. Therefore, the hardening conditions must be low temperature and short time.
Self-adjusting: In reflow soldering and pre-coating processes, the patch adhesive cures and fixes the components before the solder melts, thus preventing the components from sinking into the solder and self-adjusting. At this point, manufacturers have developed a self-adjusting patch adhesive.
Common problems, defects and analysis of SMT patch adhesive
Insufficient thrust
The thrust strength requirement of 0603 capacitor is 1.0KG, the resistance is 1.5KG, the thrust strength of 0805 capacitor is 1.5KG, and the resistance is 2.0KG. The above thrust is not achieved, indicating that the strength is not enough.
Generally caused by the following reasons:
1. The amount of glue is not enough.
2. The gel is not 100% cured.
3. The PCB board or components are contaminated.
4. The colloid itself is brittle and has no strength.
Thixotropy instability
A 30ml syringe glue needs to be used up by pressure for tens of thousands of times before it can be used up. Therefore, it is required that the patch glue itself has excellent thixotropy, otherwise the glue point will be unstable, and the glue will be too small, which will result in insufficient strength and cause peaks. When the components are soldered off, on the contrary, the amount of glue is too large, especially for the small components, which tend to stick to the pads and hinder the electrical connection.
Insufficient amount of glue or missing
Reasons and countermeasures:
1. The stencil for printing is not cleaned regularly and should be washed once every 8 hours with ethanol.
2. The colloid has impurities.
3. The stencil opening is unreasonably too small or the dispensing pressure is too small, and the designed amount of glue is insufficient.
4. There are bubbles in the colloid.
5. If the dispensing head is clogged, the dispensing nozzle should be cleaned immediately.
6. The preheating temperature of the dispensing head is not enough. The temperature of the dispensing head should be set at 38 °C.
Drawing
The so-called wire drawing is the phenomenon that the patch is not broken when dispensing, and the patch glue is connected in a filament shape in the direction of the dispensing head. There are many wires, and the patch glue covers the printed pads, which may cause poor soldering. Especially when the size is large, this phenomenon is more likely to occur when the nozzle is applied. SMD adhesive wire drawing is mainly affected by the drawing properties of the main component resin and the setting of the spot coating conditions:
1. Increase the dispensing stroke and reduce the moving speed, but it will reduce your production beat.
2, the lower the viscosity, high thixotropic material, the smaller the tendency of drawing, so try to choose this kind of patch glue.
3. Adjust the temperature of the thermostat slightly higher, and forcefully adjust it to a low-viscosity, high-thixotropy patch adhesive. At this time, consider the storage period of the patch adhesive and the pressure of the dispensing head.
Collapse
The fluidity of the patch adhesive is too large to cause collapse. The common problem of slumping is that it will cause slump after being placed for a long time after the spot coating. If the patch adhesive spreads to the pad of the printed circuit board, it may cause poor soldering. Moreover, for the components with relatively high pins, the collapsed patch adhesive can not contact the main body of the component, which may result in insufficient adhesive force. Therefore, the patch adhesive which is easy to collapse has a slump rate. Predicted, so the initial setting of its spotting amount is also difficult. In response to this, we have to choose those patch adhesives that are not easy to collapse, that is, the higher the melting rate. For the slump caused by the long time after the spot coating, we can avoid the glue and solidification of the patch in a short time after the spot coating.
Component offset
Component offset is a prone to the high-speed placement machine. The main reasons are:
1. It is the offset generated by the X-Y direction when the printed board moves at a high speed. This phenomenon is likely to occur on components with a small application area of the SMT. The reason is that the adhesive force is not caused.
2. The amount of glue on the components is inconsistent (for example: 2 glue points under the IC, one glue point is larger than the glue point), the glue is not balanced when it is heated, and the end with less glue is easily offset.
Over wave soldering
The reasons are complex:
1. The adhesive strength of the patch adhesive is not enough.
2. It has been hit before wave soldering.
3. There are more residues on some components.
4, the colloid is not resistant to high temperature shock
Patch glue
The patch adhesives of different manufacturers have great differences in chemical composition. Mixed use is easy to produce many defects: 1. Hardening is difficult; 2. Adhesive strength is insufficient; 3. Over-wave soldering is serious.
The solution is to thoroughly clean the parts that are easy to cause mixing, such as stencils, scrapers, and dispensing heads, and avoid mixing different brands of patch adhesive.
---Becky Su
Becky@hysmt.cn
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