SMT glue, also known as smt adhesive, SMT red glue, is usually red (also yellow or white) paste with hardener, pigment, solvent, etc. The device is fixed on the printed board and is generally dispensed by dispensing or stencil printing. Put the components into the oven or reflow oven to heat and harden after pasting the components. It is different from solder paste in that it solidifies after being heated, and its freezing point temperature is 150°C, and it will not melt when heated again. That is to say, the thermal hardening process of the adhesive is irreversible. The use effect of SMT patch adhesive will vary due to different heat curing conditions, connected objects, equipment used, and operating environment. When using it, the patch adhesive should be selected according to the printed circuit board assembly (PCBA, PCA) process. .

The characteristics, application and prospect of SMT adhesive

SMT patch red glue is a kind of polydilute compound, the main components are base material (that is, the main polymer material), filler, curing agent, other additives, etc. SMT patch red glue has viscosity fluidity, temperature characteristics, wetting characteristics and so on. According to this characteristic of red glue, the purpose of using red glue in production is to make the parts stick firmly to the surface of PCB and prevent them from falling off. Therefore, patch adhesive is a process product that is purely consumed and not necessary. Now, with the continuous improvement of PCA design and process, through-hole reflow soldering and double-sided reflow soldering have been realized. The PCA mounting process using patch adhesive is present. less and less trend.

Purpose of use of SMT adhesive

① Prevent components from falling off during wave soldering (wave soldering process). When using wave soldering, in order to prevent the components from falling when the printed board passes through the solder bath, the components are fixed on the printed board.

②Prevent components on the other side from falling off during reflow soldering (double-sided reflow soldering process). In the double-sided reflow soldering process, in order to prevent the large components on the soldered side from falling off due to the melting of the solder, SMT adhesive is required.

③Prevent component displacement and standing (reflow soldering process, pre-coating process). Used in reflow soldering process and pre-coating process to prevent displacement and stand-up during placement.

④ Mark (wave soldering, reflow soldering, pre-coating). In addition, when printed boards and components are changed in batches, use patch glue for marking.

SMT adhesives are classified according to the way they are used

a) Squeegee type: sizing by printing and scraping. This method is widely used and can be used directly on solder paste printing machines. The opening of the laser stencil depends on the type of part, the performance of the substrate, its thickness and the size and shape of the hole. Its advantages are fast speed, high efficiency and low cost.

b) Glue dispensing type: Glue is applied on the printed circuit board by dispensing equipment. Special dispensing equipment is required, and the cost is relatively high. The dispensing equipment uses compressed air to point red glue on the substrate through a special dispensing head. The size and number of glue spots are controlled by parameters such as time and pressure tube diameter. The glue dispenser has flexible functions. For different parts, we can use different dispensing heads, set parameters to change, and also change the shape and quantity of glue points to achieve the best effect. The advantages are convenience, flexibility and stability. The disadvantage is that it is easy to have wire drawing and air bubbles. We can adjust the operating parameters, speed, time, air pressure, and temperature to minimize these shortcomings.

Management of SMT adhesive

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

1) Red glue must have a specific serial number, which is numbered according to the quantity, date, and type of feed.

2) The red gum should be stored in a refrigerator at 2-8°C to prevent its properties from being affected by temperature changes.

3) Red glue should be warmed up at room temperature for 4 hours, and used in the order of first in first out.

4) For the dispensing operation, the red glue in the hose should be defoamed, and the unused red glue should be stored in the refrigerator. Old glue and new glue cannot be mixed.

5) It is necessary to accurately fill in the temperature recovery record form, the temperature recovery person and the temperature recovery time. The user needs to confirm that the temperature recovery is completed before using it. Generally, red gum cannot be used beyond its expiry date.

Process characteristics of SMT adhesive

Connection strength: SMT adhesive must have a strong connection strength, after being hardened, it will not peel off even at the temperature of solder melting.

Dispensing property: At present, dispensing is mostly used for the distribution of printed boards, so the glue is required to have the following properties:

① Adapt to various placement processes

②It is easy to set the supply amount for each component

③Easy to adapt to replace the variety of components

④The amount of dispensing is stable

Adapt to high-speed machines: The patch adhesives currently used must meet the high-speed requirements of dispensing and high-speed placement machines. The viscosity of the adhesive should ensure that the components do not move.

Drawing and slumping: Once the SMT glue sticks to the pad, the components cannot be electrically connected to the printed board. Therefore, the SMT glue must be free from wire drawing during coating and without slumping after coating. so as not to contaminate the pads.

Low-temperature curability: When curing, the non-heat-resistant plug-in components that have been soldered by wave soldering first have to pass through the reflow soldering furnace, so the hardening conditions must meet low temperature and short time.

Self-adjustment: In the reflow soldering and pre-coating process, the patch adhesive is cured and fixed before the solder melts, so it will prevent the components from sinking into the solder and self-adjusting. In response to this point, manufacturers have developed a self-adjustable patch adhesive.

Common problems, defects and analysis of SMT adhesive

Not enough 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. If the above thrust cannot be reached, it means that the strength is not enough.

Generally caused by the following reasons:

1. The amount of glue is not enough.

2. The colloid is not 100% cured.

3. The PCB board or components are polluted.

4. The colloid itself is brittle and has no strength.

Thixotropic instability

A 30ml syringe adhesive needs to be hit by air pressure tens of thousands of times before it can be used up, so the patch adhesive itself is required to have excellent thixotropy, otherwise it will cause unstable glue points, too little glue will lead to insufficient strength and cause peaks Components fall off during soldering. On the contrary, too much glue, especially for tiny components, is easy to stick to the pad and hinder the electrical connection.

Insufficient glue or missing spots

Reasons and countermeasures:

1. The stencil used for printing is not cleaned regularly and should be cleaned with ethanol every 8 hours.

2. The colloid has impurities.

3. The opening of the laser steel mesh is unreasonably too small or the dispensing pressure is too small, and the designed glue volume is insufficient.

4. There are air bubbles in the colloid.

5. If the dispensing head is blocked, clean the dispensing nozzle 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 glue cannot be broken when dispensing, and the patch glue is connected in a silky shape in the moving direction of the dispensing head. There are many wires connected, and the patch adhesive covers the printed pad, which will cause poor welding. Especially when using a larger size, this phenomenon is more likely to occur when dispensing nozzles. The drawing of patch glue is mainly affected by the drawing property of its main component resin and the setting solution of dispensing conditions:

1. Increase the dispensing stroke and reduce the moving speed, but it will reduce your production tempo.

2. The lower the viscosity, the higher the thixotropic material, the smaller the tendency of drawing, so try to choose this kind of patch adhesive.

3. Slightly increase the temperature of the thermostat, and forcefully adjust it to a low-viscosity, high-thixotropy adhesive. At this time, the storage period of the adhesive and the pressure of the dispensing head should also be considered.

collapse

Excessive fluidity of the patch adhesive will cause slump. The common problem of slump is that it will cause slump after being placed for too long after dispensing. If the patch adhesive extends to the pad of the printed circuit board, it will cause poor soldering. Moreover, for those components with relatively high pins, the collapsed patch adhesive cannot touch the main body of the component, which will cause insufficient adhesion. Therefore, the collapse rate of the chip adhesive that is easy to collapse is difficult Prediction, so the initial setting of its dispensing amount is also very difficult. In view of this, we have to choose those patch adhesives that are not easy to slump, that is, those with higher thixotropy. For the slump caused by leaving it for too long after dispensing, we can avoid it by completing the adhesive mounting and curing of the patch within a short time after dispensing.

Component offset

Component offset is a bad phenomenon that is prone to occur in high-speed placement machines. The main reasons are:

1. It is the offset in the X-Y direction when the printed board moves at high speed. This phenomenon is easy to occur on components with a small coating area of the patch adhesive. The reason is that the adhesion is not good.

2. The amount of glue under the components is inconsistent (for example: 2 glue spots under the IC, one glue point is larger and the other is smaller), the strength of the glue is not balanced when it is heated and cured, and the end with less glue amount is easy to shift.

Dropped parts by wave soldering

The reasons for this are complex:

1. The adhesion of the patch adhesive is not enough.

2. It has been impacted before wave soldering.

3. There are many residues on some components.

4. The colloid is not resistant to high temperature shock

SMD glue mixed

The chemical composition of patch adhesives from different manufacturers is very different, and mixed use is prone to many defects: 1. Difficult curing; 2. Insufficient adhesion; 3. Severe parts falling off after wave soldering.

The solution is: thoroughly clean the laser stencil, scraper, dispensing head and other parts that are likely to cause mixed use, and avoid mixing different brands of patch adhesives.

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