Abstract: This paper mainly introduces the characteristics of patch adhesive. It addresses the usage methods for various processes, common defects and their countermeasures, and briefly analyzes the future development trend of patch adhesive.

Keywords: Patch adhesive, thixotropy, stringing, dispensing volume.

1. Usage method of patch adhesive.

1.1 Brief introduction of patch adhesive.
Patch adhesive, also known as SMD adhesive, is a hardener uniformly distributed in a red paste, along with pigments, solvents, and other binders. It is mainly used to fix components onto printed circuit boards, typically applied by dispensing. After components are placed, they are heated and hardened in an oven or reflow soldering machine. It differs from so-called solder paste; once hardened by heat, it will not dissolve even with further heating, meaning the thermal hardening process of patch adhesive is irreversible. The application effect of patch adhesive varies depending on the thermal profile conditions. The appropriate patch adhesive should be selected according to the production process.

2. The use of patch adhesive and its application process differ based on the characteristics of the production process.

Purposes of patch adhesive by process:
① Wave soldering process: Prevents components from falling off during wave soldering.
② Double-sided reflow soldering process: Prevents components on the other side from falling off.
③ Reflow soldering process: Prevents component displacement and tombstoning.
④ Pre-coating process.
⑤ Marking for wave soldering, reflow soldering, and pre-coating.
① In wave soldering, patch adhesive fixes components onto the printed circuit board to prevent them from falling off when the board passes through the solder wave.
② In double-sided reflow soldering, patch adhesive is used to prevent large components from falling off due to solder melting from heat.
③ In reflow soldering and pre-coating processes, it prevents displacement and tombstoning during assembly.
④ Additionally, when replacing printed circuit boards and components in batches, patch adhesive is used for marking.

3. Characteristics of patch adhesive regarding bond strength:
SMT patch adhesive must have strong bond strength. After hardening, it should not peel off even at the melting temperature of solder.
※ Dispensing: Currently, dispensing is the main method for applying adhesive on printed circuit boards. Therefore, the adhesive is required to have the following properties:
① Adaptability to various assembly processes.
② Easy to set the supply amount for each component.
③ Simple to adapt to changes in component types.
④ Dispensing stability.
Adaptability to high-speed machines: Patch adhesive used today must meet the high-speed requirements of dispensers and high-speed placement machines. Specifically, high-speed dispensing must be free of stringing. Additionally, during high-speed placement, the viscosity of the adhesive on the printed circuit board must ensure that components do not move during transport.
※ Stringing and Collapse:
Once patch adhesive gets onto the solder pads, components cannot achieve electrical connection with the printed circuit board. Therefore, patch adhesive must be applied without stringing. It should not collapse after application to avoid contaminating the solder pads.
Low-temperature curing: During curing, heat-sensitive through-hole components that go through wave soldering first must also pass through the reflow oven. Hence, the hardening conditions must meet the requirements of low temperature and short time.
Self-adjustment: During reflow soldering, the patch adhesive cures before soldering, which can hinder component settling. To address this, manufacturers have developed patch adhesives that allow for self-adjustment. The main characteristics of patch adhesive are shown in Table 2.
Process differences: Wave soldering process, reflow soldering process, pre-coating process.
- Low temperature, short-time hardening does not hinder self-adjustment function.
- Good dispensing performance, stable dispensing volume, less stringing.
- High viscosity before curing, good component holding force.
- Good performance at room temperature.
- Strong adhesion at high temperatures (260°C).
- Long shelf life and long on the machine.

4. Selection of patch adhesive:
Patch adhesives are available in acrylic types (for UV and thermal curing) and epoxy types (for thermal curing only). Acrylic patch adhesives have advantages in short-time hardening and low-temperature curing. Epoxy patch adhesives that cure at low temperatures in a short time have also been developed to replace acrylic patch adhesives with poor adhesion. The selection of patch adhesive should fully consider the factors mentioned above for each process, such as the heat resistance of the components used and the on the machine, and the appropriate product should be chosen. The so-called on the machine refers to the storage period when exposed to the production line environment at low temperatures.

2. Troubleshooting for patch adhesive defects.
Typical defects of patch adhesive include the following:

① Empty dots (insufficient adhesive).
Too much adhesive, unstable adhesive distribution, too much or too little dispensing. If the adhesive amount is too low, the strength will certainly be insufficient, causing components to fall off in the solder pot during wave soldering. Conversely, too much patch adhesive, especially for tiny components, can hinder electrical connection if it gets onto the solder pads.
Causes and countermeasures:
a. Large particles mixed into the adhesive clog the dispenser nozzle, or there are bubbles in the adhesive, causing empty dots. The countermeasure is to use film-type adhesive that removes excess particles and bubbles.
b. When the viscosity of the film adhesive is unstable, the dispensed amount becomes unstable. Prevention method: Before each use, place the adhesive in an airtight container to prevent condensation, let it sit for about 1 hour, then install the dispensing head. Start dispensing only after the nozzle temperature has stabilized. A temperature control device during use is even more effective.
c. If the dispensing head has not been used for a long time, the solubility of the patch adhesive needs to be restored. At the beginning, insufficient dispensing will certainly occur. Therefore, use several dispensing cycles at each start.

② Stringing:
Stringing refers to the phenomenon where the patch adhesive does not break off during dispensing, and a filament of adhesive follows the movement direction of the dispensing head. Excessive stringing causes the patch adhesive to cover the solder pads on the printed circuit board, leading to poor soldering. This phenomenon is especially more likely to occur when using precise dispensing nozzles. Stringing of patch adhesive is mainly affected by the stringing tendency of its main component resin and the settings of the dispensing conditions.
Solutions:
a. Increase the dispensing head stroke and reduce the movement speed, but this will lower the production pace.
b. The lower the viscosity, the lower the tendency for stringing with highly soluble materials. So, try to choose such patch adhesives.
c. Slightly increase the temperature setting on the temperature controller to adjust to a low-viscosity, highly soluble patch adhesive. At this time, the storage period of the patch adhesive and the pressure of the dispensing head must be considered.

③ Collapse due to excessive fluidity of the patch adhesive:
There are two types of collapse. One is collapse caused by leaving the adhesive for too long after dispensing. If the patch adhesive spreads on the printed circuit board, it will cause poor soldering. Furthermore, for components with relatively high leads, collapsed patch adhesive cannot contact the component body, leading to insufficient adhesive relay. Because patch adhesive collapses easily, its collapse rate is difficult to predict, making the initial setting of the dispensing amount very difficult. Therefore, we must choose patch adhesives that do not collapse easily, i.e., those with relatively high solubility. For collapse caused by leaving the adhesive for too long after dispensing, we can complete the component placement and curing shortly after dispensing to avoid this situation.

④ Component shift:
Component shift is a defect prone to occur with high-speed placement machines. One type is θ-angle shift when the component is pressed into the patch adhesive. Another type is X-Y direction shift when the printed circuit board moves at high speed. This phenomenon is easily seen on components with a small patch adhesive coating area. The cause is poor adhesion. The corresponding measure is to select patch adhesives with high solubility and high viscosity. One test has shown that if the placement speed reaches 40 m/s, the adhesive strength of the patch adhesive must be achieved.

⑤ Components falling into the wave soldering pot:
Sometimes, for QFP, SOP, and other large components, during wave soldering, due to their own weight and the stress from the solder in the pot exceeding the adhesive strength of the patch adhesive, they can fall into the pot. The causes are an insufficient amount of patch adhesive or a drop in adhesive strength due to high temperature. Therefore, when selecting patch adhesive, more attention should be paid to its adhesive strength at high temperatures.
During wave soldering, to improve production efficiency, even LEDs, aluminum electrolytic capacitors, and other electronic components with poor heat resistance also pass through the flow solder oven together for curing. At this time, if the curing temperature of the adhesive is high, the aforementioned components will be damaged due to exceeding their heat resistance temperature. In this case, our approach is either to install low-heat-resistant components or to select a patch adhesive with an temperature for curing.

3. Due to the high speed of SMT production and the increasing assembly density of printed circuit boards, patch adhesives are required to adapt to the characteristics of various processes and meet the requirements of high-speed dispensers and high-speed placement machines. Furthermore, new shapes are also required for printed circuit boards and SMD products.
1. To meet the requirements of high-speed placement machines and improve placement production efficiency, the operating speeds of dispensers and placement machines are increasing. Starting from an initial dispensing of 0.2 seconds/cycle, the placement speed has now developed to 0.1 seconds/cycle. Consequently, phenomena that were difficult to in the past have emerged, specifically: ① Stringing caused by high-speed dispensing, ② θ-angle deviation caused by high-speed placement, ③ X-Y direction shift.
① To overcome stringing, the settings of the high-speed temperature controller can be adjusted manually to change the physical properties of the patch adhesive, solving the problem without affecting the production speed.
② θ-angle deviation generally occurs on the nozzle of the placement machine when the component is pressed onto the adhesive applied to the large printed circuit board, determined by the characteristics of the placement machine. It is difficult to solve this problem without changing the placement speed. Therefore, the method is to choose a patch adhesive suitable for high-speed placement machines.
③ Suction on the placement head.