1. In addition to the scale effects of automated production, SMT offers several technical advantages: components can be mounted on both sides of the PCB, enabling high-density assembly; even the smallest components can be precisely placed, resulting in high-quality PCB assemblies.

2. However, in certain situations, these advantages can be compromised when the adhesion of components to the PCB is insufficient. Consider the example shown in Figure 1. SMT components are known for their compact design and ease of mounting. They differ significantly from through-hole connectors in both size and assembly method.

3. The PCB with SMT components (left) and a Dali through-hole connector (right)

4. Connectors used for field wiring in industrial applications are typically high-power components, capable of handling high voltages and large currents. As such, adequate electrical clearance and creepage distance must be factored into their design. These considerations directly influence the size of the component.

5. Additionally, the ease of installation and the mechanical strength of the connector are crucial factors. The connector often serves as the “interface” between the PCB motherboard and external components, and as such, it may experience substantial external forces. Components assembled using through-hole technology generally offer higher reliability than their SMT counterparts. Whether subjected to strong pulling, mechanical pressure, or thermal shock, through-hole components are less likely to detach from the PCB.

6. From a cost perspective, SMT components typically account for approximately 80% of the components on most PCBs, with production costs representing about 60%. In contrast, through-hole components make up around 20% of the components but contribute to 40% of the production costs, as shown in Figure 2. This indicates that the production cost of through-hole components is relatively high. For many manufacturing companies, one of the key challenges moving forward will be to develop PCBs that rely entirely on SMT technology.


**PCB with Through-Hole Parts and SMT Components**

Due to production costs and their impact on the PCB, current methods like SMT + wave soldering and SMT + press-in technology (press-in) are not entirely optimal. This is because the existing SMT process requires secondary operations and cannot complete the assembly in a single step.

This leads to the following requirements for components using through-hole technology: both through-hole components and SMT components should be assembled simultaneously, using the same equipment and processes.

**Integrating THR with SMT**

The technology developed to meet these requirements is known as Through-Hole Reflow (THR), also referred to as the “Pin-in-Paste” (PIP) process, which is a type of through-hole reflow soldering technology.

The “lead immersion solder paste” method applies a standard SMT production process to PCBs with plated through-holes, achieving satisfactory results. However, when using this method, relevant parameters must be adjusted according to the specific components and conditions during processing.

**Characteristics of Connector Components Suitable for the THR Process**

**Steps to Complete the THR Process**

1. **Confirm Whether the Through-Hole Connector Can Adopt the THR Process**

“True” THR connector components must meet specific characteristics.

2. **PCB Design Adaptation to New Process Requirements**

1) **Aperture Size**

There are two main principles for selecting aperture size: firstly, it should allow solder to easily flow into the hole (capillary effect); secondly, it must ensure reliable assembly (component tolerance).

2) **Pad Ring Design**

The recommended width for the pad ring is 0.5mm, as shown in Figure 6. This width ensures proper evaluation of the solder joint meniscus. If larger spacing and creepage distances are used, following the above process, the pad ring width should be reduced to 0.2mm.

3. **Appearance of High-Quality Solder Joints**

THR is an independent soldering process, and the quality of the solder joints can be inspected according to the IPC-A-610C standard. When comparing solder joints formed by wave soldering and THR, THR joints may appear “tin-deficient” and exhibit only a small meniscus. This is a characteristic of the THR process, and the quality assurance team should determine if the joints meet the required standards.

**Pad Ring Design**

4. **Using a Template Suitable for the THR Process and Pressure on Solder Paste**

The standard stencil thickness is 150-120μm, and excessive coating pressure is generally unnecessary.

The following formula ensures proper contact between the pad ring and the template, while applying sufficient pressure to the solder paste without increasing the frequency of stencil cleaning:

[ ds = di + 2R – 0.1 ]

where (di) is the aperture, and (R) is the pad ring width.

The solder paste should have good fluidity, wettability, and adhesion during the coating process, ensuring that the paste flows into the hole and adheres well to the pins.

5. **Applying Sufficient Solder Paste to the PCB Pads**

The ideal THR solder paste coating pressure is shown in Figure 7. The amount of solder paste applied to each pad should be twice the volume of the corresponding hole.

The required solder paste volume can be achieved by adjusting the printing speed and the squeegee angle. For example, changing the squeegee angle can increase the pressure applied to the paste, as shown in Figure 8 (assuming constant speed).

Another method is the closed coating system, which directly applies pressure to the solder paste. By adjusting this pressure, the correct paste volume can be applied.

Both methods deliver good results in practice, but occasional under-application of solder paste may occur. If this happens, the following measures can help improve the situation: use the maximum allowable template thickness, reapply solder paste, increase the solder paste volume locally, apply paste on both sides (for double-sided reflow), or increase the coating pressure. Additionally, using tighter tolerances may help.

6. **Choosing the Optimal Packaging Form**

“Tape packaging” is commonly used in SMT processing, and THR connectors typically follow this packaging standard. The width of the tape usually ranges from 32mm to 88mm.

THR components are compatible with most standard feeders, but certain components, especially vertical ones, require checking the radius provided by the feeder to ensure proper fit at the feed and discharge points.

Many machines also support waffle tray or tube packaging, meeting a variety of needs, including dedicated or “unfinished” components.

7. **Check the Soldering Condition According to IPC-A-610C Standards**

THR connectors can be inspected according to the IPC-A-610C standard. As long as the pins extend from the PCB, the solder joints on the soldering surface can be evaluated.

**Reducing Production Costs with the THR Process**

A major consideration in adopting THR technology is finding a balance between production costs (low) and component costs (high). THR connectors are typically more expensive than standard components due to higher material and packaging costs.

Cost reduction potential depends on several factors: the degree of production automation, order volume, the ability to replace other through-hole components, and whether new products need to be designed or existing products redesigned.

Not all manufacturers recommend using THR connectors, but if all components except the PCB connector are already being processed via SMT, then adopting THR technology for the connector is undoubtedly the best option.
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