1. The amount of solder paste is particularly large.

2. There are many residues formed after flux volatilization, which will pollute the machine and PCB.

3. The probability of voids in solder joints increases. Because the through-hole components have to go through the whole reflux temperature curve, they must withstand the thermal shock with the peak temperature of 240 ℃ and 30s. The adaptability of reflow soldering must be considered for through-hole components. The components shall be made of resins that do not deteriorate within 183 ℃ (preferably 220 ℃ for 40s) and 235 ℃ and (60 ~ 90) s. Component manufacturers also need standards on bending, dimensional stability, shrinkage and dielectric properties.

4. Qj165b's standard for through-hole welding points is the existence of welding wetting angle on PCB welding surface (surface B) and solder filling through-hole with at least 100% plate thickness. The main technical challenge of thr is how to print enough solder paste in and around the through hole with high-density pin components to form an acceptable welding point on side B to meet the requirements of qj165b. Because in THR, it is not a problem to form a welding wetting angle on side a, because the solder paste is printed from side a.

5. Since the volume of metal components in the solder paste accounts for about 50% of the solder paste volume, it is necessary to optimize the through-hole pad design and template design to ensure sufficient amount of solder paste and prevent less tin and holes.

6. Compared with ordinary wave soldering process, thr has higher technical difficulty: PCB thickness, plated hole size, PCB pad size, solder paste printing amount, component lead diameter, component pin spacing, metal content of solder paste, design of printed screen board, component installation and solder joint detection will affect the shape of solder joint. In addition, the filling amount of solder in the metallized hole is also related to the opening size of the printed screen.

7. Thr has special requirements for component installation design

1) Metallized hole design

(1) Round lead: the diameter of standard metallized hole is round lead diameter + 0.254mm.

(2) Rectangular lead: the aperture of standard metallized hole is diagonal + 1.27mm.

2) Pad size

When considering the minimum pad size, the metallization hole size, PCB manufacturing tolerance and the required minimum pad size must be considered. The minimum weld ring is defined as the minimum amount of metal from the edge of the metallized hole to the outer diameter of the pad. Minimum pad diameter = last metallization hole diameter + 2 (minimum ring) + PCB manufacturing tolerance. The minimum pad size reduces the amount of solder paste required to form the wetting angle of face a and face B.

3) Gap between pad and surrounding adjacent conductor

There shall be enough gap between the metallization hole component pad and the surrounding adjacent metal conductor. If there is too close metallization hole, the solder will be evenly divided between the metallization hole and the pad, causing bridging. There shall be no other exposed metal, hole or solder mask in the required gap area around the bonding pad of metallized hole components. Legend solder mask may affect the flow of solder paste during reflow and separate it to form solder beads.