Pin-in-paste reflow soldering is an emerging technology in international electronic assembly applications. When a PCB has both surface-mount components and a small number of through-hole components such as sockets on the same side, we generally adopt the approach of first mounting the SMT components and passing them through the reflow oven, then manually inserting the through-hole components and passing them through wave soldering. However, if pin-in-paste reflow soldering is adopted, it is only necessary to insert the through-hole components after SMT placement and before entering the reflow oven, and then pass them through the reflow oven together.
? ?Through this comparison, the superiority of pin-in-paste reflow soldering over traditional processes can be seen. First, it reduces the number of process steps, eliminating the wave soldering step, which naturally saves considerable cost. It also reduces the number of required workers and improves efficiency. Second, compared with wave soldering, reflow soldering has a much lower possibility of bridging, thus improving the first-pass yield. Pin-in-paste reflow soldering technology has great advantages over traditional processes in both economy and advancement. Therefore, pin-in-paste reflow soldering technology is an innovation in electronic assembly and will inevitably be widely applied. However, to apply pin-in-paste reflow soldering technology, it is also necessary to put forward some requirements different from traditional processes regarding components, PCB design, stencil design, and other aspects.
I. Components:
? ?Through-hole components are required to withstand the reflow temperature profile of the reflow oven, with a minimum of 230°C for 65 seconds. This process includes coating solder paste over the holes (which will flow into the holes during the reflow process). To make this process feasible, the component body should be 0.5 mm from the board surface, and the lead length of the selected component should be equivalent to the board thickness, with a square or U-shaped cross-section (preferably over a rectangular one).
II. Calculating Hole Size
? ?The finished hole size should be 0.255 mm (0.010 inch) larger in diameter than the maximum measured dimension of the lead, usually using the diagonal of the lead cross-section, excluding retention features. The drilled hole size is 0.15 mm (0.006 inch) larger than the finished hole, which is the plating compensation. The hole calculated in this way is the acceptable minimum size.
III. Calculating the Stencil: (Solder Paste Volume)
? ?The first part of the calculation is to determine the solder paste volume required for soldering: the volume of the hole minus the volume of the lead plus the volume of the solder fillet (what kind of solder fillet is needed). The required solder volume multiplied by 2 gives the required solder paste volume, because the metal content in the solder paste is 50% by volume (taking ALPHA's UP78 solder paste as an example). During the printing process, the solder paste is printed onto the PCB through the stencil apertures. Due to pressure, the solder paste can generally be pressed into the hole by 0.8 mm (when the squeegee is at a 45-degree angle to the stencil). We calculate the volume of solder paste entering the hole, subtract it from the required solder paste volume, and obtain the volume of solder paste remaining in the stencil apertures. This volume divided by the stencil thickness gives the required area of the stencil apertures.
IV. Stencil Design:
? ?The position of the stencil will depend on the following factors:
? ?1. The minimum distance from one side of the aperture to the center of the hole is required to be equal to the radius of the drilled hole.
? ?2. The aperture is always larger than the pad, so the solder paste will be applied over the solder mask. After reflow soldering, it must be confirmed that no solder paste residue remains on the solder mask pad. The edges of the aperture are required to be straight, because when the solder paste flows into the hole during the reflow process, there will be no solder paste on the surface to undergo reflow.
? ?3. The mold shape of the bottom surface of the component has design limitations. A space of 0.2 mm is required between the bottom surface and the printed solder paste (this must be included in the design).
? ?4. On sockets, many apertures provide straight and narrow printing, so component positioning and test points next to through-hole sockets should leave some space for the solder paste layer.
? ?5. For general components such as crystal oscillators, there is sufficient space under the component to meet the area required for printing, which means it will not be necessary to apply solder paste outside the component.
V. Preparation of Component Leads:
? ?It is very important that the leads have the correct length. Before they enter this process, they must be pre-cut to a condition of being 1.5 mm longer than the board thickness. All variations in lead dimensions and aperture dimensions will be accommodated by the amount of solder fillet, so some variation will be reflected in the height variation of the solder fillet. The reflow oven temperature profile is required to be set as follows: smoothly ramp up to 165+20°C within 4.5 minutes, pass through only one temperature zone from 165 to 220+5°C, and hold at 220+5°C for 50 seconds.
VI. Soldering:
? ?For practical reasons, there is always variation in solder paste during pin-in-paste reflow soldering, so a solder fillet is designed to accommodate a range of variations. The varying fillet is always located beneath the component, ranging from flat to full, to meet inspection standards.
? ?Pin-in-paste reflow soldering is relatively simple in application, but compared with the "insertion-wave soldering" process, it requires more preparatory work. The key point of pin-in-paste reflow soldering is the design of the aperture shape and size, and this calculation process must also consider factors such as the solid content of the solder paste, the volume of the leads, and the hole size. Therefore, to achieve a perfect solder joint, careful calculation and repeated experimentation are required. This article only explains the basic technical requirements of pin-in-paste reflow soldering, and many detailed issues remain to be further tested and summarized.
