Combinations of mixed technologies are used for more suitability with a product’s circuit board design. Each basic attachment method, whether single-sided through-hole, double-sided plated through-hole, single-sided surface-mount, and double-sided surface-mount, has different advantages and limitations. This is of course the reason why developers have used mixed methods or the combinations of the basic attachment methods in printed circuit boards. In mixed assembly PCBS, several variations can be made. They can be one of the following configurations:
• Single-sided boards containing both the through-hole components (TH) and surface-mount (SM) components on one side only.
• Double-sided boards containing mixed through-hole (TH) and surface-mount (SM) components on one side and surface-mount (SM) components on the other.
• Double-sided boards containing the leaded through-hole (TH) components on one side and the surface-mount (SM) components on the other side.
Single-Sided Mixed Assembly
This method combines through-hole and surface-mount technologies on single-sided printed circuit boards. The assembly process involves two steps. First is to screen-print the solder pastes on the board’s surface, place the surface-mount components on them, and then let them undergo reflow soldering. Second is to insert all the leaded through-hole components, and solder them via wave soldering or hand soldering. The conventional flow of the single mixed assembly process is to surface-mount first, then to through-hole.
This is done to manage some issues. One of them is that TH components can be too large that they can obstruct the application of solder paste and the pick-and-place process for SM components. Aside from component size issues, SM components sometimes undergo repeated reflow soldering and the TH components could not take excessive thermal excursions. Similarly, some TH components cannot handle the heat of the reflow oven without getting damaged. This is why assembly process is done in the SMT then THT order. Then, during the TH selective, manual, or wave soldering, SM parts are protected by Wave Fixtures to keep them from falling off the board.
If desired, the order of the assembly process can definitely be reversed. However, the specifications of the TH components must be compatible with SMT. The order of assembly is important for mass production optimization. Mixed method assembly may also include surface-mounting components onto PGA sockets and attaching PGAs to printed circuit boards by through-hole technology. The single sided through-hole with surface-mount is the most common method used in electronic product assembly.
Double-Sided Mixed Assembly
Double-sided mixed assembly can be categorized into two. First is the separated SM and TH parts on each side of the board. Second is the mixed SM and TH parts on one side and SM parts on the back side of the board. For this discussion, the topic will be focused on the second category. The package may be described as the combination of double-sided SM and single-sided TH assembly. In most cases, the SM components on the mixed side of the board are passive components, such as resistors and bypass capacitors since they can withstand TH wave soldering process. The assembly process for the double-sided mixed assembly follows the same order as that of the single sided mixed boards, which is SMT first then THT.
For the complete assembly process, there are three steps. First is the placement of the SM parts and reflow solder process on the front side. Next is the placement of the SM parts and reflow solder process on the back side of the board. Lastly is the insertion of the TH parts on the front side and wave solder process. This type of assembly can sometimes be prone to assembly defects due to the extra exposure to heat and operational procedures. It is also time consuming, highly technical, and delicate. Nevertheless, double-sided mixed assembly for printed circuit boards guarantees enhanced performance.