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Figure 8: Dipping station: Rotary drum (left), linear squeegee (right)
Advantages of PoP include:
- Miniaturization as PCB area for only one part is needed
- Allows direct electrical path between microprocessor and memory on the component level
Disadvantages of PoP include:
- Top and bottom part of a PoP must match together to avoid yield issues (co-planarity, warpage, CTE)
- Extra process steps required
- Extra cost for equipment
- May require nitrogen reflow
The recommended PoP assembly is the in-line process on the PCBA, using existing SMT machines with the addition of a dipping station inside the machine. This method lowers the total number of process steps involves, thus achieving lowest total cost.
To ensure a successful PoP assembly, there are several critical process controls to be implemented. The repeatability and reproducibility for solder paste printing, and inline solder paste inspection must be verified. The SMT machine placement must also have good repeatability and reproducibility. The flux height and flux volume at the dipping station should be checked once an hour. The SMT machine placement force for the top part must be set to a low setting and calibrated. The top and bottom part of the PoP is sensitive to moisture and we need to ensure that MSD control is strictly followed.
Cavity Assembly
The assembly method involves creating a cavity during the PCB manufacturing process. The cavity allows overall thickness reduction by allowing components to sit below the top layer of the PCB. There are many challenges with this assembly method, which we will discuss in further detail.
Figure 9: Cavity assembly.
The cost of the PCB will increase due to the extra steps required to create a cavity, and to plate the pads inside the cavity.
To deposit solder paste into the cavity area, a special stencil is needed if doing one pass printing, or two separate stencils for double printing. The thickness tolerance of the PCB must be fully understood to design the correct pocket depth for cavity printing. This is to create a tight seal in the cavity area for proper printing.
If using the one pass printing stencil, the automatic stencil cleaner on the screen printer will not work due to the gap created by the pocket on the cavity stencil. Due to this reason and the tolerance of the PCB on cavity depth, the double printing method is preferred for cavity assembly.
Figure 10: Automatic stencil cleaning issue with one pass
Double printing requires the use of two stencil printers. The first machine will host the stencil with the pocket to print the cavity area first. A slitted squeegee blade is needed to ensure the solder paste is deposited via the opening in the stencil. The slitted squeegee blade needs to have a front squeegee which is performing the actual printing, and a back squeegee to act like a pusher to ensure the front squeegee falls into the cavity.
Figure 11: Slitted squeegee with front and back blade.
After solder paste is deposited in the cavity area, the PCBA will go through the second stencil printing machine to print the regular PCBA surface. The cavity area will be blocked on the stencil.
The components in the cavity are placed by SMT machine by increasing the z-distance travel of the placement.
The addition of a stencil printer to the SMT line will require additional floor space to accommodate.
Future
Molded Interconnect Device
The 3D assembly method discussed previously uses a combination of rigid and flexible PCBA, or takes advantage of the z-axis by either adding components on top of each other (PoP), or reducing the PCB.
Molded interconnect device (MID) does away with traditional PCBA, but allows circuitry to exist on top of the injection molded plastic. The process involves injection molding of the plastic part with additives, laser activation of the artwork on the plastic part, then plating of copper and surface finish on the unit.
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