What methods are feasible for directly mounting a smaller PCB onto a larger base PCB, with the following conditions:
- No spacing or gap between the two PCBs
- Electrical contacts in addition to physical attachment
- The top PCB is approximately one-third the size of the bottom PCB
I am in the early design phase of a project and exploring various options, so I’m open to recommendations for both standard methods and innovative ideas. I’m already familiar with edge castellations (AKA “half vias”), so other suggestions would be particularly helpful.
Is it feasible to design the top PCB with pad contacts only on the bottom (similar to QFN/QFP style) that can be soldered directly onto pads on the bottom PCB?
The purpose of stacking the boards this way is that the base PCB will remain constant across different product variants, while the top PCB will vary in components, size, and contact configuration. The goal is to create a flexible system where different top PCBs can be easily attached to a standardized base.
Regarding the use of pads under the PCB, I assume you’re referring to a setup similar to the Telit HE910 module shown in the image below:
![Telit DE910](https://global.discourse-cdn.com/sierraconnect/original/1X/0b79e8c181b7a66745069707b092e5acef3484b3.jpeg)
This module reflow solders directly onto a PCB. As shown in the picture, the gap between the module and the main PCB is minimal, certainly less than 1mm. This technique is effective because the components inside the module can typically withstand multiple reflow processes—usually at least two, one for each side of the board. Since these modules only have components on one side, they likely undergo only one reflow.
You might consider using a hot plate to solder a module like this, as it allows soldering without overheating the components inside the module. However, I would advise against this method. When the solder solidifies, the mother PCB will be significantly hotter than the daughter PCB. As the mother PCB cools and shrinks, it will create shear forces in the solder joints, potentially causing warping.
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castellations might be prefered especially during debugging but QFN style pads are an option. You might need to try multiple test boards to figure out which format and pad geometries work best for the design and manufacturing processes.
Be aware that having minimal space between the daughter board and parent board might allow unexpected issues when it comes to flux residues for both cleaning and no-clean types.
One thing you need to be aware of is the design should avoid having different signal potentials on each board that is only separated by solder mask. Solder mask is not a guaranteed electrical insulator since its only job is to control solder application.
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