It is important that you design your picosatellite to include room for the sensor or engineering payload. While this is obvious, it is so easy to get caught up in building efficient power systems and programming the CPU, only to realize you don’t have enough space for your mission.
Imaging sensors—sensors that require a line of sight outside the satellite shell—require the most difficult placement. In-situ sensors, which measure the satellite’s ambient environment, must be placed where noise from the onboard electronics do not contaminate the signal. Therefore, isolation and shielding are considerations, which will affect both your spatial and weight budgets.
Shielding is a complex topic. Here, we are concerned about shielding your components to prevent cross-talk—to prevent any of your components from causing interference with the others. This is something you can and will test for when you integrate your pieces into your final build.
There is also shielding from the external environment expected in space. LEO is in the ionosphere and there are radiation and electric/magnetic field concerns. Unfortunately, shielding against those are not a simple matter. First, shielding requires adding material, which adds weight.
Second, while high-energy particles will do more damage than low-energy particles (typically), electronics are susceptible to specific energies more than others. Finally, shielding will block low-energy particles but it will also slow down (or attenuate) higher-energy ...