App note from Kionix on things to look for and placement evaluation before placing magnetometer on PCBs. Link here (PDF)
Simulations of the magnetic field environment to provide magnetometer part placement recommendations need considerable technical information to assist the process. Magneto-static and magnetic noise information of the materials and magnetic field sources need to be collected from the design candidate’s 3D geometry, materials, and functional proposals. With this information, magneto-static simulations can be done to guide the optimal part placement in a system.