Laser trimming of surface mount resistors to achieve tight tolerances has been an accepted process for many years.  Board designers and PWB manufacturers have been developing and validating processes to embed resistive materials into inner layer circuit traces of the PWB.  Both screened-on thick film pastes and deposited thin film materials have proven successful.  The initial goals of embedding these passive components are to reduce the size and weight of complex boards, often through an available reduction in the number of layers required.  Depending on the application and design, additional benefits can be improved electrical performance, increased design flexibility, and better reliability (fewer solder joints).  Once implemented in volume, the cost for the finished board will also decrease through reduced part count and assembly costs.

Single-plunge trim in thick-film paste.

Serpentine trim on thin-film resistor.