  The highly granular silicon-tungsten electromagnetic calorimeter (SiW ECAL)
is the reference design of the ECAL for the International Large Detector (ILD)
concept, one of the two detector concepts proposed for the future International
Linear Collider (ILC). Prototypes for this type of detector are developed
within the CALICE Collaboration. This contribution reports on the development
of an ultra-thin PCB called Chip-on-Board (COB) that is equipped with
wirebonded ASICs and pixelated silicon wafers. This set will form the basic
unit of detection of the SiW ECAL. It will favour the design of an ultra
compact calorimeter since these boards feature a thickness of 1.2 mm to be
compared with the 3 mm of the default solution variant using ASICs in BGA
packaging. We report also on the first results obtained with the COB boards on
a beam test at DESY.
