ob-fpc: Flexible printed circuits for the alice tracker Main Requirements. The OB FPC must meet demanding requirements: Material: Low material budget Electrical: impedance of differential lines @ 100W, transmission control and readout signals up to 400Mb/s without significant deterioration, minimal voltage drop along power supply planes, Dimensional: tight total dimension and holes/vias tolerance Modules and stave assembly requires tight dimensional constraints on vias/holes position, length, etc

ob-fpc specifications FPC Dimensions: total 23 x 6 cm^2 Consists of: main part (TAB+FOPC) + small independent components

STACK-up specifications Substrate: Polyimide, 75 um Metal: Cu, 18 um FINAL (top & bottom) Coverlay: 20 um flex solder mask Plating (not shown) on selected areas. Aimed tolerance (impedance and mat. budget): 10% separately for each layer. Layer Thickness (um) Tolerance Dielectric substrate 75 +-10 % Copper (FINAL) 18 +- 10% Coverlay (flex Solder mask) 20

STACK-up specifications MATERIALS: Chosen: Pyralux AP8535R by Dupont (double-sided, copper-clad laminate, Cu thickness 18 um, polyimide core 75 um)  Brand: DUPONT Name: Pyralux AP8535R Dielectric constant: 3.4 @1 MHz (~3.4 @ ~500MHz) Dissipation factor: 0.002@1 MHz (~0.004 @~500MHz ) Dielectric strength [kV/mil] 6-7 In-plane CTE: 25 ppm/C (T<220 C) Operating temperature: 180 C max Pyralux was chosen for its thermal, mechanical and electrical characteristics. Alternative materials can be proposed if with similar characteristics

STACK-up specifications Starting thickness 18 um Cu (Pyralux) requires selective metallization Thickness 18 um everywhere except around vias Where metal thickness allowed up to 35um max. Anyhow it must not exceed coverlay height. Alternatively Starting with thinner Cu layers (or thinning 18 um Cu …) allows for uniform Cu deposition What tolerances can be achieved? Alternative solutions can be proposed by the firms given that the final product complies with all the electrical, geometrical and mechanical requirements.

Specifications for FPC Production Surface Finishing The finishing must be chosen in order to guarantee a surface of high quality for wire bonding Tinplating PB pads, lateral pads, bridges (see sec) should be tinplated for easier later soldering.

Dimensional tolerances FPC contains: main circuit (OB-FPC + TAB section) + other structures Dimensional tolerances are a crucial requirement for the assembly of the module and later for the assembly of modules on staves. This implies tight tolerances on positions of holes, vias, pads, length and width TAB OB-FPC Line of cut

Dimensional tolerances FPC edge TAB cut line Edge distance Referencepoint: Dowel pin hole center -Position of right FPC edge -Position of all holes/vias centers -Position of all pads to be all defined with respect to the actual center of the central dowel pin hole

Other specs Numbers under review! Tolerance for right FPC edge: +-25um. Tolerance for upper and lower edges: +-50um. Position and dimension of holes/vias (all shapes): +-50um Lateral Pads (37+37): +-50 um Other pads (PB, reference, sub/pw) : +- 50um Dowel pin diameter: +-20um Data lines impedance requirement: 100W +- 5% Implications on  requirements on lines width 100 mm and distance 200 mm ± 5 μm dielectric, Cu and cover layer thickness and characteristics Controlled impedance?

Other specs Flatness Essential for proper assembling. Need for FPC identifier (DB)

oblong holes (see fig.) : 1600 um (major axis) , 400 um (minor axis) Ground (vias) and power (holes): oblong holes (see fig.) : 1200 um (major axis) , 400 um (minor axis), with asymmetric crown (crown width equal to 150um on one side, 100um on the other side). Signal holes: oblong holes (see fig.) : 1600 um (major axis) , 400 um (minor axis)

Components mounting SMD components (88 of 0603 and/or 0402 type) connectors (2) to be mounted on each FPC

Preparation of Specifications for FPC Production Quality assurance Several of our requirements difficult/impossible to be verified by companies Asking for a minimal set of tests: Firms are required to provide circuits tested for Isolation of lines Continuity of lines Flatness of FPC General optical inspection (Plating quality,…) cleanliness Proper connection of surface m. components, if applicable All other requirements to be verified by us after delivery: Full tests on pre-production (a preproduction should be required before starting full pr.) Systematic tests on all produced FPCs Full tests on samples (batch validation)