1. 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 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

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

3. 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

4. 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: MHz ~500MHz)
Dissipation factor: MHz )
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

5. 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.

6. 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.

7. 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

8. 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

9. 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?

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

11. oblong holes (see fig.) : 1600 um (major axis) , 400 um (minor axis)
Ground (vias) and power (holes):
oblong holes (see fig.) : 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.) : um (major axis) , 400 um (minor axis)

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

13. 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)