# Wedge Assembly and Testing

D. Winter, Columbia University

## Assembly

### Summary

The wedge is the basic unit of the FVTX detector, covering 7.5^o^ of
azimuth. It consists of a carbon-fiber (TPG) backplane, a High-Density
Interconnect (HDI), readout chips (the FPHX), and silicon sensor. There
are two sizes of wedge, "A" and "B". Wedge type A is 4.990 cm in radial
dimension and type B is 1.268 cm. Apart from length, the two wedges are
identical. The assembled FVTX will contain 96 Type A wedges and 288 Type
B wedges, for a total of 384 wedges.

The current plan has the assembly taking place at FNAL's Sidet facility,
which has many years' experience in microassembly and silicon detectors.
Sidet will handle both mechanical assembly and wirebonding. They also
will provide space to set up test equipment for FVTX collaborators to
test the wedges. We have worked with Sidet to develop the current
procedure and will continue to work closely with them to finalize it.
The procedure has been developed to achieve the needed mechanical
tolerances, while minimizing complexity and maximizing robustness.
Testing at various stages of the assembly is integral to the process.

The scope of wedge assembly and testing consists of WBS items 1.4.1.3.3,
1.4.1.3.4, 1.4.1.3.5 and 1.4.1.3.6. In this document, we describe the
current assembly and testing plans. The wedge assembly starts with
delivery of the TPG backplanes, HDIs populated with the required
surface-mount components and certified as working by the vendor,
sensors, and FPHX chips.

### Procedure

The assembly will take place using only pre-tested HDIs, FPHXs, and
sensors.

The steps in the assembly procedure consist of:

1.  Bond HDI to backplane, using dowel-pin placement. The backplane is
    held in place using a vacuum.

2.  Bond FPHX chips to HDI, using vacuum fixture to transfer chips from
    alignment fixture to assembly fixture.

3.  Wirebond FPHX output pads to HDI

4.  Test electronic integrity of FPHX chips

5.  Bond sensor to HDI, using vacuum fixture (similar to FPHX transfer).

6.  Wirebond sensor outputs to FPHX input pads

7.  Test electronic integrity of wedge

8.  Encapsulate wirebonds

9.  Bend HDI

The wirebonding procedure will include both the actual wirebonding as
well as inspection of the wirebonds. The wire bond material is 1% Si and
99% Al. Al is the preferred material for the current HDI vendor under
consideration. There are 157440 wirebonds for 96 Type A wedges and
1385472 wirebonds for 288 Type B wedges, for a total of 1.4 million
wirebonds.![](media/image1.jpeg){width="2.7576388888888888in"
height="1.8416666666666666in"}![](media/image2.jpeg){width="2.7506944444444446in"
height="1.8527777777777779in"}

The equipment required for wedge assembly is expected to be fairly
standard. Vacuum fixtures are being designed by HYTEC in conjunction
with Sidet, and will be fabricated by an outside vendor. Optical
placement of FPHX chips in the fixtures will require a microscope. Other
parts will be placed using a CMM. The planned adhesives require no
special equipment for curing. Sidet has all needed equipment (including
wirebonding stations) aside from the vacuum fixtures.

Initial designs for the assembly fixtures have been developed with the
help of the Sidet team. The current design for the fixtures can be seen
in Figure 0-1. There are placement jigs for the sensor and FPHX chips
that define the relative placement of the parts to one another. Vacuum
pickup jigs are then used to transfer the parts to the assembly jig.
Both the backplane and HDI will be placed using dowel-pin alignment
(relying on hole and slot to eliminate binding). Each wedge contains one
TPG backplane, one HDI, and one sensor. Wedge Type A has 10 FPHX chips
(5 per side) and Wedge B has 26 FPHX chips (13 per side).

All parts will be bonded using Arclad 8026 transfer adhesive, based on
the recommendation of the Sidet team. This is a non-conducting adhesive
tape, requiring no curing time. Using procedures developed by Sidet, the
sensor will be bonded using a thin band of 8026 along the edges
(extending under all bond pads). A conducting epoxy (such as TRA-DUCT
2902) will be used under the center region of the sensor to provide
electrical connection to the bias plane on the HDI.

Currently Sidet is being contracted to develop the prototype assembly
procedure. This includes feedback for the design of the assembly
fixtures and dicing of dummy silicon parts for mockups of the FPHX chips
and sensor. FVTX will supply Sidet with prototype (and/or mockup
equivalents) of the backplane and HDI. The primary goal is to confirm
the validity of the proposed assembly procedure, as well as produce an
estimate for the cost and schedule of the production assembly.

## Testing

### Summary

Testing refers to the electronic testing of the wedge, at various points
in the assembly process. The goal is to ensure that the wedge is working
to spec at each of the critical stages. In this way we can identify
problems as early as possible, as well as certify that the unit works as
expected before shipping to the cage assembly location.

Testing will include the use of a "mini-DAQ" which will interact with
the wedge in the same way as the PHENIX DAQ. Thus tests will be based on
the integrity of the readout chain.

### Test Procedures

Testing will take place at three stages of the assembly: after FPHX
chips have been wirebonded to HDI, after sensor has been wirebonded to
the FPHX chips, and then finally when the wedge is completed (ie. after
the final encapsulation). All results will be recorded in a database.
All tests will require the use a mini-DAQ similar to the planned readout
chain for PHENIX.

The first set of tests will be after the FPHX chips have been mounted on
and wirebonded to the HDI. Such tests will include (but not necessarily
be limited to):

1.  Successful power-on of the FPHX chips.

2.  Read back of initial state of the FPHX chips to confirm they have
    powered on in the expected state.

3.  Downloading a standard configuration to all the FPHX chips.

4.  Read back of the configuration from all the FPHX chips to confirm
    the successful download.

5.  Noise floor measurement for all FPHX chips.

6.  Threshold (turn-on curve) measurements of the ADC for all FPHX
    chips, pulsing the channels using an external signal.

The above set of tests have been developed using the FPIX chip. Since
the FPHX chip has been designed using the principles of the FPIX, this
test suite is considered appropriate and easily adapted to the FPHX. The
procedure will be extended and tailored as needed once FPHX chips are
available.

Based on procedures developed with the FPIX chip, we estimate the above
procedure will take approximately 2 hours per wedge (large wedge) and 50
mins per wedge (small wedge). In reality the testing should take
significantly less time, since the FPHX chip has 22 times fewer channels
to test.

The second set of tests will be essentially the same, but with the
sensor now attached:

1.  Repeat the above set of tests, to confirm that all previously
    existing parts still operate to spec.

2.  Execute the power-on sequence with sensor bias included, repeat
    tests to confirm that all parts still operate to spec.

3.  Apply external stimulation to the sensor (LEDs) to measure response.
    In addition to the mini-DAQ, such tests will require a dark box in
    which the wedge can be mounted and exposed to the attached LED.
    Standard logic (such as NIM) will be needed to trigger the readout
    in coincidence with the LED.

The third set of tests (after final encapsulation) will be the same as
or a subset of the second set. The goal is not to determine if the parts
are working, but rather to confirm that no damage has occurred during
the encapsulation.

Sidet has ample laboratory space adjacent to the assembly area for FVTX
collaborators to set up and perform the above tests, and has offered it
for our use (this is a standard procedure for Sidet customers). The FVTX
project will have to supply the mini-DAQ as well as other test
equipment.

## Repairs

Every effort possible will be made to ensure that no damage is done
during the assembly process. All fixture surfaces will be polished or
coated in Teflon, and all wedge parts (HDI, FPHX, and sensors) will be
certified as working before being received. Nevertheless, it is
unavoidable that some problems may arise, and we need to plan for
possible repairs to the parts.

### HDI

Repairs of the HDI may or may not be possible. This will be the subject
of R&D. If an HDI is found to be non-functioning at any stage of the
assembly, every effort will be made to salvage the remaining parts if
possible.

### FPHX

The question of repairing FPHX chips falls into two categories: before
and after sensor placement. Should any FPHX chip be damaged during
placement or be found to be defective during the first round of tests,
it will be removed and replaced. Sidet has experience with this
particular problem and understands the technical issues of such
procedures. After the sensor has been placed, replacement of an FPHX
chip is much more difficult. A proper procedure for this will be
developed during the prototype assembly development.

### Sensors

If a sensor is damaged or found to be defective, replacement will depend
on the extent of the problem. It is expected that if a sensor is removed
from the assembly, it will be unusable. Extraction of the sensor will
require care to ensure no damage takes place to the HDI, FPHX chips, or
wirebonds.

### Wirebonds

Should a wirebond be damaged in the process of assembly or testing, it
is straightforward to have it repaired at Sidet. After encapsulation,
the possibility of damage should be minimal. Even so, Sidet has
experience with repairing encapsulated wirebonds, and can do so if
needed.

Once the wedges have been shipped, should there be damage to a wirebond,
there are facilities at BNL that can be contracted to do the repair. If
that facility is not available, the unit may also be shipped back to
Sidet. It may also be possible to include FVTX institutions in the list
of potential repair shops, especially if we find the average repairs are
simple enough to be feasible.

## Documentation

All stages of assembly and testing will be documented. The exact
implementation of the documentation will be developed so as to make it
as efficient as possible for the testers and assemblers as well as
provide a seamless connection with the PHENIX databases. PHENIX relies
on both MySQL and Postgres databases to record configuration state data.
Both are open source and free, and therefore have been proven to be
flexible and easy to use. There is a slight preference for Postgres,
since that is the database that stores calibration information for the
experiment. Regardless of the choice of technology, suitable interfaces
will developed for input and export of data from the database.

"Traveler" files will be developed to document all activity for each of
the wedges. This file will be updated with each modification or test
(including times, tests, assembly steps, part serial numbers, and other
relevant information), and will travel with the wedge from cradle to
grave. Currently it is planned that this file be paper forms to be
filled out by the user and later entered into the database for online
archiving. If it proves to be more beneficial to enter traveler
information directly into the database (updating paper copies as changes
take place), it will be trivial to swap the order. In the past, Sidet
has maintained a traveler (paper binder) for each assembled module, so
this plan is consistent with previous experience.

Ultimately the history of the wedge will be incorporated into the
configuration database for data production.

## Shipping

Standard COTS containers will be purchased to house the assembled units.
Enough containers will be purchased to house all units (including
spares). These containers will provide the needed mechanical and
electrostatic protection to allow them to be used both as storage boxes
and shipping containers. Units will be shipped from FNAL to BNL in
batches as they pass their final tests and inspections.

## HDI Bending

HDI bending refers to the fact that the ends of the HDI (the "ear",
containing the connector) will need to be bent at 90^o^. This
requirement is because the wedges are mounted on a plane transverse to
the beamline, but the signals in and out are carried on flex cables
running parallel to the beamline.

Because the width of wedges extends beyond the 7.5^o^ of the active area
of the sensor, the wedges will also be mounted on their support planes
staggered at two different heights (from the support disk), and
alternating in which side they face. The fact that half the wedges will
be facing opposite directions means that the HDI connector will be bent
either up or down (relative to the face of the sensor), depending on
where it will be mounted. The bending procedure is still a subject of
R&D.

The original plan for the HDI bending includes it as part of the
contract with the HDI vendor. However, in planning the prototype
assembly, it was decided that the prototype would not be bent prior to
the prototype assembly. Sidet recommends that the bending take place as
late in the assembly as possible. Doing so minimizes stress on the HDIs.
The wedges can be shipped unbent, and bending would take place at the
cage assembly area, prior to mounting of the wedge. This approach also
allows greater freedom for choosing where each wedge is located, rather
than committing a wedge to one orientation or another prior to assembly.
Part of the bending R&D will include an assessment of the tradeoffs
between having the vendor perform the bending and impact prebent wedges
have on the assembly, storage, and shipping processes.

The radius of curvature for the bend is estimated at 1.0 cm. Since the
HDI is .300mm in thickness, this is a ratio of 33:1, which is not
considered aggressive. We will work closely with the HDI vendor to
ensure that the proper bending procedure has been implemented. In
addition, we will capitalize on the experience the VTX pixel project
gains, since they will be faced with a similar task in assembling their
ladders (the VTX plan has the bending taking place after ladder
assembly, at the equivalent of the cage assembly location).

## Schedule

The scope of wedge assembly and testing consists of WBS items 1.4.1.3.3,
1.4.1.3.4, 1.4.1.3.5, and 1.4.1.3.6. During schedule discussion,
additional WBS items may be mentioned as appropriate.

A summary of the current schedule for the wedge assembly WBS items is
shown here:

  ------------ ------------- ----------- ------------ ---------- ------------
  WBS          Description   Start       End          Duration   Duration

  1.4.1.3.3    Attach flex   4/10/09     11/5/09      36 weeks   84 weeks
               cable (HDI)                                       

  1.4.1.3.4    Attach sensor 4/17/09     11/12/09     36 weeks   

  1.4.1.3.5    Wirebond      4/10/10     11/15/10     37 weeks   
               Assembly                                          

  1.4.1.3.6    Test chip,    5/18/10     12/13/10     28 weeks   
               sensor, +                                         
               cable                                             
               assemblies                                        
  ------------ ------------- ----------- ------------ ---------- ------------

Preliminary discussions with Sidet have allowed for an estimate of
assembly time. With a current understanding of the tasks involved, we
estimate an assembly time of 11 hours/wedge. This includes all
lamination, wirebonding, and encapsulation, but does not include testing
time. For 400 wedges, this requires 15 weeks to perform the entire
production. The current project schedule includes 84 weeks from start to
finish, which includes significant contingency.

## Manpower

Manpower requirements for assembly prototyping and production will be
handled by Sidet. FVTX manpower will be applied to the testing and
oversight of the assembly production. The prototype assembly will
involve only Sidet personnel, and the process will help us understand to
what tasks FVTX manpower can be applied (as cost and time contingency).

Sidet's preliminary plans include one station for assembly, which
includes HDI lamination and chip and sensor placement. Each assembly
step will require a single technical person, either a Technician I,
Senior Technician, Technician Specialist, or Technical Supervisor. It is
anticipated that as experience is gained in the assembly, as much as
possible will be performed by lower-ranked individuals.

There is room for more than one assembly station although the later
steps (chip and sensor placement, for example) are highly specialized
and it may only be possible for technical individuals to be involved.
Additionally, there is only one wirebonding and encapsulation station
available for the wedges.

We plan to train FVTX collaborators to contribute to the assembly. The
first several steps, up to and including the bonding of the HDI, could
be done by trained FVTX contributors. This is one possible place to
incorporate some schedule contingency. The more specialized work (as
mentioned) will be done by the Sidet personnel. All testing will be done
by FVTX collaborators.