DFPS Base and Service Board (A3043)

© 2022-2023 Kevan Hashemi, Open Source Instruments Inc.




[19-DEC-22] The Direct Fiber Positioning System (DFPS) Base and Service Board (A3043) consists of a base board that holds sixteen fiber positioners and a service board that provides sixteen twelve-way connectors for fiber controllers. The two boards are joined by a flex cable that carries sixty-four electrode voltages for the sixteen positioners. Sixteen DFPS Fiber Controllers (A3045A) plug into one side of the service board. On the other side is a ten-way connector that plugs into a DFPS Backplane (A3044A).

Figure: Base and Service Board. On the left are sixty-four pins for sixteen actuators. On the right are sixteen connectors for fiber controllers.

The base board and service board are roughly 19 mm square. They must fit inside a 20-mm square envelope so that we can maintain a 5-mm × 5-mm density of fibers. The sixteen connectors for the fiber controllers are packed onto the service board in a staggered arrangement that allows all sixteen fiber controllers to fit into the 19-mm square.

Figure: Sixteen Connectors for Fiber Controllers. These connectors are twelve-way plugs with 0.025" pitch. On the underside is a ten-way, two-row header with 0.050" pitch.

The fiber positioners load into holes on the base board, where we align their electrodes with pins that rise up through the base board top layer. The bottom layer of the base board makes contact with the heads of these pins. Each of these electrode connections passes along the flex cable to the service board. The base board bottom layer maintains isolation between the electrode voltages and the base plate. There are no exposed pads or tracks on the bottom side of the bottom layer, which is the surface we glue to the base plate.

Figure: Sixty-Four Pins for Actuator Electrode Connection. Each pin will be hand-soldered to an actuator electrode.

We build the Base and Service Board (A3043) on a rigid-flex printed circuit baord (PCB). The base board portion has a separate top-layer circuit board holding electrode pins that we solder to the base-board section of a six-layer rigid-flex board. For this we must use an SMT re-flow oven, to solder the pin heads to pads that are hidden within the two-layer structure. The flex cable arises from the two center layers of the six-layer PCB. The service board portion consists of a rigid square with sixteen connectors soldered on the PCB top-side and one connector soldered on the PCB bottom-side. When we load the A3043 into our positioning system, we bend the flex so that the PCB top-side of the service board becomes the under-side with respect to vertical. The fiber controllers plug into the underside while the connetor on the upper-side plugs into the under-side of the backplane.

Figure: Three 4×4 Positioner Cells On Bridge with Frame. The frame bolts to our motorized, two-axis gimbal. Note that the flex cable is shown divided into three strips in this view, but in our actual construction, they are combined into one cable.

The flex cable of the base and service board carries all sixty-four electrode potentials of the sixteen fiber positioners mounted on the base board section. The tracks carrying these potentials are only 5 mil wide (125 μm). They alternate from top to bottom layer on the flex cable. Their pitch is 10 mils (250 μm). We must not run the tracks opposite one another on the flex cable, or else they are likely to short to one another when we bend the flex cable sharply where it emerges and enters the rigid sections. With this spacing, we are approaching the highest density we can achieve with a flex cable and still have adequate insulation between ±250 V signals. If our cell were more than four positioners deep, we would have a hard time running the electrode signals into the base board.


The following versions of the DFPS Base and Service Board (A3043) exist.

Version Actuator Geometry Controller Comment
A Any with 3.8-mm OD actuator 4 × 4 on 5-mm grid A3045A For sixteen-fiber cells.
Table: Versions of the DFPS Base and Service Board (A3043).


A3043A_Drawing.gif: Base and Service Board Drawing.
A3043A_Bottom.gif: Drawing of fiber controller connectors.
A78554.pdf: Twelve-way, female, dual-row, horizontal, connector for controllers.
A78557.pdf: Twelve-way, male, dual-row, vertical, connector for service boards.
S3043A_1.gif: Schematic of Base and Service Board Version A.
A304301A.zip: Base and Service Board PCB Version A.
A304301A.jpg: Annotated photograph of A204301A.
A304301A_Top.svg: Top view of A304301A.
A304301A_Bottom.svg: Bottom view of A304301A.
A304302Y.zip: Base Board PCB Version Y.
A304302Y_Top.svg: Top view of A304302Y.
A304302Y_Bottom.svg: Bottom view of A304302Y.
A304302Y.dxf: Drawing of A304302Y.
A304301X.zip: Bottom layer of base board Version X.
A304301X_Top.svg: Top view of A304301X.
A304301Y.zip: Bottom layer of base board version Y.
A304301Y_Top.svg: Top view of A304301Y.
A304301Y_Bottom.svg: Bottom view of A304301Y.
A304301Y.dxf: Drawing of A304301Y.
A304302X.zip: Top layer of base board version X.
A304302X_Top.svg: Top view of A304302XR2.
A304302X_Bottom.svg: Bottom View of A304302X.


[08-JUN-22] Draft layouts of top and bottom layer 61-mil thick circuit boards that we will glue together to form the 125-mil thick base board with counter-sinks for the positioner end caps.

[23-JUN-22] Our A304301X board looks good, see photo below. Our A304302XR1 has a problem: our Gerber files included a copper annulus around the actuator holes, which will short the electrodes. We prepare A304302XR2 with no copper around the actuator holes top or bottom, and no solder mask clearance on the top side.

Figure: A304301X, Top Side. Shown with 1.25-mm diameter ferrule.

[15-AUG-22] We are trying to devise a way to assemble the two layers of the base board, allowing for the top layer to be rigid only, the bottom layer to be rigid-flex, in which the top and bottom layers are well-aligned with pins, the actuators are provided with protruding pins for their electrode contacts, the electrode connections are made through the caps of these actuator pins being surface-mount (SMT) soldered in our SMT reflow oven, and we glue the sixteen positioners into the base board at the same time we glue the base board to the base plate in our assembly jig.

Figure: Base Board Layers Fastened with Pins. Four pins hold the base board layers together. Sixteen pins provide electrode voltages for actuators.

After soldering all the actuator pins to the top side of the top layer, we apply solder paste to the caps of these pins, fasten the bottom layer to the top layer with four pins that we hand-solder to the top side of the top layer and the bottom side of the bottom layer, then place in the reflow oven to get the actuator pin heads to weld to the pads on the top side of the bottom layer. One concern is the distance between the flange of the ferrule that will mount in the top layer hole.

Figure: Ferrule Gap Close-Up. Actuator pin cap to metal of flange appears to be ≥0.5 mm.

Another concern is our ability to solder the actuator pins to the electrodes of the actuators in the center of the 4 × 4 array. One plan is to hand-solder with a soldering tip that is less than 1.4 mm wide. Another is to apply solder paste to all the pins with a syringe tip 1-mm in diameter, then place in the reflow oven. In both cases, we remove the entire positioner cell from our assembly jig, so that we have the base plate, base baord, and its sixteen positioners glued together. We place on our work surface and rotate to give ourselves access from all sides.

[16-AUG-22] We submit A304301Y and A304302Y for fabrication, width 0.8 and 1.6 mm respectively, with CNC machining for the borders.

[25-AUG-22] We receive A304301Y and A304302Y. We assemble with the sixteen electrode pins and four fiducial pins. Alignment of the ferrule and flange holes appears perfect. We are able to solder positioners to the pins easily. We did not add paste to the electrode pin heads. Next time, we will add the paste before joining the two boards, place in oven, and see if we can get all sixty-four pins to weld to the pads on the lower layer.

[09-SEP-22] Base and Service Board rigid-flex PCB A304301A layout is done. We are checking the netlist. The board is six layers with 4-mil tracks, 5-mil spacing, and a two-layer central flex portion that joins the base and service sections. Circuit diagram is A3043A_1.gif.

[05-OCT-22] We have our A304301A, see here. We load P1 and P17. We believe we can load all connectors by hand.

[07-OCT-22] We rip off P17 by accident, pulling off a fiber controller. We load two other connectors and insert two fiber controllers. We observe eight ramping electrode voltages on the pads on the base board, carried there by the flex cable. We apply black potting epoxy to the base of the controller connectors, in the hope that this will secure them better.

[19-OCT-22] We assemble a complete sixteen-positioner base board with the help of the reflow oven. We check all connections between the electrode pins and the contacts on the service board connector footprints. We check for contact between neighboring electrodes. We have no short circuits. We have no open circuits. For photograph of pin array here.

[26-OCT-22] We load sixteen controller connectors by hand with the help of a new narrow-shaft soldering bit. We connect two fiber controllers programmed to generate ramps with their amplifiers, power up with 500 V and observe the ramps on our pins. For photograph of connectors see here. The positioning of the connectors is accurate to ±0.2 mm, which is good enough for the controllers to fit.

[21-DEC-22] The base and service board plugs into a Backplane (A3044) for the first time, mating with its 0.050" header, and all connections are correct.

[01-FEB-23] We are loading fiber controllers onto our service board, shown below. Connector P12 we ripped off a while back. We secured the rest of the connectors in place with black epoxy. But now we find that five of the remaining fifteen controller plugs are missing their 0V connection.

Figure: Service Board Faults. Purple: No 0V on plug. Blue: Actuator disconnected. Green: Fiber controller identifiers.

We plug a controller into each of the fifteen plugs in turn. We look for a DAC square wave on the controller and we check for connection between the controller's 0V pad and our command transmitter 0V. We see a DAC square wave if and only if we have 0V connection. Otherwise we see 1.2 V on each DAC output with no 0V connection. Looking at the connector layout, we suspect that the track marked below is broken.

Figure: Connector Arrangement with Top Layer Copper. Showing location where we suspect broken track.

Damage to the track may have occurred when we ripped the P12 plug off the board. The track supplying 0V to all five faulty plugs may have ripped off the 0V next to P9. We now try the fiber controller in each of the working sockets, applying ±250V and watching for movement of the masts. Of the nine good connectors, we see movement in eight masts. The mast that does not move is the one connected to P4. The fiber controller is producing all electrode voltages, but none of them are appearing on the actuator. We check three of the actuator's electrode pins and find 0 V on all three. We suspect that the pin heads are not soldered to the base board. We see roughly 254 μm of movement in one fiber, while 15 mm separation of masts appears as 950 μm on the image sensor. The mast tips is moving roughly 4 mm. A few of the fibers move the full 4 mm. Some move diagonally in our field of view, which suggests that one or more of their electrode potentials are missing. We repair A123 by replacing F1, U1 and U2. We program with ID E542. We load onto our service board and see movement of its fiber.