Four-side buttable radiation detector unit and method of making thereof

1. A radiation detector unit, comprising:

an interposer;

at least one radiation sensor bonded to a front side of an interposer;

an application-specific integrated chip (ASIC) bonded to a backside of the interposer by first solder balls;

a carrier board bonded to the backside of the interposer by second solder balls and located on a backside of the ASIC; and

at least one flex cable assembly attached to a respective side of the carrier board,

wherein the second solder balls have a reflow temperature that is lower than a reflow temperature of the first solder balls.

2. The radiation detector unit of claim 1 , further comprising an engagement block including a flat front side surface attached to a backside surface of the carrier board by a thermally conductive adhesive and a non-planar backside surface including at least one engaging surface feature, the engagement block configured to operate as a heat sink.

3. The radiation detector unit of claim 2 , wherein each of the interposer, the ASIC, the carrier board, and the engagement block has a respective cross-sectional area within two-dimensional planes that are parallel to a front side surface of the interposer such that lateral extents of the interposer, the ASIC, the carrier board, and the engagement block are entirely within an area defined by a cross-sectional area of the at least one radiation sensor within a two-dimensional plane that is parallel to the front side surface of the interposer.

4. The radiation detector unit of claim 3 , wherein:

the at least one flex cable assembly contains signal wires and at least one power wire;

the interposer is configured to transmit event detection signals from the at least one radiation sensor to the ASIC, and the ASIC is configured to convert the event detection signals from the at least one radiation sensor to digital detection signals;

the interposer is configured to transmit power from the carrier board to each of the at least one radiation sensor and to the ASIC;

the signal wires are configured to transmit the electronic detection signals and the at least one power wire is configured to provide electrical power to the interposer; and

the at least one flex cable assembly comprises a pair of flex cable assemblies attached to two opposing sides of the carrier board, wherein the pair of flex cable assemblies have a lateral extent within horizontal planes that are parallel to the front side surface of the interposer such that the lateral extent is located entirely within the area defined by the cross-sectional area of the at least one radiation sensor within the two-dimensional plane upon extension away from the at least one radiation sensor.

5. The radiation detector unit of claim 4 , wherein each pair of flex cable assemblies includes:

a respective snap-in connector at a distal end that is distal from the carrier board;

a first bend region connected directly to a respective side of the carrier board and bending away from the interposer;

a first straight region connected to the first bend region extending away from the at least one radiation sensor along a normal direction that is perpendicular to the front side surface of the interposer;

a second bend region connected to the first straight region and bending toward another of the pair of flex cable assemblies;

a second straight region connected to the second bend region and positioned non-perpendicularly to a backside of the engagement block;

a third bend region connected to the second straight region and bending away from the engagement block; and

a third straight region connected to the third bend region and extending away from the backside of the engagement block directly or indirectly connected to the snap-in connector.

6. The radiation detector unit of claim 5 , wherein the snap-in connectors of the pair of flex cable assemblies include two sets of connection pins that face each other about a two-dimensional plane that is parallel to the two opposing sides of the carrier board and bisects the carrier board.

7. The radiation detector unit of claim 2 , wherein the backside of the ASIC is thermally connected to a front side of the carrier board by a thermally conductive paste.

8. The radiation detector unit of claim 1 , wherein:

the second solder balls are disposed in an array that laterally surrounds the ASIC; and

the at least one radiation sensor is bonded to the front side of the interposer by a respective array of electrically conductive epoxy portions having a setting temperature than is lower than the reflow temperature of the second solder balls.

9. The radiation detector unit of claim 1 , wherein:

the at least one radiation sensor comprises a pair of radiation sensors having a respective rectangular shape and adjoined to each other with no gap or with a gap less than 3 mm; and

the at least one radiation sensor comprises a radiation-sensitive material selected from cadmium zinc telluride, cadmium telluride, gallium arsenide, silicon, and a scintillator material.

10. A detector module, comprising:

radiation detector units, each radiation detector unit including an interposer, at least one radiation sensor, an application-specific integrated chip (ASIC), a carrier board, an engagement block including a first engaging surface feature, and a pair of flex cable assemblies connected to respective sides of the carrier board; and

a circuit board including second engaging surface features disposed on an edge thereof and at least one row of board-side connectors on at least one major surface thereof,

wherein:

the radiation detector units are arranged in a row along the edge of the circuit board, with the first engaging surface features interlocking with the second engaging surface features; and

the flex cable assemblies are is connected to a respective board-side connector through a respective snap-in connector.

11. The detector module of claim 10 , wherein:

the pair of flex cable assemblies is attached to two opposing sides of the carrier board; and

a first row of board-side connectors is located on a first major surface of the circuit board;

a second row of board-side connectors is located on a second major surface of the circuit board that is located on an opposite side of the circuit board;

a first flex cable assembly of each pair of flex cable assemblies is connected to a respective one of the board-side connectors within the first row of board-side connectors; and

a second flex cable assembly of each pair of flex cable assemblies is connected to a respective one of the board-side connectors within the second row of board-side connectors.

12. The detector module of claim 10 , wherein each radiation sensor has a front side surface that is substantially perpendicular to each of the at least one major surface of the circuit board.

13. The detector module of claim 10 , wherein:

a front side surface of the at least one radiation sensor within each radiation detector unit has a rectangular shape having a pair of lengthwise edges along a lengthwise direction and a pair of widthwise edges that are shorter than the lengthwise edges along a widthwise direction; and

the radiation detector units of are assembled such that lengthwise edges of each pair of neighboring the radiation detector units abut each other.

14. A detector array comprising an assembly of multiple detector modules of claim 10 , wherein rows of radiation sensors attached to neighboring detector modules abut each other.

15. A computed tomography system comprising:

a radiation source configured to emit an X-ray;

the detector array of claim 14 , wherein the detector array is configured to receive the X-ray from the radiation source through an intervening space configured to contain an object therein; and

an image reconstruction system including a computer configured to run an automated image reconstruction algorithm on detection signals generated from the detector array.

16. A method of forming a radiation detector unit, comprising:

bonding an ASIC to a backside of an interposer at a first reflow temperature using first solder balls;

bonding a carrier board to the backside of the interposer at a second reflow temperature lower than the first reflow temperature; and

after the steps of bonding the ASIC and bonding the carrier board, bonding at least one radiation sensor to a front side of the interposer using electrically conductive epoxy portions at a setting temperature lower than the second reflow temperature.

17. The method of claim 16 , wherein:

the setting temperature is less than 60 degrees Celsius;

at least one flex cable assembly is attached respective sides of the carrier board before bonding the carrier board to the backside of the interposer;

the interposer is configured to transmit event detection signals from the at least one radiation sensor to the ASIC, and the ASIC is configured to convert the event detection signals from the at least one radiation sensor to digital detection signals;

the interposer is configured to transmit power from the carrier board to each of the at least one radiation sensor and to the ASIC; and

the at least one flex cable assembly comprises a pair of flex cable assemblies attached to two opposing sides of the carrier board.

18. The method of claim 17 , further comprising:

snapping in the snap-in connectors of each pair of flex cable assemblies into a respective board-side connector located on a circuit board;

attaching the backside surface of the carrier board to a flat front side surface of an engagement block; and

attaching the radiation detector units to an edge of the circuit board by engaging at least one engaging surface feature of the engagement block with a corresponding engaging surface feature provided at the edge of the circuit board.

19. The method of claim 16 , wherein the first bonding structures comprise first solder balls and the first temperature comprises a first reflow temperature of the first solder balls.

20. The method of claim 16 , wherein the first bonding structures comprise copper pillars and the first temperature comprises a temperature of a thermo-compression method used to bond the ASIC to the backside of the interposer.