Electronic Device With Flexible Printed Circuit Strain Gauge Sensor

• US 20150296607A1
• Filed: 04/11/2014
• Published: 10/15/2015
• Est. Priority Date: 04/11/2014
• Status: Abandoned Application

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Yang, Henry H., Last, Matthew E., Jiang, Tongbi T.
• Application Number:
  US14/251,227
• Publication Number:
  US 20150296607A1
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01L 1/205 using distributed sensing elements
• CPC Class Codes: G01L 1/2287 constructional details of the strain gauges adjustable resistors H01C10/00
• CPC Class Codes: G06F 3/00 Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
• CPC Class Codes: G06V 40/12 Fingerprints or palmprints
• CPC Class Codes: H05K 1/028 Bending or folding regions of flexible printed circuits H05K1/0283 takes precedence
• CPC Class Codes: H05K 1/167 incorporating printed resistors
• CPC Class Codes: H05K 2201/0154 Polyimide
• CPC Class Codes: H05K 2201/0391 Using different types of conductors
• CPC Class Codes: H05K 2201/09263 Meander
• CPC Class Codes: H05K 2201/10151 Sensor

Abstract

An electronic device may be provided with a flexible printed circuit. The flexible printed circuit may have layers of metal and dielectric. Strain gauge resistors may be formed from a strain gauge metal such as constantan. The strain gauge metal may be formed within the flexible printed circuit layers. A strain gauge may include strain gauge circuitry coupled to a strain gauge bridge circuit. Strain gauge resistors for the bridge circuit may be formed from traces that follow parallel meandering paths in the flexible printed circuit layers. A component such as a fingerprint sensor may overlap the strain gauge resistors. Strain gauge resistors may be formed in different overlapping metal layers in the flexible printed circuit layers or may be formed from the same metal layer. Electroplating techniques may be used to form metal traces to which solder balls or wire bonds are coupled.

Claims

1. A flexible printed circuit, comprising:

flexible printed circuit layers including a metal trace; and

a strain gauge resistor formed from a meandering path of strain gauge metal in the flexible printed circuit layers, wherein the metal trace is coupled to the strain gauge metal.

2. The flexible printed circuit defined in claim 1 wherein the flexible printed circuit layers include a polymer substrate layer and wherein the strain gauge metal is formed on the polymer substrate layer.

3. The flexible printed circuit defined in claim 2 wherein the polymer substrate layer comprises a polyimide substrate layer.

4. The flexible printed circuit defined in claim 3 wherein the strain gauge metal comprises constantan.

5. The flexible printed circuit defined in claim 4 wherein the metal trace comprises copper.

6. The flexible printed circuit defined in claim 4 further comprising a polymer cover layer having an opening, wherein a portion of the metal trace is exposed in the opening.

7. The flexible printed circuit defined in claim 6 further comprising a wire bond on the portion of the metal trace that is exposed in the opening.

8. The flexible printed circuit defined in claim 6 further comprising solder on the portion of the metal trace that is exposed in the opening.

9. The flexible printed circuit defined in claim 6 further comprising an electrical component mounted on the flexible printed circuit layers that overlaps the strain gauge resistor.

10. The flexible printed circuit defined in claim 9 wherein the electrical component comprises a fingerprint sensor.

11. The flexible printed circuit defined in claim 10 wherein the strain gauge resistor comprises a portion of a strain gauge bridge circuit.

12. The flexible printed circuit defined in claim 11 wherein the strain gauge bridge circuit comprises an additional resistor and wherein the strain gauge resistor and the additional strain gauge resistor have parallel meandering paths.

13. The flexible printed circuit defined in claim 12 wherein the strain gauge resistor and the additional resistor comprise constantan foil attached to the polyimide substrate with a layer of adhesive.

14. The flexible printed circuit defined in claim 11 wherein the strain gauge bridge circuit comprises an additional strain gauge resistor and wherein the strain gauge resistor and the additional strain gauge resistor are formed in overlapping layers within the flexible printed circuit layers.

15. A flexible printed circuit, comprising:

a flexible polymer substrate layer;

a metal layer on the flexible polymer substrate layer;

a strain gauge resistor formed from strain gauge metal on a first portion of the metal layer; and

an electroplated metal trace on a second portion of the metal layer.

16. The flexible printed circuit defined in claim 15 wherein the strain gauge metal comprises constantan.

17. The flexible printed circuit defined in claim 16 wherein the second portion of the metal layer serves as an electroplating seed layer and wherein the electroplated metal trace comprises copper on the electroplating seed layer.

18. The flexible printed circuit defined in claim 17 further comprising an additional strain gauge resistor, wherein the strain gauge resistor has a first meandering trace and wherein the additional stain gauge resistor has a second meandering trace that runs alongside of the first meandering trace.

19. A flexible printed circuit, comprising:

a flexible polymer substrate layer;

a metal layer on the flexible polymer substrate layer, wherein a first portion of the metal layer is patterned to form a strain gauge resistor having a meandering trace and wherein a second portion of the metal layer serves as an electroplating seed layer; and

an electroplated metal layer on the second portion of the metal layer.

20. The flexible printed circuit defined in claim 19 wherein the metal layer comprises constantan.

21. The flexible printed circuit defined in claim 20 further comprising a fingerprint sensor overlapping the strain gauge resistor.