Design and characterization of flexible circuit for disk drive application

Design and Characterization of Flex On Suspension (FOS) Circuit for HDD Head Gimbal Assembly

Design Challenges

Fundamental Problem - To read and write smaller bits or write bits faster (at GHz rate)

Current Suspension Design Improvements - Reduce Track Pitch, Improve Positioning Accuracy

Design Constraints :	CONNECTION TO	GMR HEAD SLIDER
1.) Mechanical	PREAMP	ASSEMBLY

2.) Process Capability

3.) Electrical

FLEX ON SUSPENSION (FOS)

SPINDLE

MOTOR

Design Guidelines

1.) Mechanical

- Stiffness of HGA: minimize in Gimbal region. Why? Improve Compliance in HGA during flight. How? - reduce trace thickness. Effects? - Circuit Impedance

2.) Process Capability

- 3M design guidelines : 25 microns tracewidth and spacing

3.) Electrical

- Driver chip output impedance and GMR head impedance : controlled impedance throughout flex circuit

GMR HEAD

DISK

FLEX

Electrical Analysis of Flex

Objective : To reduce design cycle time

To achieve fastest time to market

Requirement : Short, well defined current transitions from Write Driver to Write Coil. Write Coil excited by large dI/dt. Any abrupt changes in impedance especially inductance distort the signal shape or attenuation by reflections. The same applies to the Read channel.

Verification : Test and Measurement methodology. Impedance profile along the flex circuit. Time domain measurements such as impedance and crosstalk using Time Domain Reflectometry (TDR) instrumentation.

TDR Measurement Setup

Time Domain Reflectometry - High speed digitizing oscilloscope with a built-in step generator capable of launching fast edge into a device under test (DUT)

Function and Outputs - Monitor reflected wave from various discontinuities in the DUT. Identify areas along DUT contributing excessive inductances and capacitances.

Ei Er

TDR

LOAD

DUT

Step Output - 35ps, 400mV

Flex Circuit Structure

Total structure length = 52mm

PRE-AMP

GIMBAL

BODY	SWAGE PLATE

Virtual TDR using Ansoft Spicelink

DIVIDE and CONQUER

"Identify each and every unique 2 Dimensional cross sections

"Perform Resistance, Inductance, Capacitance, Conductance 2D extraction using Ansoft’s SI2D parasitic extractor

"Create Spice Equivalent Circuit for each and every unique section

"Sitch all subcircuits in Ansoft’s Schematic Capture and perform Time Domain Simulations

Differential Signaling

"2 signals are transmitted between preamplifier and the GMR head : READ and WRITE

"Differential System

"responds only to potential difference between its balanced input teminals

"suppress outputs from common mode voltages - better noise immunity

" inputs must be balanced	READ pair WRITE pair

Unique HGA Cross Sections

Covercoat

Copper Traces

Substrate

Adhesive

Steel

SI 2D

For illustration purposes, extraction was done for the Write signal pair, a similar extraction was done for the Read pair

R, L, C, G Extractions and Field Plots

V1	= Z11I1	+ Z12I2	Zodd = (Z11	- Z12)	Zdiff = 2 x (Z11 - Z12)
V2 = Z21I1		+ Z22I2	Zeven = (Z11	+ Z12)