Apparatus for testing multiple magnetic recording heads

A test apparatus for testing a plurality of magnetic recording heads. The apparatus includes a tool which supports a bar that contains a number of individual magnetic recording heads. Alternatively, the tester may contain a tool which supports a plurality of individual sliders. The tool locates the heads within a magnetic field emanating from a coil wound magnet. The magnet has a length and permeability that provides a uniform magnetic field for each slider in the entire bar. The test apparatus includes a probe card that has a plurality of probe needles which make simultaneous contact with the bonding pads of each head. The needles are coupled to a test circuit that applies a constant current to the magnetic recording heads and measure a change in voltage as a function of a varying magnetic field. The tester can sequentially test the heads without spatially moving the bar, thereby minimizing the test cycle time of testing the heads. Alternatively, the tester may test two or more of the heads simultaneously.

BACKGROUND OF THE INVENTION 
1. Field Of The Invention 
The present invention relates to an apparatus for testing magnetic 
recording heads. 
2. Description Of Related Art 
Magnetic recording heads are used in hard disk drives to magnetically 
record information on a rotating disk. Magnetic heads are typically 
constructed on a wafer that is sliced into separate rows or bars. Each bar 
has a number of individual recording heads. The individual heads are 
eventually sawed from the bars into elements called sliders and later 
assembled to a head gimbal assembly (HGA) of a hard disk drive. 
Each bar of heads is typically tested before being sawed into individual 
components to insure that the magnetic heads comply with manufacturing 
specifications. For example, the magnetic heads may have a 
magneto-resistive (MR) element that has a resistance that will change with 
a varying magnetic field. MR heads can be tested by providing a constant 
current to a magnetic head, exposing the head to a time varying magnetic 
flux and measuring the change in voltage of the MR device. The intensity 
of the magnetic field is typically varied to obtain a number of data 
points for each head. 
The heads are typically manually tested by placing an individual head in a 
magnetic field generated by Helmholtz coils. Manually testing individual 
heads increases the test cycle and ultimate cost of producing the parts. 
It would be desirable to provide an apparatus and technique that reduces 
the cycle time required to test magnetic recording heads and the cost of 
mass producing the heads. 
After the bars are sawed, the individual magnetic recording heads are again 
tested for various characteristics including the flying height created by 
the air bearing surface of the head. Non-conforming heads are sometimes 
re-polished to comply with manufacturing specifications. The re-polished 
heads are again tested within a magnetic tester. Each head has an etched 
serial number to identify the part. To keep track of the parts, the 
sliders are held under a microscope and visually inspected to read the 
serial numbers. The manual reading operation is time consuming and prone 
to human error. Additionally, the individual components are typically 
manually handled with tweezers that may damage the head. It would be 
desirable to provide a test apparatus that can both scan the serial number 
and electrically test individual magnetic recording heads in a manner that 
does not damage the parts. 
SUMMARY OF THE INVENTION 
The present invention is a test apparatus for testing a plurality of 
magnetic recording heads. The apparatus includes a tool which supports a 
bar that contains a number of individual magnetic recording heads. 
Alternatively, the tester may contain a tool which supports a plurality of 
individual sliders. The tool locates the heads within a magnetic field 
emanating from a coil wound magnet. The magnet has a length and 
permeability that provides a uniform magnetic field for each slider in the 
entire bar. The test apparatus includes a probe card that has a plurality 
of probe needles which make simultaneous contact with the bonding pads of 
each head. The needles are coupled to a test circuit that applies a 
constant current to the magnetic recording heads and measures a change in 
voltage as a function of a varying magnetic field. The tester can 
sequentially test the heads without spatially moving the bar, thereby 
minimizing the test cycle time of testing the heads. Alternatively, the 
tester may test two or more of the heads simultaneously.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings more particularly by reference numbers, FIGS. 1 
and 2 show a magnetic recording head tester 10 of the present invention. 
The tester 10 is used to test the magnetic characteristics of a plurality 
of magnetic recording heads 12 located within a bar 14. The magnetic 
recording heads 12 are typically devices which have a magneto-resistive 
element that changes resistance with a varying magnetic field. A constant 
current is provided to the MR head and the change in voltage is measured 
in conjunction with a varying magnetic field. The tester 10 can provide a 
plot of the output voltage as a function of a variation in the intensity 
of the magnetic field. 
The bar 14 is supported by a tool 16 that locates the magnetic recording 
heads 12 within a gap 18 of a magnet 20. Wrapped around the magnet 20 is 
an excitation coil 22 that is connected to a magnetic driver circuit 24. 
The driver circuit 24 provides a current to the coil 22, which generates a 
magnetic field H that flows across the gap 18 and the heads 12. The magnet 
20 has a length and permeability that provides a uniform magnetic field 
across the bar 14, so that each magnetic recording head 12 is exposed to 
the same magnetic field intensity. With a uniform magnetic field intensity 
the tester 10 can simultaneously test two or more magnetic recording heads 
12. The driver circuit 24 is connected to a computer 26 which can vary the 
amplitude of the current provided to the coil 22. 
FIGS. 3a-d show different waveforms generated by the computer 26 and the 
magnetic driver circuit 24. When operating within a hard disk drive, the 
field strength of the disk drive disk does not typically induce a 
substantial Faraday effect on the heads 12. Because of the size of the 
magnet 20 and the location of the heads within the magnetic field, the 
tester 10 may create a Faraday voltage that distorts the output of the 
heads 12. Faraday voltage is proportional to the time rate of change of 
the magnetic flux density. To compensate for the Faraday effect of the 
magnetic field on the heads 12, the computer 26 preferably provides a 
linearly varying waveform to the coil 22 and the magnetic heads 12. The 
linear waveform produces constant Faraday voltages which are substracted 
from the measured MR voltages of the head. 
The linear waveform is to be distinguished from the sinusoidal waveforms 
used in the prior art. The subtraction of the Faraday voltage from the 
measured MR voltage may introduce a small timing error. The voltage error 
is proportional to the time derivative of the Faraday voltage. The second 
time derivative of a sinusoidal waveform is a sine wave. A sinusoidal 
waveform will therefore produce a sinusoidal error. The second time 
derivative of the linear waveform is zero, except at the peaks which are 
typically disregarded in MR head testing. The linear waveform introduces 
no timing errors. The tester 10 may create the sawtooth waveforms shown in 
FIGS. 3a and 3c, or clipped sawtooth waveforms shown in FIG. 3b and 3d. 
The clipped waveforms allow the Faraday voltage to return to zero at the 
peak amplitudes of the magnetic field. Although clipped and unclipped 
sawtooth waveforms are shown and described, it is to be understood that 
the computer 26 and driver circuit 24 may provide any programmable, or 
fixed, arbitrary waveform. 
The tester 10 includes a sensor 28 that is located within the gap 18 to 
measure the strength of the magnetic field flowing across the heads 12. By 
way of example, the sensor 28 may be a hall sensor. The sensor 28 is 
located in a location within the gap 18 that has the same magnetic field 
intensity as the magnetic field that flows across the heads 12 so that the 
sensor 28 senses the same magnetic field amplitude as the recording heads. 
The tester 10 can be calibrated to insure that the magnetic field at the 
heads and the sensor are the same. 
Referring to FIG. 2, the sensor 28 is coupled to a sensor circuit 30 which 
contains circuitry to both drive the sensor and measure the strength of 
the magnetic field. The output of the circuit 30 is provided to the 
computer 26. The computer 26 typically contains an analog to digital (A/D) 
converter and digital to analog (D/A) converter, respectively to receive 
data and provide commands to the other circuits of the tester 10. 
The tester 10 includes a probe card 32 which contains a plurality of probe 
needles 34. The probe needles 34 are placed in contact with the contact 
pads 36 of the magnetic recording heads 12 to couple the heads to the 
tester 10. The probe card 32 is attached to a z-axis actuator 38 which can 
move the card 32 relative to the magnet 20 and the bar 14. The needles 34 
are connected to an electrical tester circuit 40. The tester circuit 40 
typically contains a programmable constant current source 42 which 
provides a constant current to the heads 12 through the needles 34. In the 
preferred embodiment, the tester circuit contains a pre-amplifier circuit 
44 that is coupled to a precision resistor 46. The pre-amplifier circuit 
44 amplifies the voltage across the resistor 46 and provides the amplified 
voltage to the computer 26. The heads 12 typically contain a 
magneto-resistive element which has a resistance that will vary with a 
changing magnetic field. The changing resistive values of the heads vary 
the voltage of the precision resistor 46. The voltage of the resistor 46 
is amplified and provided to the computer. The computer can plot an output 
voltage versus magnetic field curve for each head 12. 
The probe card 32 contains needles 34 that can be coupled to at least two 
magnetic recording heads. In the preferred embodiment, the probe card 32 
has enough needles 34 to test every magnetic recording head in a bar of 
heads without moving the bar. The needles 34 may be coupled to the tester 
circuit 40 by a multiplexor 48 which multiplexes the circuit 40 to one of 
the heads 12. The computer 26 can control the multiplexor 48 so that each 
head is subsequently connected to the tester circuit 40 for a test 
routine. By way of example, the computer 26 may switch the multiplexor 48 
to connect the tester circuit 40 to the first head 12 on the bar. A test 
routine is then performed on the first head. The computer 26 then switches 
the multiplexor 48 to a different input channel so that the next adjacent 
magnetic recording head is coupled to the tester circuit 40. The second 
head is tested and the computer 26 switches to the next channel. The cycle 
of switching channels and testing heads is repeated until all of the heads 
on the bar 14 are tested. Although a multiplexor 48 and a single tester 
circuit 40 is shown and described, it is to be understood that the tester 
may be constructed with multiple tester circuits, with or without a 
multiplexor. The multiple tester circuits would allow multiple heads to be 
tested simultaneously. 
Referring to FIG. 1, the tester 10 preferably includes a bearing assembly 
50 that allows the tool 16 to be rolled into and out of the magnet 20. The 
tool 16 may be coupled to an actuator 52 that pushes the heads 12 into the 
magnet gap 18. The tester 10 may have a second actuator (not shown), or a 
spring return, that pushes the tool 16 out of the magnet 20 when the 
actuator 52 is de-activated. The actuator(s) allows the heads 12 to be 
automatically loaded and unloaded from the magnet 20. 
The tester 10 may also contain a vacuum pencil 54 that can move a bar or 
individual heads from a tray 56 to the tool 16. The vacuum pencil 54 is 
preferably coupled to an x-y table 58 and a z-actuator 60 which can 
accurately place the pencil 54 into the tray 56 and over the tool 16 to 
load and unload the bar, or individual heads. The position of the vacuum 
pencil 54 can be controlled by an operator through an input device (not 
shown) such as a joystick. The vacuum pencil 54 captures the bar/heads 
while minimizing the probability of marking or otherwise damaging the 
devices. 
FIG. 4 shows an alternate embodiment of a tool 16' that can support a 
number of individual magnetic recording heads 12. It is sometimes 
desirable to test the heads individually even after testing in bar form. 
The tool 16' has a plurality of grooves 62 that each hold an individual 
head component. Each groove 62 preferably contains a pair of apertures 64 
that are coupled to a vacuum source 66. The vacuum source creates an air 
flow that pulls the heads into the grooves 62. The pull of the vacuum 
source insures that the corners of the magnetic recording heads 12 are 
accurately located at the corners of the grooves 62, so that the probe 
needles 34 make contact with the contact pads 36 of the heads 12. There is 
preferably an aperture at each axis of the groove corner to insure contact 
between the heads and each side of the grooves. The grooves 62 are 
preferably separated by a section 68 that has a width which allows the 
same probe card 32 to be used for either the bar tool 16 or the individual 
head tool 16'. The tools are preferably constructed from a material that 
is hard enough to withstand the constant striking of the heads with the 
corners of the grooves. 
FIG. 5 shows an alternate embodiment of a tester which contains an optical 
scanner 70. The magnetic recording heads 12 are typically manufactured 
with a serial number on the side of the die opposite from the contact pads 
of the heads 12. It is desirable to read the serial number to accurately 
correlate the test data with the head being tested. In one embodiment, the 
tool 16 is constructed from a transparent material that allows the serial 
numbers to be optically scanned while the heads are electrically tested. 
To minimize space, the tool 16 may contain prisms 72 which allow the 
optical scanner 70 to be placed at a right angle relative to the serial 
numbers of the heads. The heads 12 can be scanned during a magnetic test 
routine to minimize the test cycle. Although a prism 72 is shown and 
described, it is to be understood that the scanner 70 can read the serial 
number directly through a transparent portion of the tool without a prism. 
In operation, the tool 16 is initially located out of the magnet 20. The 
vacuum pencil 54 automatically moves a bar 14 into the tool 16. By way of 
example, the testing of a bar will be described, although it is to be 
understood that the same routine may be applied to test a number of 
individual heads 12. After loading the bar 14 into the tool 16, the 
actuator 52 is activated to push the heads 12 into the gap 18 of the 
magnet 20. The probe card 32 is then moved in a downward direction so that 
the needles 34 are in contact with the contact pads 36 of the heads 12. 
The tester 10 may undergo an initialization routine to calibrate the 
various circuits. By way of example, the computer 26 may vary the current 
generated by the current source to obtain a desired calibrated reading. 
The computer switches the multiplexor 48 to couple one of the heads to the 
tester circuit 40. The computer 26 then provides commands to apply and 
vary the magnetic field generated by the magnetic driver circuit 24, coil 
22 and magnet 20. The computer 26 may obtain multiple samples of head 
output voltage data and magnetic field intensity, and then average the 
data to minimize any noise in the test. 
After the test cycle is completed the multiplexor 48 is switched to couple 
the next head to the tester circuit 40 and the test routine is repeated. 
The cycle of coupling heads and performing test routines may be conducted 
for each head of the bar. Simultaneous tests may be performed on a 
plurality of heads for a tester with multiple tester circuits 40. By 
placing the needles in contact with every head on the bar, the testing of 
all the heads can be completed electronically without spatially moving the 
bar, thereby greatly decreasing the cycle time required to test a bar of 
heads. The computer 26 accumulates the data and may plot a voltage versus 
magnetic field graph as shown in FIG. 6. 
While certain exemplary embodiments have been described and shown in the 
accompanying drawings, it is to be understood that such embodiments are 
merely illustrative of and not restrictive on the broad invention, and 
that this invention not be limited to the specific constructions and 
arrangements shown and described, since various other modifications may 
occur to those ordinarily skilled in the art.