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SEMI G30-88 © SEMI 198 6, 1988 3 recommended that the therm o couple be secured into the mount ing ba se wi th a the rmal cond uctin g adhe sive ( or solder) and that pa rticular attention be paid to mini mizi ng air voi…

SEMI G30-88 © SEMI 1986, 1988 2

efficient interface between the package and the primary

heat sink. The mounting surface temperature is

determined with a thermocouple attached from the side

or bottom of the adapter with a thermal conducting

adhesive or grease at or near the interface between the

adapter and the package. It is at this point that the

device-under-test temperature is specified and

controlled. The adapter also contains the socket or other

electrical interconnection scheme. A thin coating (about

25 – 50 mm thick) of a thermal heat-sinking compound,

such as zinc-oxide-loaded silicone thermal grease, is

used at the interface to provide a reliable thermal

contact.

2.3 Fluid Bath Assembly — A typical temperature-

controlled fluid bath for thermally characterizing the

microelectronic device under test is shown in Figure 2.

In this figure, the package is mounted in a fluid bath

separate from the fluid circulator, although it can be

immersed directly in an integrated fluid circulator/bath

unit. The fluid in the bath should be continuously

stirred or agitated to ensure the required temperature

stability. Since this working fluid is being used as an

infinite heat sink, the case-to-fluid (ambient)

temperature difference at the case temperature reference

point of interest should be minimized, i.e., less than or

equal to 20°C. For case-to-fluid temperature differences

greater than 20°C, accuracy and repeatability

difficulties may occur due to a large variable

temperature gradient in the fluid film boundary layer at

the package-fluid interface. The case-to-fluid

temperature difference can be minimized by increasing

the fluid velocity and by decreasing the power density

seen by the fluid. The device under test should be

mounted such that heat transfer to the fluid is not

impeded. For leaded devices, the leads should be

oriented in such a manner so as not to interfere with the

heat transfer to the fluid and provide freedom to any

thermal currents caused by the power dissipation within

the package. The case temperature of the device under

test should be measured with a thermocouple that is

attached to the package and should not be assumed to

be at the fluid temperature. Care should be taken to

minimize exposure of the thermocouple bead to the

high temperature gradient in the fluid film boundary

layer at the package-fluid interface. The working fluid

should have a thermal conductivity at 25°C of at least

0.0006 W/cm°C. Working fluids such as inert

fluorocarbon liquids and silicone oils are suitable as a

cooling media.

3 Procedure

3.1 Direct Measurement of Refere nce Point

Temperature — T

. For the purpose of measuring a

microelectronic device thermal resistance, the reference

point temperature shall be measured at the package

location of highest temperature which is accessible

outside the package. This reference point location is

determined with the device operating in free air and

with no external heat-sinking. In general, this reference

point is found to be on the outside surface of the

package substrate directly underneath the chip in the

major path of heat flow from the chip to the heat sink or

ambient. Examples of the reference point location for

both cavity-up and cavity-down ceramic packages are

depicted in Figure 3. The package surface may be

altered to facilitate this measurement, provided that

such alteration does not affect the original heat transfer

paths and, hence, the thermal resistance, within the

package by more than a few percent. For packages with

an integral heat dissipater attached to the outside

surface of the package substrate, the case temperature

reference point shall be on the surface of the heat

dissipater at a point opposite the backside of the chip as

indicated in Figure 4.

3.1.1 Case temperature, T

. The microelectronic

device under test shall be mounted under specified

conditions so that the case temperature can be held at

the specified value. A thermocouple shall be attached

on the surface of the device package directly under the

chip. A conducting epoxy may be used for this purpose.

The thermocouple bead should be in direct mechanical

contact with the case of the microelectronic device

under test. For devices which, in their normal

application, are intimately connected (by pressure

contact, adhesive, soldering, or other means) to an

external heat sink, the mounting surface temperature, as

measured directly below the primary heat removal

surface of the case, may be used as the equivalent case

temperature.

If it is found that attaching the thermocouple directly to

the case is impractical, an alternate approach utilizing a

thermocouple welded to one side of a thin metal disk

should be used. This can be accomplished by parallel

gap welding the crossed thermocouple wires to one side

of a 0.25 cm (0.094 in) diameter, 0.02 cm (0.008 in)

thick beryllium-copper disk and then, with a thin layer

of adhesive, bonding the other side of the disk to the

case at the point of interest.

3.1.1.1 Mounting surface temperature , T

. The

mounting surface temperature is measured directly

below the primary heat removal surface of the case. It is

measured with a thermocouple at or near the mounting

surface of the heat sink. A typical mounting

arrangement is shown in Figure 5. The surface of the

copper mounting base shall be nickel plated and free of

oxides.

The thermocouple hole shall be drilled into the

mounting base such that the thermocouple lead is

directly below the area on the case of interest. It is

SEMI G30-88 © SEMI 1986, 19883

recommended that the thermocouple be secured into the

mounting base with a thermal conducting adhesive (or

solder) and that particular attention be paid to

minimizing air voids around the ball or the

thermocouple. A thermal conducting compound (or

adhesive) should be used at the interfaces of the

mounting base and the device under test. The mounting

surface technique is application oriented in that it takes

into account the mounting surface interface.

3.2 Thermal Resistance, Junction-t o-Specified

Reference point, R

ΘJR

3.2.1 General Considerations — The thermal

resistance of a semiconductor device is a measure of the

ability of its carrier or package and mounting technique

to provide for heat removal from the semiconductor

junction. The thermal resistance of a microelectronic

device can be calculated when the case/mounting

surface temperature and power dissipation in the device

and a measurement of the junction temperature are

known.

When making the indicated measurements, the package

shall be considered to have achieved thermal

equilibrium when halving the time between the

application of power and the taking of the reading

causes no error in the indicated results within the

required accuracy of measurement.

3.2.2 Indirect Measurement of Junction Temperature

for the Determination of R

ΘJR

— The purpose of the test

is to measure the thermal resistance of integrated

circuits by using particular semiconductor elements on

the chip to indicate the device junction temperature. In

order to obtain a realistic estimate of the operating

junction temperature, the whole chip in the package

should be powered in order to provide the proper

internal temperature distribution. During measurement

of the junction temperature the chip heating power

(constant voltage source) shall remain constant while

the junction calibration current remains stable. It is

assumed that the calibration current will not be affected

by the circuit operation during the application of

heating power.

The temperature-sensitive device parameter is used as

an indicator of an average (weighted) junction

temperature of the semiconductor element for

calculations of thermal resistance. The measured

junction temperature is indicative of the temperature

only in the immediate vicinity of the element used to

sense the temperature.

The temperature-sensitive electrical parameters

generally used to indirectly measure the junction

temperature are the forward voltage of diodes and the

emitter-base voltage of bipolar transistors. Other

appropriate temperature-sensitive parameters may be

used for indirectly measuring junction temperature for

fabrication technologies that do not lend themselves to

sensing the active junction voltages.

3.2.2.1 Steady-state technique for mea suring T

. The

following symbols shall apply for the purpose of these

measurements:

Measuring current in milliamperes.

MH-

Value of temperature-sensitive parameter in

millivolts, measured at I

, and corresponding to the

temperature of the junction heated by P

MC-

Calibration temperature in degrees Celsius,

measured at the reference point.

MC-

Value of temperature-sensitive parameter in

millivolts, measured at I

, and specific value o

The measurement of T

using junction forward voltage

as the TSP is made in the following manner:

Step 1 — Measurement of the temperature coefficient

of the TSP (calibration).

The coefficient of the temperature-sensitive parameter

is generated by measuring the TSP as a function of the

reference point temperature, for a specified constant

measuring current, IM, by externally heating the device

under test in an oven or in a fluid bath. The reference-

point temperature range used during calibration shall

encompass the temperature range encountered in the

power application test (see Step 2). The measuring

current is generally chosen such that the TSP decreases

linearly with increasing temperature over the range of

interest, and that negligible internal heating occurs in

the silicon and metal traces. For determining the

optimum TSP calibration or measuring current, V

vs.

log I

curves for two temperature levels that encompass

the calibration temperature range of interest should be

plotted. The optimum measuring current, I

, is then

selected such that it resides on the linear portion of the

two V

vs. log I

curves that were generated. A

measuring current ranging from 0.05 to 5 mA is

generally used, depending on the specifications and

operating conditions of the device under test, for

measuring the TSP. The value of the TSP temperature

coefficient, V

, for the particular measuring

current used in the test, is calculated from the

calibration curve, V

vs. T

. At least three points

should be used to generate the voltage vs. temperature

curve for the determination of the TSP temperature

coefficient.

Step 2 — Power application test.

The power application test is performed in two parts.

For both portions of the test, the reference point

temperature is held constant at a preset value. The first

measurement to be made is that of the temperature-

sensitive parameter, i.e., V

, under operating

conditions with the measuring current, I

, used during

the calibration procedure. The microelectronic device

under test shall then be operated with heating power

) applied. The temperature-sensitive parameter,

, shall be measured with constant measuring

current, I

, that was applied during the calibration

procedure (See Step 1).

The heating power, P

, shall be chosen such that the

calculated junction-to-reference point temperature

difference as measured at V

is greater than or equal

to 20°C. In accomplishing this, the device under test

should not be operated at such a high heating power

level that the on-chip temperature-sensing and heating

circuitry is no longer electrically isolated. Care should

also be taken not to exceed the design ratings of the

package-interconnect system, as this may lead to an

overestimation of the power being dissipated in the

active area of the chip due to excessive power losses in

the package leads and wire bonds. The values of V

, and P

are recorded during the power application

test.

The following data shall be recorded for these test

conditions:

a. Temperature-sensitive electrical parameters (V

, or other appropriate TSP).

b. Junction temperature, T

, is calculated from the

equation:

= T

+ V

− V

()

∆V

∆T

−1

where T

= T

or T

c. Case or mounting surface temperature, T

or T

d. Power dissipation, P

e. Mounting arrangement (including package

mounting force).

3.3 Calculations of R

ΘJR

3.3.1 Calculations of Package Therm al Resistance —

The thermal resistance of a microelectronic device can

be calculated when the junction temperature, T

, has

been measured in accordance with procedures outlined

in Sections 3.1 and 3.2.

With the data recorded from each test, the thermal

resistance shall be determined from:

−

H(package)

, junction - to reference point,

where R

QJR

QJC

or R

QJM

and T

or T

, respectively.

4 Summary Report

The following details shall be specified as appropriate:

a. Description of package, including thermal test chip,

location of case or chip carrier temperature

measurement(s), and heat sinking arrangement.

b. Test condition(s), as applicable (see Section 3).

c. Test voltage(s), current(s), and power dissipation of

test chip.

d. Recorded data for each test condition, as applicable.

e. Symbol(s) with subscript designation(s) of the

thermal characteristics determined.

f. Accept or reject criteria.

RELATED REFERENCES

1. Unencapsulated Thermal Test Chip, SEMI G32-86

Guideline, Book of SEMI Standards, Packaging

Volume.

2. Accepted Practices for Making Microelectronic

Device Thermal Characteristics Test — A User’s

Guide. JEDEC Engrg. Bull. No. 20, Jan. 1975

(Electronic Industries Assoc., Washington, D.C.).

3. Thermal Characteristics, Method 1012.1, MIL-STD-

883C Test Methods and Procedures for

Microelectronics, Nov. 4, 1980 (Rev. Aug. 15,

1984).