Data processing: generic control systems or specific application – Specific application – apparatus or process – Specific application of temperature responsive control system
Reexamination Certificate
1999-09-23
2002-09-17
Picard, Leo P. (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Specific application of temperature responsive control system
C700S207000, C700S209000, C700S212000, C700S033000
Reexamination Certificate
active
06453219
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved method for controlling temperature response of a part in a conveyorized thermal processor by means of closed-loop feedback.
BACKGROUND OF THE INVENTION
Thermal processing involves a series of procedures by which an item may be exposed to a temperature-controlled environment, and is used in a variety of manufacturing procedures such as heat treating, quenching and refrigerated storage. One example of a thermal processor is a reflow oven. The production of various goods such as electronic circuit boards in solder reflow ovens frequently entails carefully controlled exposure to heating and/or cooling for specific periods. The elevated temperature conditions needed to solder component leads onto printed circuit boards must be gradually and uniformly applied to minimize thermal expansion stresses. For this reason, convection heat transfer may be employed in these solder “reflow” operations. The connecting solder paste incorporates an amalgam of substances that must undergo phase changes at separate temperature levels. Solder reflow may be performed by sequentially passing a part (such as a printed circuit board to become a processed product) through a series of thermally isolated adjacent regions or “zones” in the reflow oven, the temperature of each being independently controlled.
Convection heat transfer chambers or zones are typically set to a fixed control temperature throughout a thermal process. A zone may have one or more controlled thermal elements, these each having a corresponding control monitoring location. A thermal element may be defined as either a heat source for heating or a heat sink for cooling and may be commanded to a control temperature. The temperatures commanded at the control monitoring locations form a “control temperature profile” along the reflow oven. The temperature exposure of the part may be governed by the processor temperature of the air in each zone and the exposure time within each region. The temperature of the air along the zones forms a “processor temperature profile”. The series of instantaneous part temperature values as the part travels along the conveyor and through the oven may be called a “part temperature profile” and if based on measured data may be called a “measured part temperature profile”. The temperature response of the part must satisfy a manufacturer's part specification requirements, which include allowable tolerance bands or tolerance limits around target values that have been defined. A measured value within the corresponding tolerance limit satisfies the specification. The procedure for operating the oven to obtain temperature data (used in creating a measured part temperature profile) may be called a “test process”.
The temperature response of the part may be monitored by instrumenting the part or adjacent device with one or more thermocouples (or other temperature measuring contact devices such as thermisters or resistance temperature detectors) prior to sending the part into the reflow oven or by remote observation with a thermal sensor. Alternatively, the temperature response of the part may be measured by a remote means such as an infrared or optical scanner. The thermocouple measurements can be sent to a data acquisition device through an attached cable or by a radio transmitter or by similar means. The temperature along the conveyor may also be measured by different means. Two examples are a thermocouple attached to the conveyor (though not in thermal contact) so that it moves along with the part, and a fixed probe extending along the length of the oven and positioned adjacent to the conveyor having a plurality of thermocouples disposed along the probe interior.
A thermal processor, such as a reflow oven, may be modulated by a series of n control parameters labeled C
j
numbering from j=1 to n. These control parameters may include the oven setpoint temperature at each zone, the conveyor speed, product conveyor density (number of parts per unit conveyor length), or a combination of these and other variables subject to direct adjustment or indirect influence during the thermal processor operation. Other physical influences on the thermal process include initial conditions, which may depend on the ambient temperature and humidity, as well as characteristics difficult to measure directly, such as convection rate. These may be referred to in the aggregate as noncontrollable processor parameters.
A side view diagram of a reflow oven is shown in
FIG. 1
as an example of a thermal processor. The reflow oven
10
has a conveyor
12
aligned along the length of It the oven
10
that moves in the direction towards the right from entrance
14
to exit
16
. The oven interior may be divided into two or more zones for thermal processing. In the illustration, first and second zones
18
a
and
18
b
are shown. Each zone has at least one heating and/or cooling element
20
a
and
20
b
and may feature one or more monitoring instruments
22
a
and
22
b
in proximity to the elements to monitor thermal processing. These element monitoring instruments
22
a
and
22
b
may be thermocouples or thermostats. The oven
10
may also include one or more recirculation fans
24
to increase convection. The conveyor
12
may be moved by means of a conveyor motor
26
a;
the fan may be rotated by means of a fan motor
26
b
. The settings for heating and/or cooling elements
20
a
and
20
b
, and the motors
26
a
and
26
b
are controlled by a control station
28
, which receives settings input from a receiver
30
instructed by an operator
32
or other means. Each control parameter may be commanded to a target condition at a control interface between the input receiver
30
and the control parameter. The example shown in
FIG. 1
features a first zone interface
34
a
, a second zone interface
34
b
, a conveyor interface
34
c
and a fan control
34
d
. These control parameters may be expressed as C values in a series of n dimensions where each control parameter may be identified as one of C
1
, C
2
, . . . , C
n
or as C
j
where j=1 to n. In the example shown in
FIG. 1
, n equals 4. Measured data from instruments monitoring control performance such as element monitoring instruments
22
a
and
22
b
may be received by a data acquisition device
36
a
and recorded on a storage medium
36
b.
The response of the thermal processor to the settings of the control parameters may not be identical to the C
j
values. This processor response may be characterized as processor parameters labeled C′ in a similar series of n dimensions. In conjunction with the processor parameters C′ may be measured and these data recorded. The correlation between the control parameters C and the process parameters C′ may be influenced by the rate of convection, the thermal isolation across zones and the ability of the thermal elements to attain the target control parameter values. In practice, the process parameters may fluctuate with time around a local or running average {overscore (C)}′. Process deviance tolerances may be specified or defined, which if exceeded may cause the thermal processor to discontinue. These tolerances may be expressed as absolute differences between the control parameters and their corresponding average processor parameters |C−{overscore (C)}′|
j
for series j=1 to n. After the thermal processor has had sufficient time (t→∞) for processor parameters to reach thermal equilibrium (assuming the control parameters are held to fixed values), such that the process parameters C′ vary only within a specified equilibrium range or tolerance &egr;
Cj
, that difference |C−C′|
j
may be expected to vary within that equilibrium range expressed as |C−C′|
j
≦&egr;
Cj
.
A part
38
, such as a printed circuit board, may be placed on the conveyor
12
upstream of the oven entry
14
to be transported through the oven
10
and egressing through the
Dransfeldt Eric
Kazmierowicz Philip C.
Schultz Stanley Douglas
Jarrett Ryan
KIC Thermal Profiling
Picard Leo P.
Ritchie David B.
Thelen Reid & Priest LLP
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