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
2003-05-06
Von Buhr, Maria N. (Department: 2125)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Specific application of temperature responsive control system
C700S028000, C700S032000, C700S033000, C700S209000, C700S210000, C700S212000
Reexamination Certificate
active
06560514
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved method and apparatus for optimizing control of the thermal profile for a part being processed in a conveyorized thermal processor.
BACKGROUND OF THE INVENTION
Thermal processing involves a series of procedures by which an item is 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 of time. 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 is 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 is performed by sequentially passing a part (such as a printed circuit board 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 at least one corresponding control monitoring location. A thermal element may be defined as either a heat source for heating or a heat sink for cooling and is 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 is 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 is called a “part temperature profile” and if based on measured data is called a “measured part temperature profile”. The temperature response of the part must satisfy a manufacturer's specification requirements, which include allowable tolerance bands or tolerance limits around target values. 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) is 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 of which being (a) a thermocouple attached to the conveyor (though not in thermal contact) so that it moves along with the part, or (b) 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, is 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, 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.
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 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
is moved by means of a conveyor motor
26
a
; the fan is 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 is 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 is identified as one of C
1
, C
2
, . . . , C
n
or as C
j
where j=1 to n and may be identified as a control series. 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.
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 exit
16
. The time-varying thermal exposure may be obtained by using a series of adjacent first and second zones
18
a
and
18
b
at a conveyor speed such that part location in the oven
10
may be defined by conveyor speed multiplied by time since entry. The Temperature of the part
38
may be monitored remotely by an infrared or optical scanner or else measured conductively by one or more attached thermal sensors such as a thermocouple
40
. The measured part temperature data from the thermocouple
40
may be transmitted to the data acquisition device
36
a
, either by direct connection or broadcast signal, and recorded on storage medium
36
b.
The measured part temperature data by thermocouple
40
may be compared to the specification ranges to determine whether the control commanded values from the control station
28
produce a part temperature that complies with specification. The specification ranges represent the allowable limits for calculated feedback parameters that are selected to characterize the thermal process for the part. These feedback parameters may be written as B values in a series of m dimensions where each feedback parameter is identified as one of B
1
, B
2
, . . . B
m
, or as B
i
where i=1 to m. The amount by which a measured value deviates from the middle of its specification range corresponds to a feedback index, and the maximum of these feedback indices denotes the Process Window Index, S. for that thermal process.
Temperature variation may be compared along the reflow oven's length between the commanded temperatures for the elements, the measured zone temperatures, the measured temperature response for the part and the corr
Kazmierowicz Philip C.
Schultz Steven Arthur
Jarrett Ryan
KIC Thermal Profiling
Ritchie David B.
Thelen Reid & Priest LLP
Von Buhr Maria N.
LandOfFree
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