Now .....
I've to get one of these .....
The 'poor man's reflow oven " ...
Puhui T 962 / 962A ..
( The T 962 C is really nice also .... )
http://www.ebay.com/itm/new-T962-Infrared-SMD-BGA-IC-Heater-Reflow-Oven-18X23-5CM-/271195005806?pt=LH_DefaultDomain_0&hash=item3f247b636e
And "upgrade" it with a
PID* microcontroller ...
http://www.ebay.com/itm/Reflow-Oven-Controller-PID-Ramp-rate-fits-T962-and-T962A-ovens-better-control-/190842334813?pt=LH_DefaultDomain_3&hash=item2c6f16ee5d
*PID microcontroller :
The Reflow Process
A
reflow oven is used to
reflow solder surface mount components onto a Printed Circuit Board (PCB). The basic process entails applying either a leaded or unleaded solder paste to the pads on the PCB. Next, the SMD components are placed on the board and the board put into the reflow oven. The reflow oven applies/adjusts the amount of heat in stages in order to bond (solder) the components to the board. Each stage must be performed using a strict procedure. There are 5 stages for a typical reflow system (Preheat, Soak, Reflow, Dwell and Cooling).
Stage 1 – Preheat
The temperature inside the reflow oven is raised to approximately 125°C at a rate of approximately 2°C per second. This heating rate must be gradual in order to not cause the solder to bubble and splatter. The flux becomes liquid at this temperature and the excess flux will flow away from the pads leaving behind grains of solder.
Stage 2 – Soak
The temperature is slowly raised to approximately 170°C – 175°C and maintained for approximately . At this stage, the temperature of the circuit board and components are nearly the same. Equalizing the temperature prevents cracking or warping of the PCB and/or components during soldering. In addition, the solder flux liquefies and coats the pads.
Stage 3 – Reflow
The temperature inside the oven is ramped up to the soldering temperature of approximately approximately 220°C - 240°C as quickly as possible. At this stage, the grains of solder begin to melt and bond the metal contacts of the components to the associated solder pad.
Stage 4 – Dwell
The soldering temperature is maintained for approximately 10-30secs. The solder is now melted and drawn together by surface tension. The flux is also force outward by the surface tension leaving behind a bond between the component and PCB pad.
Stage 5 – Cool Down
The temperature inside the oven is slowly decreased to room temperature. The cool down must take place slowly to prevent potential warping or cracking of the components and/or PCB because of thermal shock.
The below figure[1]shows a typical temperature vs. time curve for the reflow process:
The actual time and temperature requirements is dependent on the type of solder paste used. I use Kester Sn63/Pb37 (tin/lead) solder paster. The recommended reflow profile is below:
In general, solder pastes can be divided into two groups based on the composition of the solder. Most lead based solder pastes consist of an allow of tin (Sn) and lead (Pb). This composition has a melting point of 183°C.
Lead-free pastes usually consist of a tin (Sn), silver (Ag) and copper (Cu) alloy. Lead-free solder paste begins to melt around 217°C with a maximum reflow temperature of 240°C. Current
Restriction of Hazardous Substances (RoHS) regulations require consumer electronics to contain lead-free solder (some exceptions apply).
One of the first questions that one may ask when considering the stages of the reflow process is how to go about implementing it. The simplest approach is to apply heat if the temperature is below the set point and turn off heating if the temperature is above the set point. This method of control is known as OFF-ON control or more simply as ‘
Bang-Bang Control‘. The problem with an OFF-ON control scheme is the temperature tends to over and undershoot the set point. This happens because the action-reaction of the reflow oven is not fast enough to regulate the temperature to maintain a constant set point. When the heating elements are turned off they do not immediately stop radiating heat. The internal temperature of the heating elements is higher than the temperature of the surrounding air so, thermal energy continues to transfer until they both are the same. When the temperature drops below the set point, the heating elements turn on but, it takes time for them to become hot enough to raise the temperature back to the set point. Thus, the temperature continues to drop until enough heat is applied to bring it back to the set point. A common way to deal with the issues of OFF-ON control is to use a PID routine which is the topic of our next discussion.
What is PID?
PID stands for “Proportional, Integral and Derivative.” Together, these three terms describe the basic elements of a PID controller. A PID controller is a type of control loop feedback system that calculates the difference between a measured input variable and a desired set point and attempts to minimize the error (how far are we away from set point) by adjusting the systems output.
What is Proportional Control?
The proportional term adds or subtracts output proportional to the current error value of the control system in order to drive the system back to the desired output (set point). The error value is given by how far the input (measured value) is away from the set point.
The proportional term is given by the following:
pTerm = Kp * error(t)
Where
Kp – A constant used to adjust the proportional response
error – Value given by subtracting the set point from the input (error = set point – input)
t – The time over which the input is sampled (sample time = 1 second for this explanation)
To summarize the proportional term:
- Error must exist in the system in order to have proportional drive.
- The system will try to correct the error by turning the heating elements ‘ON’ or ‘OFF’ in order to add or remove heat from the system.
- If the measured value is below the set point, heat will be added. If the measured value is above the set point, heat will be removed. If the measured value equals the set point, there is no error and thus, no proportional drive.
What is Integral Control?
A proportional only system would not be sufficient enough in our application to eliminate the error. Our system must be able to change its output according to the current error as well as past errors. The integral is proportional to both the magnitude (amount) of the error and the duration of the error. In other words, the integral is the sum of errors over time. This means the integral adds the amount of error as time passes and attempts to rapidly change the output to eliminate the error.
The integral term is given by the following:
iTerm = iTerm + (Ki * error)
Where
Ki – A constant used to adjust the integral response
error – Value given by subtracting the set point from the input (error = set point – input)
The integral component tracks accumulated error and attempts to accelerate the process towards the set point. This can cause the process to overshoot the set point if not kept in check. The integral component is kept in check in our application by clamping its output between a minimum and maximum value. The clamping routine is given by the following:
IF (iterm > outMax) THEN iTerm = outMax
IF (iTerm < outMin) THEN iTerm = outMin
Where
iTerm – Integral component
outMax – Maximum value the iterm can be
outMin – Minimum value the iTerm can be
To summarize the integral term:
- Error must exist in the system in order to have integral drive.
- The amount of error in the system is accumulated over time. Thus, a small error can become a large correction the longer error exists.
- The system is forced to correct the accumulated error.
- The effect from the integral component will cause the system to overshoot the set point as it attempts to reduce the accumulated error in the system if not limited.
What is Derivative Control?
The derivative term measures how quickly error changes with respect to time and affects the rate of change of the controller output. The rate of change is equivalent to measuring the slope of a line. The derivative term will attempt to reduce the magnitude of the overshoot that tends to be produced by the integral component.
The derivative term is given by the following:
dTerm = (currentTemperature – lastTemperature) * Kd
Where
currentTemperature – Current measured value
lastTemperature – Previous measured value
Kd – A constant used to adjust the derivative response
The derivative component must keep track of the previous measured value. This is accomplished by the following:
lastTemperature = currentTemperature
To summarize the derivative term:
- The system must be taking action. In other words, heat must be added or removed from the system over a period of time.
- The effect of the derivative component is based on how fast the system is heating up or cooling down within the sample period of once per second and is used to counter the overshoot produced by the integral component.
Output Drive…All Together Now!
Each of the PID components are calculated independently and added together to form the output drive. Take note that the
dTerm is subtracted from the sum of the
pTerm and
iterm because the
dTerm is derived from the measured input value(s). The output drive is what controls the process of determining how long to turn the system ON and OFF.
The output drive is given by the following:
Output = (pTerm + (iTerm – dTerm)))
Where
Output – Drive value for system control
pTerm – Proportional component
iTerm – Integral component
dTerm – Derivative component
The output drive process is visualized in the below flowchart:
The above flowchart is a simplistic view of the temperature control process. The process begins by establishing a set point. The next step in the loop is to take a temperature reading. That temperature reading is used to calculate the error or how far we are away from the set point. Next, we calculate the Proportional, Integral and Derivative components and use those values to determine how much adjustment needs to be applied to regulate the system. In the case of our reflow oven, the amount of drive is determined by how long the heating elements are ON or OFF. This type of control can be considered a different take on the PID algorithm and is know as Time Proportional Control (TPC).
Time Proportional Control
Time Proportional Control, a form of Pulse Width Modulation (PWM), is a mathematical technique that allows a feedback controller to use an ON-OFF discrete actuator, such as a relay or Solid State Relay (SSR), as if it were a continuous actuator cable of generating control efforts anywhere between 0 and 100%. The concept of TPC is to turn the actuator ON and OFF for periods proportional to the desired control effort.
Our routine is given by:
timeOn = (tCycle * Output)/100
Where
timeOn – Value indicating how long power will be applied to the heating elements
tCycle – Time span for process
Output – Calculated value from the PID routine
The routine needs to keep track of how much time has elapsed in order to stay within the set process time span. This is accomplished by the following:
Tvar = Tvar + 1
IF (Tvar >= outMax) THEN Tvar = outMin
Where
Tvar – Variable used to track how much time has elapsed
outMax – Maximum value Tvar can be
outMin – Minimum value Tvar can be
The minimum and maximum time span for our application is 0 and 10 seconds, respectively. The above routine ensures the process time span does not go below or exceed the set time span.
http://hobbybotics.com/projects/hobbybotics-reflow-controller-v8-03/
