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Heat Flow Happens

Posted on May 24, 2011

An often misunderstood concept is heat flow and how it can influence the temperature of the product being heated so here is Wikipedia’s definition of heat flow, followed by a discussion of our own on the subject.

A Little Physics

At some point in our schooling, we learned about physical laws. Physical laws are constant; in the context of our daily lives, they don’t change. Three of these laws refer to the way in which energy comes and goes relative to matter. One of those three laws says, quite simply, that when hot stuff cools down, cold stuff heats up. This means that energy is being moved from one thing to another and that energy influences the measurable temperature of those things. This is energy flow, or heat flow.

How Does Energy Flow?

Energy flow is what happens when energy is moved from one object to another through a conductor. Thus, conducted energy flow occurs between two objects when there is:

  1. An energy difference between two objects.
  2. There is a conductor to act as a bridge enabling the energy to flow.
Energy Conductor Heat Flow illustration

Energy needs a conductor in order to flow

Energy always flows through a conductor from an object of high energy to an object of low energy. In this illustration, the high-energy object is a moving hammer, the low energy object is the table and the conductor is a block sitting on the table.

When you hit the block with the hammer, the energy contained in the moving hammer is transferred to the block when it hits. Some is also conducted through the block and transferred to the table it is sitting on. However, because the block is not a perfect conductor, which is true for most things, some of the energy stays in block. That energy bounces between the molecules of the block like balls on a pool table.

Because the molecules rub up against each other, and there is friction between them, some of the moving energy of the hammer is converted to heat energy, which causes a rise in the block’s temperature. It all comes down to molecular motion in an imperfect conductor creating friction that raises its temperature. Therefore, temperature increase is a way of observing energy flow, and energy flow that causes a temperature rise is called heat flow.

What Affects Heat Flow?

Heat Cold Flow Conductor illustration

Heat "flows" from hot to cold

The amount of heat that flows is dependent on the same basic things as energy flow, only we sense its effects by measuring temperature difference. As with high-energy objects imparting their energy to low-energy objects through a conductor, heat flow happens when a hot object transfers its heat through a conductor to a cold object. The quality of the conductor determines how quickly, and how much of the heat is transferred. If no heat is lost to the outside, the objects and the conductor in the above illustration would settle to a temperature 1/2 the value of their difference above the cold object, or below the hot object. The quality of the conductor would then only affect the time it takes for the heat flow to even out the two temperatures.

Heat flow, therefore, is affected by the temperature difference between the Hot and Cold object and the quality of the conductor. The higher the temperature difference, the faster the heat flow and the higher the conductivity of the conductor, the faster the heat flow.

In A Reflow Oven, Heat Flow Matters

The main difference between a reflow oven and the heat flow description above is that the hot object has a continuous supply of energy, provided you do not try to take more from it then it can supply. That energy level is maintained by keeping the hot object at a constant temperature. Most oven controllers are very good at maintaining temperature; however, the delivery of the energy to your circuit board is dependent on the method of conduction. In other words, how good is the conductor between the hot object (the oven heater) and the cold object (your circuit boards)? Methods of improving this conductor have been played with by oven manufactures for many years. Three common ways to conduct heat between the oven heater (the heat source) and your circuit boards (the heat sink) are:

  1. Infrared Radiation (IR)

    IR depends mainly on the “sun-like” conduction of heat via electromagnetic radiation. Although some conduction occurs due to heating of the atmosphere surrounding the circuit board, it plays a lesser role than the radiant energy. This is useful when the entire surface being heated will absorb the radiant energy in exactly the same way at exactly the same rate. The main drawback is that in order to make up for the inefficiencies of this method of heat transfer, the heat source is often set to temperatures much higher than the required melting temperature for the solder on your circuit board. In addition, the components on your circuit board each absorb radiant energy at different rates, aka emissivity. While one component may heat very little, another may be over-heated because it absorbs radiant energy very well. This uneven heat absorption makes it very difficult to choose a heat-source temperature that will meet the varying absorption rates of the variety of components on a typical circuit board.

  2. Vapor Phase (Hot Liquid)

    Vapor phase heat conduction is one of the best around because of its high conductivity and heat capacity, which is released as the vapor changes from gas (vapor) to liquid. Plus, it has the ability to conform to the irregular shape of your circuit board. This method of conducting heat in reflow soldering machines is currently enjoying a comeback.

  3. Convection (Hot Air)

    Convection has been very popular because it provides “vapor like” conformance to board shape, but does not require much more than electric power to function. Here hot air (or nitrogen) is used as the conductor, but with an extra kick: convection fans that move heated air from the heat source to the heat sink. This combination of an air conductor and air movement is called convection. The greatest asset of convection is that it allows you to lower the temperature of the heat source because the movement of the air creates a more efficient heat flow system.

  4. Direct Conduction (Metal Plate)

    Using a direct conductive medium, like a metal plate, is efficient, but is only practical when the surface being heated is flat.

What about my boards?

So heat flow causes circuit boards to get hot within an oven. In this case, the oven is the heat source (like our hammers); moving air is the conductor that delivers the energized air (like moving the hammers) and pounds the heat into the circuit boards (the heat sink).

Convection Oven Heat illustration

In a convection oven, heat is "hammered" into your circuit board by moving air

Because of the increased conductivity that the moving air creates, lower oven temperatures still allow a circuit board to reach the required temperature, and more evenly because conducted heat is not as dependent on a component’s ability to absorb radiant energy. The heat is conducted to the circuit board as fast as the moving air can deliver it.

So, if you want to increase the heat flow, you just turn up the temperature, right? That is correct, but this creates a potential hazard for your circuit boards. Given enough time, your circuit boards will reach the heat source’s temperature. If that temperature is significantly higher than the specified limits for your components, you will damage them. Another way to increase the heat flow is to set the temperature at an acceptable level and only increase the airflow. In other words, add more hammers. This forces the heat into your circuit board components much more evenly and at a rate that meets your component specs. You can accomplish this at your desired process speed while lowering the risk of component damage due to over-temperature.

How can I measure heat flow?

Heat flow is easy to understand, but difficult to measure. This is because the conductor of the heat flow (air) plays as big a role as the temperature in determining how much heat actually flows. For heat, everything is a conductor. There is no simple way to say where heat is going; it goes everywhere. So you cannot know how much of the energy consumed by your heater actually gets to your circuit boards, even though heaters are nearly 100% efficient at converting electrical energy to heat. Therefore, energy into an oven is not energy transferred to your circuit boards, because a lot of it is going into the room, up the exhaust, and some into you.

As a result, heat flow measurement has taken the form of very sensitive probes that measure the temperature difference between the surfaces of thin films. These measurements translate into the amount of energy in watts per square centimeter. Typical values for heat flow in a reflow solder machine from the top heating zones to the surface of a circuit board range from 0.1 to 0.7 watts/sq. cm. This value drops as it proceeds through the oven because circuit boards heat up and reduce the temperature difference between the heater and the circuit boards. This naturally reduces the heat flow because heat flows from hot to cold and if there is no more cold, there is no place for the heat to flow – no heat flow.

Problem Solved

Understanding that oven temperature alone “does not a temperature profile make” is vital toward making your oven consistently reproduce a good temperature profile. The temperature profile your circuit board experiences as a result of your oven settings is dependent on the temperature settings AND the heat flow capacity of your oven. To illustrate this fact, two thermal profiles were run on the same circuit board where the speed of convection air in the oven was the only parameter changed. As you can see, the thermal profile at low convection is dramatically lower then the same profile at high convection, while the oven’s zone temperature settings remained the same.

This does not mean that the heat flow capacity should be large, but it must be consistent hour-to-hour, day-to-day, and week-to-week. Small variations in heat flow can cause temperature variation on your circuit board, even though the oven temperature remains the same. A measuring device that can produce meaningful information relative to heat flow in addition to the temperature will give you a complete understanding of your oven and its performance over time. Of course I would not tell you this if I did not know of the tool that can measure your oven’s ability to “hammer heat” into your boards; that being the OvenRIDER NL 2.

Conclusion

Controlling the reflow oven soldering process requires monitoring not only temperature but also heat flow. Since heat flow is what causes objects to increase in temperature, measuring your oven’s ability to increase the temperature of fixed thermal mass objects will allow you to detect variation in heat flow. By observing the temperature rise in the mass over a fixed amount of time, you can discover if the same amount of heat has been transferred into the mass. Such a measurement, combined with ambient temperature, yields a much more complete profile of a reflow oven. Only by knowing the heat flow capacity of an oven can you be assured that the oven will heat your circuit boards at the same rate and to the same temperature, every time.

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