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Step 9Using the Hot Plate Soldering System
The requirements for reflow soldering are fairly simple. Basically, the board and components must get hot enough to flow (melt) the solder. Along the way, the temperature must increase at the correct rate. Too hot and the flux boils and disturbs placed parts. Too slow and the solder oxidizes and won't flow correctly. The desired temperatures and the times associated with them constitute what is known as a Solder Temperature Profile. Have a look at Figure 3 and Table 2 in the Intersil apps note that you downloaded back in Step 7. These show a generic temperature profile. I'll show you how I interpreted this target profile and detail exactly the profile I used successfully. Remember, all my work is with lead-free solder paste. If you're using lead solder, you'll have to figure out the best profile yourself. Fortunately, it's not too difficult to provide a satisfactory profile. But first we have to have some way to know what temperatures our hot plate is reaching.
In this step, I will describe experiments that you will need to perform as you set up the Hot Plate Solder System. You should have several extra circuit boards that you don't care about for experiments. I had 8 boards and used 4 of them for experiments before I built a keeper. Two of my experimental boards had solder cleaned off them and were used a second time. Don't be afraid to experiment. It's the key to success.
Let's get started with the experiments. Hook up your hot plate and control system. In Step 6 we experimented a little with this, but now we'll proceed more methodically. I found that a cycle length of 10 seconds worked well, so we'll set that value for Cycle Length and not change it. We'll only change the On Time of the control pulse.
To control the soldering process we need to know what temperature the surface of the hot plate is reaching. The problem is that the hot plate doesn't have a temperature sensor, so we have to find one. The best sensor is probably a non-contact, infrared temperature gun. This can give a quick, accurate reading of the temperature of your hot plate. The SparkFun unit can only measure a maximum of 428F/220C which is not quite high enough (we'd like to measure up to 260C), but it's probably adequate. I managed to rent a unit locally with greater range, but two and half rentals would pay for the SparkFun unit.
A cheaper, but less accurate, approach is to use an oven thermometer (Picture 26) and bend the stand so it can lay flush on the hot plate. While this unit will not provide an accurate measurement of temperature, it will provide a repeatable reading and can be a useful reference. I'll explain how to do that. As you experiment, you'll want to record your settings, what temperature is indicated on the thermometer, and how long you wait between readings, for future reference. If you have an infrared sensor, record it's readings also. Don't put a PCB on just yet – that's the next step.
Bend the stand so the thermometer sits as flat as possible on the hot plate. Let it sit on your hot plate with the Hot Plate Control Unit's On Time set to 2. The temperature indicator will probably not quite reach the scale. Now increase the On Time to 4 and wait a few minutes until the temperature is stable and waiting longer doesn't show an increase. Reaching an equilibrium temperature took about 30 to 40 minutes on my hot plate.
Continue increasing the On Time in increments of 2, then taking readings after 10 minutes, 20 minutes, 30 minutes, and 40 minutes, then increasing the On Time by 2, taking readings again, until the indicated temperature reaches at least 200C on the thermometer or 280C on the infrared sensor. If you're using the SparkFun unit, you'll have to stop at 220C on the IR, but you'll have a good bit of data by then to guide you. You should get a good idea of two things: First, what is the difference in temperature readings between the oven thermometer and the infrared sensor? Second, how soon does the infrared sensor indicate that the hot plate is at equilibrium as opposed to when the thermometer reaches equilibrium? This data will be extremely helpful as you try to create a temperature profile for soldering.
The next step is to try melting solder on an actual circuit board. My experience is that I get more repeatable results if I let the hot plate cool down fully between soldering sessions. This takes about an hour, so you have plenty of time to stencil solder on to one of your spare boards. Don't put any parts on it just yet, we're just going to learn when the solder melts.
Now that the hot plate's cooled down, let's warm it up again. Based on my experience this warm up makes the rest of the cycle more repeatable. I use an On Time of 5 and wait 30 minutes for the warm up. The temperature on my oven thermometer reaches a nearly stable reading of 70C, the first mark on the temperature scale. Don't worry about the temperature profile just yet – that's later. First we need to know when the solder flows.
Place your board on the warm hot plate and let it warm up also, say 5 minutes or so. Then gradually increase the temperature. I found that my lead-free solder flowed when the thermometer read about 140C to 160C on the oven thermometer, so be careful as you approach this temperature range. Record the thermometer reading when your solder flows. You'll probably note that the solder paste will first spread out on your circuit board – mine actually spread out and touched between pads. Don't panic, this is OK. (This is also the reason I don't worry if I smear a little solder paste. Surface tension takes over when the solder flows and pulls the solder onto the pads.) Then you'll notice that the paste will turn a darker gray. Finally, it'll get shiny silver as it flows and beads up on the pads. That's the point when you want to record the temperature. Picture 27 shows a board with the solder just melted. Don't increase the temperature any further or you'll risk damaging (burning) your board. The temperature may climb a little as the hot plate overshoots. That's OK.
You should now have sufficient data to determine a satisfactory soldering temperature profile. Refer to the first paragraph in this section if you've forgotten what a soldering temperature profile is. The soldering profile I used is pretty simple. Here it is:
- Starting with the hot plate at room temperature, set the On Time to 4 and wait 30 minutes. The oven thermometer should read 70C or very close. The IR sensor will probably read between 110C and 120C. If you're soldering a board, put it on the plate and wait two minutes for it to heat up.
- Set the On Time to 20 (remember to push the button) and wait until the oven thermometer reads 120C (six to six and a half minutes). The IR sensor will read about 220C.
- Set the On Time to 48 and wait until the oven thermometer reads 150C (eight to eight and a half minutes.) The IR sensor will read about 300C.
- Turn everything off. The oven thermometer will continue to increase, but shouldn't go over 160C. The IR gun could read as high as 315C. If you're soldering a board, wait about 30 seconds, no more than one minute, and slide the board off the hot plate onto the carrier to finish cooling.
Note: The temperature of the surface of the circuit board will be lower than the temperature of the hot plate. Based on when solder flowed, I estimate the temperature of the board to be about 20% lower than the IR temperature readings.
Don't worry if your IR sensor won't read over 220C. You should have enough comparison data between it and your oven thermometer to know if you're tracking my readings. If you are, then use my times and see what the results are. If you're readings don't track mine, you'll need to do some more experiments.
I've included results from the experiments I did. Please note that for the results shown I kept the hot plate on for nine minutes so the temperatures overshot what I would normally use (off after eight minutes). When I turn off at eight minutes, the shape of the ramp is nearly identical to that shown for nine minutes, the peak just stays below 260C (board temperature).
Look first at Temp Profiles. The top line is the IR sensor readings and the bottom line is the oven thermometer readings. Based on when the solder flows, I estimate the board temperature to be about 20% lower than the IR readings. That's the middle line. Again, I'd turn things off at eight minutes if I were actually soldering a board and the maximum IR temperature would not go over 315C.
Now let's compare what I measured to the target from the Intersil apps note. Look at Target Profiles. The upper and lower target profiles from Intersil are shown along with the board temperatures from my experiments. Making the adjustment for 8 minutes versus nine minutes, we see that the profile is pretty close to the target. Not bad!
Two things should be pointed out. First, the cool down rate is kind of arbitrary. It's not clear to me what the impact of a rate considerably slower that Intersil suggests would be. But I probably don't care. What's most important is that letting the board cool briefly on the hot plate, then sliding it onto a carrier where it can finish cooling pretty quickly is probably just fine. Cooling too quickly could be bad, but putting the board on an insulator (like piece of wood or perf board) should be OK. Watch out for static electricity if you do this.
Second, the profile could be tuned more. Using a higher value for the initial On Time (say 24 instead of 20) would raise the initial rate of temperature increase. Since I've gotten satisfactory results, I haven't seen fit to do that. Please experiment to your heart's content.
Once you have a profile that seems like it's close to the target, it's time to experiment by using it to solder. Stencil paste onto another board – still no parts. Use the profile you've determined to flow the solder on this test board and run your profile. Does solder flow when you expect it to (about when the oven thermometer reads 140C – about eight minutes)? If so, you're ready to solder some parts. If not, you need to run another experiment or two.
Now for the real thing. Stencil on the paste and place your parts. (If you're nervous and want to experiment a bit more, leave off any expensive or scarce parts the first time.) Be sure your hot plate has fully cooled, then warm it as discussed above. Put the board with parts onto the plate. Picture 28 shows my board with the paste heated. Picture 29 shows the board when the solder flowed. Run your solder cycle. When all the solder has flowed, turn off the hot plate, let it cool for 30 seconds to a minute (it'll still be mighty hot), and slide your circuit board off onto your carrier so it can finish cooling. How's it look? Great? OK, dance around the house some – you've just performed magic! (I still get a kick out of this process.) Picture 30 shows my final product.
In this step, I will describe experiments that you will need to perform as you set up the Hot Plate Solder System. You should have several extra circuit boards that you don't care about for experiments. I had 8 boards and used 4 of them for experiments before I built a keeper. Two of my experimental boards had solder cleaned off them and were used a second time. Don't be afraid to experiment. It's the key to success.
Let's get started with the experiments. Hook up your hot plate and control system. In Step 6 we experimented a little with this, but now we'll proceed more methodically. I found that a cycle length of 10 seconds worked well, so we'll set that value for Cycle Length and not change it. We'll only change the On Time of the control pulse.
To control the soldering process we need to know what temperature the surface of the hot plate is reaching. The problem is that the hot plate doesn't have a temperature sensor, so we have to find one. The best sensor is probably a non-contact, infrared temperature gun. This can give a quick, accurate reading of the temperature of your hot plate. The SparkFun unit can only measure a maximum of 428F/220C which is not quite high enough (we'd like to measure up to 260C), but it's probably adequate. I managed to rent a unit locally with greater range, but two and half rentals would pay for the SparkFun unit.
A cheaper, but less accurate, approach is to use an oven thermometer (Picture 26) and bend the stand so it can lay flush on the hot plate. While this unit will not provide an accurate measurement of temperature, it will provide a repeatable reading and can be a useful reference. I'll explain how to do that. As you experiment, you'll want to record your settings, what temperature is indicated on the thermometer, and how long you wait between readings, for future reference. If you have an infrared sensor, record it's readings also. Don't put a PCB on just yet – that's the next step.
Bend the stand so the thermometer sits as flat as possible on the hot plate. Let it sit on your hot plate with the Hot Plate Control Unit's On Time set to 2. The temperature indicator will probably not quite reach the scale. Now increase the On Time to 4 and wait a few minutes until the temperature is stable and waiting longer doesn't show an increase. Reaching an equilibrium temperature took about 30 to 40 minutes on my hot plate.
Continue increasing the On Time in increments of 2, then taking readings after 10 minutes, 20 minutes, 30 minutes, and 40 minutes, then increasing the On Time by 2, taking readings again, until the indicated temperature reaches at least 200C on the thermometer or 280C on the infrared sensor. If you're using the SparkFun unit, you'll have to stop at 220C on the IR, but you'll have a good bit of data by then to guide you. You should get a good idea of two things: First, what is the difference in temperature readings between the oven thermometer and the infrared sensor? Second, how soon does the infrared sensor indicate that the hot plate is at equilibrium as opposed to when the thermometer reaches equilibrium? This data will be extremely helpful as you try to create a temperature profile for soldering.
The next step is to try melting solder on an actual circuit board. My experience is that I get more repeatable results if I let the hot plate cool down fully between soldering sessions. This takes about an hour, so you have plenty of time to stencil solder on to one of your spare boards. Don't put any parts on it just yet, we're just going to learn when the solder melts.
Now that the hot plate's cooled down, let's warm it up again. Based on my experience this warm up makes the rest of the cycle more repeatable. I use an On Time of 5 and wait 30 minutes for the warm up. The temperature on my oven thermometer reaches a nearly stable reading of 70C, the first mark on the temperature scale. Don't worry about the temperature profile just yet – that's later. First we need to know when the solder flows.
Place your board on the warm hot plate and let it warm up also, say 5 minutes or so. Then gradually increase the temperature. I found that my lead-free solder flowed when the thermometer read about 140C to 160C on the oven thermometer, so be careful as you approach this temperature range. Record the thermometer reading when your solder flows. You'll probably note that the solder paste will first spread out on your circuit board – mine actually spread out and touched between pads. Don't panic, this is OK. (This is also the reason I don't worry if I smear a little solder paste. Surface tension takes over when the solder flows and pulls the solder onto the pads.) Then you'll notice that the paste will turn a darker gray. Finally, it'll get shiny silver as it flows and beads up on the pads. That's the point when you want to record the temperature. Picture 27 shows a board with the solder just melted. Don't increase the temperature any further or you'll risk damaging (burning) your board. The temperature may climb a little as the hot plate overshoots. That's OK.
You should now have sufficient data to determine a satisfactory soldering temperature profile. Refer to the first paragraph in this section if you've forgotten what a soldering temperature profile is. The soldering profile I used is pretty simple. Here it is:
- Starting with the hot plate at room temperature, set the On Time to 4 and wait 30 minutes. The oven thermometer should read 70C or very close. The IR sensor will probably read between 110C and 120C. If you're soldering a board, put it on the plate and wait two minutes for it to heat up.
- Set the On Time to 20 (remember to push the button) and wait until the oven thermometer reads 120C (six to six and a half minutes). The IR sensor will read about 220C.
- Set the On Time to 48 and wait until the oven thermometer reads 150C (eight to eight and a half minutes.) The IR sensor will read about 300C.
- Turn everything off. The oven thermometer will continue to increase, but shouldn't go over 160C. The IR gun could read as high as 315C. If you're soldering a board, wait about 30 seconds, no more than one minute, and slide the board off the hot plate onto the carrier to finish cooling.
Note: The temperature of the surface of the circuit board will be lower than the temperature of the hot plate. Based on when solder flowed, I estimate the temperature of the board to be about 20% lower than the IR temperature readings.
Don't worry if your IR sensor won't read over 220C. You should have enough comparison data between it and your oven thermometer to know if you're tracking my readings. If you are, then use my times and see what the results are. If you're readings don't track mine, you'll need to do some more experiments.
I've included results from the experiments I did. Please note that for the results shown I kept the hot plate on for nine minutes so the temperatures overshot what I would normally use (off after eight minutes). When I turn off at eight minutes, the shape of the ramp is nearly identical to that shown for nine minutes, the peak just stays below 260C (board temperature).
Look first at Temp Profiles. The top line is the IR sensor readings and the bottom line is the oven thermometer readings. Based on when the solder flows, I estimate the board temperature to be about 20% lower than the IR readings. That's the middle line. Again, I'd turn things off at eight minutes if I were actually soldering a board and the maximum IR temperature would not go over 315C.
Now let's compare what I measured to the target from the Intersil apps note. Look at Target Profiles. The upper and lower target profiles from Intersil are shown along with the board temperatures from my experiments. Making the adjustment for 8 minutes versus nine minutes, we see that the profile is pretty close to the target. Not bad!
Two things should be pointed out. First, the cool down rate is kind of arbitrary. It's not clear to me what the impact of a rate considerably slower that Intersil suggests would be. But I probably don't care. What's most important is that letting the board cool briefly on the hot plate, then sliding it onto a carrier where it can finish cooling pretty quickly is probably just fine. Cooling too quickly could be bad, but putting the board on an insulator (like piece of wood or perf board) should be OK. Watch out for static electricity if you do this.
Second, the profile could be tuned more. Using a higher value for the initial On Time (say 24 instead of 20) would raise the initial rate of temperature increase. Since I've gotten satisfactory results, I haven't seen fit to do that. Please experiment to your heart's content.
Once you have a profile that seems like it's close to the target, it's time to experiment by using it to solder. Stencil paste onto another board – still no parts. Use the profile you've determined to flow the solder on this test board and run your profile. Does solder flow when you expect it to (about when the oven thermometer reads 140C – about eight minutes)? If so, you're ready to solder some parts. If not, you need to run another experiment or two.
Now for the real thing. Stencil on the paste and place your parts. (If you're nervous and want to experiment a bit more, leave off any expensive or scarce parts the first time.) Be sure your hot plate has fully cooled, then warm it as discussed above. Put the board with parts onto the plate. Picture 28 shows my board with the paste heated. Picture 29 shows the board when the solder flowed. Run your solder cycle. When all the solder has flowed, turn off the hot plate, let it cool for 30 seconds to a minute (it'll still be mighty hot), and slide your circuit board off onto your carrier so it can finish cooling. How's it look? Great? OK, dance around the house some – you've just performed magic! (I still get a kick out of this process.) Picture 30 shows my final product.
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Aug 13, 2009. 6:34 PMtundrawolf
says:
This is fantastic! I used to work with used test equipment and related equipment and wave soldering always fascinated me. I would like to see more steps and WAY more pictures. It is a lot for me to digest. But it is great nonetheless.
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