Soldering an ESP32 with a hot air gun

The ESP32-C6 microcontroller comes in a QFN32 package whose central ground pad cannot be soldered from the top with a standard iron. Instead, we describe how to use a hot air gun to solder this device to a crown QFP32 0.5mm adapter. We use a two stage soldering process to prevent the small IC package from been blown away while being heated by the air gun.

Tin the pads and contacts

We first prepare the adapter board by tinning all the pads of the IC footprint, then wicking off the solder from all but the central ground pad. Although we wick off most of the solder, this process still leaves a thin layer on the surface of each pad. This thin layer of solder will make it easier to bond the IC contacts to the pads, compared with a bare gold surface.

It is important leave only a small amount of solder on the central ground pad, otherwise the IC will sit too high, and the contacts along the perimeter will not connect to their corresponding pads on the board. To control the amount of solder on the central ground pad we first tin the pad, so that the applied solder forms a convex blob, then wick part of it off from each of the corners. The result is shown below. We also clean the flux residue off after each soldering step, so the pictures in this post are all residue-free.

We now prepare the IC by first tinning all its contacts, then wicking off the solder from only the central ground contact. This is the opposite arrangement of solder that we added to the adapter board above. For the IC we retain solder only on the perimeter contacts as these contacts are smaller than the corresponding pads on the adapter board. For the adapter board we retain the solder on only the central ground pad as the pad is smaller than the corresponding ground contact on the IC.

To tin the contacts on the IC we first apply extra flux with a flux pen, add solder to the tip of the iron, then drag then iron across the contacts. This process leaves a small raised ball of solder on each of the contacts. Notice that the perimeter contacts wrap around the edge of the IC, so for each contact there is a blob of solder on the bottom as well as the sidewall.

First reflow secures the IC

Next, we hold the adapter board in a vice, add more flux to the tinned pads, then align the IC with the pads. To align the IC we take a piece of 5mm Kapton tape, place the IC on the middle of the tape, hold the tape with both hands, then set the alignment by moving the tape. Using Kapton tape like this is easier than trying to align the IC with tweezers or similar. Taping the IC to the vice also prevents the IC from blowing away when we use the hot air gun to reflow the solder. Once the IC is aligned we apply an extra piece of tape around the outside of the vice to hold everything in place.

The following picture shows that the tape securing the IC should make contact with the vice along the vice's outside edge, rather than on its top surface. Ensuring that the tape contact line is parallel to the edge of the IC helps keep the IC aligned on the adapter board.

As a negative example, consider what would happen if we stuck the tape directly to the top surface of the vice. When we heat the area the tape is likely to pull up, and the contact line will no longer be parallel to the surface of the board, causing the IC to become misaligned:

The following picture shows what happens when the tape pulls up. Notice how the contacts on the right of the IC are raised relative to the surface of the board:

We adjust the tape position to keep the IC flat against the board. At this stage the contacts need to be reasonably well aligned with their corresponding pads, but the horizontal alignment does not need to be perfect. We will adjust the alignment in the second reflow stage later. For now, the following situation is good enough:

We now heat up the IC and surrounding area with the heat gun. We use leaded solder and have the heat gun set to 320C at 30% air flow rate. For lead free solder we would use 340-350C and a similar flow rate. This is the temperature of the heating element in the heat gun, for this particular heat gun. The final temperature of the IC and board will be lower.

We swirl the heat gun around to heat up the area evenly. We test when the solder on the central ground pad melts by gently touching the top of the IC with a pair of tweezers. When the solder melts the IC noticeably drops down into the melt, and we can feel it wiggling around under the tape.

After allowing the board to cool we remove the tape and check alignment of the contacts. The purpose of the first reflow stage was to attach the central ground contact to its corresponding pad. Provided the contacts around the perimeter of the IC do not bridge multiple pads, then the alignment shown below is good enough. Some excess solder has been squeezed out on the right hand side of the IC, but we will fix that later.

Second reflow aligns the contacts

Now that the ground contact is attached and the tape is removed, we apply more flux around the perimeter of the IC and heat the area again. This time, when the solder melts we gently nudge the IC with tweezers to fix the alignment. The surface tension of the solder on the ground pad prevents the IC from blowing away under the heat gun.

We make sure to check the alignment on all four sides. Here is the alignment on the left hand side:

Here is the alignment on the bottom:

In the picture above, notice that while the contacts are aligned with the pads, there appears to be a small gap between the contacts on the right and their corresponding pads. It may be that these contacts are properly connected to their corresponding pads under the IC body. However, as we are doing this by hand rather than with an optimised manufacturing process, we will also add a fillet of solder to the sidewalls to be sure of a good connection.

The crown adapter board is set up for hand soldering and has long pads to allow extra solder fillets to be added for the sidewalls. If we were using a standard size footprint with short pads then it would be difficult to get the iron in to add these fillets.

Solder the sidewalls

We use a soldering iron with a chisel tip, add more flux around the perimeter of the IC, add a small amount of solder to the tip of the iron, then drag the iron from the edge of the IC to the outer edge of the pads. Using enough flux should prevent the solder from bridging multiple pads, but if a bridge is created it is easy to wick this solder off and re-do one side of the IC. We are left with a nice fillet of solder from the sidewall of each contact to its corresponding pad:

Check continuity

For the last step we use a multimeter to check continuity, and to ensure that no contacts have been accidentally shorted to the central ground pad under the body of the IC package. We can ensure that each of the perimeter contacts are positively connected to their corresponding pads just by visual inspection.

Accidental shorts are more likely if there was too much solder on the central ground pad at the start. In this case the excess may have squeezed out to touch one of the perimeter pads. The second solder reflow stage helps to correct this issue. When we wick away excess solder and re-heat the area, the surface tension of the solder will tend to pull it back to the area over the individual pads, rather than bridging multiple pads. 

Equipment used

I used the following equipment for this post. This is just what I used.

1. Atten ST-8902D 2 in 1 Hot Air Rework Station with a right angle nozzle. If you get the same station then check that your nozzle has the correct inner diameter. There are several versions of this rework station including an "Update Version" that has a different nozzle size.
   
2. Atten T990-D16 chisel tip for the soldering iron, with tip dimensions 1.6x0.5mm.
3. Relife RL-601I mini rotary fixture. This vice has a magnetic swivel base and works well with the prototype boards.
4. Titanium tweezers with a curved tip.
5. Flux-cored solder wire, 0.5mm diameter.
6. Goot CP-20B desoldering wick, 2mm width.
7. Circuitworks CW8100 flux pen. You still need to clean the flux off afterwards, even though it says "No Clean". The "No Clean" part only applies if you put the whole board through a reflow oven and all the applied flux is heated correctly. For hand soldering you should clean off the ugly sticky residue anyway.
8. Chemtools Kleanium Flux Remover. This is a mixture of isopropyl alcohol and hexane. The addition of hexane does a better job than plain isopropyl alcohol.
9. Kapton tape, 5mm width.
10. Thin cotton tips / q-tips for cleaning.

I did the soldering under an AmScope SM-4 stereo microscope, mainly for eye protection. The alignment process could be done with good lighting and a magnifying glass.

Close-up photos were taken with an Olympus TG-6 camera in focus bracketing mode.