Simple PCB Reflow Without Solder Paste

Introduction

This post will cover a simple process for building small quantities of surface mount PCBs with a minimum of expensive, single-purpose equipment. It additionally avoids using solder paste, which is expensive and potentially hazardous if mishandled. The method presented is suitable for single-sided PCBs and most SMD packages. This post is primarily aimed at hobbyists looking to build PCBs at home.

Figure 1. Assembled SMD PCB

Equipment

There are a few pieces of equipment that are necessary for the method presented here:

• Soldering iron - An iron with good temperature control and a fine tip is preferred, but almost any iron can be used. The iron used in this tutorial is a Weller WMRP paired with a WD1M power supply. A variety of chisel style tips are used.

Figure 2. Weller WMRP soldering iron and WD1M power supply

• Solder - The solder used in this tutorial is 0.015” diameter #245 rosin flux core Kester 63/37 (63% Tin, 37%Lead). This solder is constructed as a tube with flux filling its center. The flux helps solder joints to reflow more easily.[1]  Kester #245 flux is a no-clean flux, which means that it will not leave a sticky residue after soldering.[2] The 0.015” diameter is thin enough to give good control. In general, thin solder wire will be the easiest to work with when making small joints. Leaded solder is easier to work with and has a lower melting temperature than lead-free equivalents, but lead-free solder might be required in some applications.

Figure 3. Spool of Kester 63/37 leaded solder

• Flux - Flux is a chemical used to remove oxides from metal surfaces to make solder joints reflow more easily. A Kester #951 flux pen is used in this tutorial. This is a no-clean flux that is compatible with the solder used. Solder manufacturers publish compatibility tables for their different types of flux and solder. Flux pens provide an easy way to buy a small amount of flux, but they do not allow for precise application. Refillable squeeze bottles with fine tips provide more control and can be bought on Ebay cheaply.

Figure 4. Kester 951 solder flux pen

• Hot plate - Several heat sources are commonly used for hobbyist PCB assembly.[3] In this tutorial, a hot plate is used. It is important that the hot plate has temperature control and that it has a solid metal heating surface rather than an exposed coil. These requirements make it possible to deliver a controlled amount of heat uniformly to the PCB. In this tutorial, an Oster hot plate form Target was used.

Figure 5. Hot plate with a solid metal surface and temperature control knob

• Tweezers - A set of fine-tip tweezers is necessary for placing and adjusting small components on PCBs. Tweezers should be made from metal so that they do not melt when used with a soldering iron. Aven 5-SA tweezers are used in this tutorial. These are anti-magnetic stainless steel tweezers with very fine tips. Good tweezers are cheap and they make working with small parts easier, so they are worth buying.

Figure 6. Fine tip steel tweezers

There are several optional pieces of helpful equipment:

• Magnification - SMD packages are often very small, so it is useful to have some kind of magnification device in order to see parts more easily and inspect joints. In this tutorial, a 1x to 3x stereoscopic microscope with 10x eyepieces is used, but a large magnifying glass or video microscope could be used as a cheaper alternative. Stereoscopic microscopes provide depth perception, which is their primary advantage over video microscopes. 30x magnification is sufficient to work with the smallest surface mount components.

Figure 7. Stereoscopic microscope for SMD rework

• Fume extractor - A fume extractor is a fan with a filter attached to it for capturing solder fumes. The fume extractor used in this tutorial is an Aoyue AO486.

Figure 8. Basic fume extractor

• Vise - It is helpful to have a device to hold PCBs in place. The vise used in this tutorial is a Stick Vise.

Figure 9. A Stick Vise can be used to hold PCBs

• Solder wick - Solder wick is used to remove excess solder. It is helpful for correcting mistakes.

Figure 10. Solder wick can be used to remove solder

• Pen knife - A pen knife can be useful for scraping away solder mask or cutting PCB traces if needed. An Aven Technik pen knife is used in this tutorial.

Figure 11. Aven Technik pen knife

Techniques

The process presented here has four parts:

1. Tinning
2. Placement
3. Reflow
4. Inspection and Correction

Tinning

The first stage of the PCB assembly process is to “tin” the boards. Tinning refers to the process of melting solder onto the pads of the PCB. This solder is then melted again to form the final solder joints. This differs from traditional SMD rework, which uses solder paste instead.

The technique varies based on the size of the pads being tinned. For small, isolated pads, place the tip of the soldering iron flat on the pad to heat it to soldering temperature. Feed solder into the pad until a droplet forms and covers it entirely. Remove the iron and solder simultaneously. The pad should be covered with a small, round bubble of solder as in figure 12. If the solder bubble has a point or if there is a large amount of solder on the pad, brush the tip of the iron quickly across the solder bubble to remove the excess solder.

Figure 12. Properly tinned PCB pads

For lines of small pads, such as those found on many IC footprints, it can be faster to build a ball of solder on the iron tip, then drag the ball across the pads. Solder bridges may form between pads as in figure 13. To clear these solder bridges, brush the tip of the iron perpendicular to the line of pads.

Figure 13. A solder blob bridges several contacts. It can be cleared by brushing the iron across the solder bridge.

For very large pads, such as grounding pads on IC footprints, place the iron tip on the pad and feed in a small amount of solder. Spread this solder around the pad uniformly with the tip of the iron as in figure 14. It is important to control the amount of solder added to large ground pads because if there is too much solder, it can lift up the part and cause it to slide or tilt during rework.

Figure 14. Lightly tinned center ground pad to avoid displacing the component

Placement

After all of the pads on the board have been tinned, place a small drop of solder flux onto each one.
This serves two purposes:

1. Clean contacts - Flux removes oxides, allowing the joint to reflow properly.
2. Temporarily adhere parts - Flux is slightly tacky, which means that it can be used to hold parts in place.

Use tweezers to place all components onto their corresponding pads as in figure 15.

Figure 15. Components placed on a tinned PCB

To make placement more efficient, it is often useful to find the required components and put them into an organizer. It is also helpful to print out a drawing of the PCB and label it with part values to reference quickly as in figure 16.

Figure 16. PCB drawing labeled for reference during assembly

Reflow

Industrial solder reflow uses a tightly controlled oven to follow an optimal temperature profile. These temperature profiles are designed to minimize thermal shock and to stay within the temperature limits of components.[4] Profiles are specified as a series of ramps and holds, where the ramps follow set rates of temperature increase or decrease measured in ℃/s. There are four regions in a typical reflow profile:

• Preheat - The board is brought to a temperature that is well below soldering temperature but high enough that solvents in the flux evaporate. This stage prevents the violent evaporation of solvents at higher temperatures. The temperature ramp rate during this phase is typically limited to 3 ℃/s
• Soak - The board is held at a temperature somewhat lower than the melting point of the solder. This allows temperature to equalize between the board and different components.
• Reflow - The temperature is ramped to around 25 ℃ above the solder melting point and held there. A ramp of 2 ℃/s is typical of this phase and the board is usually held at the maximum temperature for around 20 seconds.
• Cool down - The board is cooled at a rate of around 4 ℃/s, allowing the solder to solidify.

Figure 17. Representative PCB reflow temperature profile

In practice, a precise profile is difficult to follow with a manually controlled hot plate, but it is possible to approximate an appropriate profile.

The first step is to determine the appropriate reflow temperature setting for the hot plate. This is done by placing a tinned scrap PCB onto the hot plate and slowly ramping up the temperature until the solder liquifies and becomes shiny. Mark where this temperature setting is and turn the hot plate off.

Allow the hot plate to cool back to room temperature, then place the board onto the hot plate and turn it to a medium-low temperature. This simulates the preheat phase. Allow the board and hot plate to come up to temperature for several minutes to simulate the soak phase. Turn up the temperature to the previously determined reflow setting to simulate the reflow phase. After the solder liquifies, wait 20 seconds and then turn off the hot plate. Use a pair of pliers or tweezers to remove the board from the hot plate to cool. It may be necessary to leave the hot plate on longer after the solder liquifies depending on the solder used.

It is possible to refine the process by using an infrared thermometer to find appropriate temperature settings for the hot plate. To find the reflow set point, turn on the hot plate and adjust it so that its temperature is stable at 25 ℃ above the solder’s melting temperature. For leaded solder, an appropriate temperature might be 225 ℃. To find the set point for the simulated soak phase, repeat this procedure with a setpoint of 160 ℃. Test the upper setpoint using a scrap PCB to verify that the solder melts at the temperature it is expected to melt at and adjust the temperature setpoint as needed.

Inspection and Correction

After the board cools to room temperature, inspect it for parts that are not properly soldered. The most common problems are parts turning or sliding.

If any parts are incorrectly soldered, it is necessary to reflow either all or part of the board to correct them depending on the problem.

For small parts where only one joint has been formed, it is possible to use a fine tip soldering iron to manually re-solder.

Figure 18. A SMD resistor that turned during rework

Hold the part lightly with tweezers and touch the tip of the soldering iron to the connected solder joint.

Figure 19. Desoldering an incorrectly reflowed resistor

The part will come free.

Figure 20. Removed SMD resistor

Move it to the correct location and hold it with tweezers while melting the solder ball on one of the pads to form a joint.

Figure 21. Re-soldering a SMD resistor by hand

Repeat this for the other joint.

Figure 22. Corrected SMD resistor

If a component has multiple incorrectly formed joints, it is necessary to heat it and the surrounding area of the board to soldering temperature so that it can be moved. This can be achieved either by following the hot plate reflow process to bring the entire board up to temperature or by using a hot air reflow gun to heat only the desired area. Once the component reaches soldering temperature, use a pair of tweezers to push it into place. Allow the board to cool and re-inspect the joint. If the entire board was heated to soldering temperature, re-inspect the entire board. If the component is small, it may be easier to remove it, place it again under a microscope, and then reflow it again.

Once the board looks visually correct, run electrical tests to verify that it works and make any further corrections that might be necessary.

Designing for Assembly

When designing PCBs to be assembled with this method, there are a few concepts to keep in mind to ease assembly.

Avoid BGA and QFN packages. These packages have pads that are entirely covered. This makes assembly more difficult because differences in solder ball size can prevent connections to some pins. Additionally, it is more difficult to inspect solder joints on BGA and QFN components because all joints are covered.

If a BGA package must be used, make sure that there are no straight-line connections between the center pad and any of the outside pins. The large center pad will wick all of the solder to its center, which makes it almost impossible to solder any pins connected to it without solder mask to block the solder flow. As an alternative, connect the outside pin to a via, route a trace on the bottom of the board, and connect it to another via in the middle of the center pad.

When designing with small passive components, make an effort to keep package sizes at 0402 or larger. Smaller packages become increasingly difficult to work with, especially when placed without a microscope.

Place components far enough apart that they can be accessed with a soldering iron to simplify any corrections that might need to be made after reflow.

References

[1] https://www.circuitspecialists.com/blog/choosing-soldering-flux/
[2] https://www.kester.com/products/product/245-flux-cored-wire/
[3] https://www.sparkfun.com/tutorials/category/2
[4] https://www.compuphase.com/electronics/reflowsolderprofiles.htm

Impulse Response

I saw the Robojackets 20th Anniversary Competition listed on one of the various competition listing sites and impulsively signed up for it. So began my latest beetleweight combat robot, Impulse Response. My initial plan for this competition was to resurrect Event Horizon with a new design that more heavily leveraged 3D printing, since I acquired a used Prusa i3 MK3 3D printer over winter break. After messing around with that design for a while and even buying some parts for it, I decided that I wanted something simpler and easier to maintain. This led to the conclusion that I should make a 3D printed undercutter.

Design

The vast majority of the design process for Impulse Response took place over the course of two days in late January. The design goals were simple:

• Minimize the number of machined parts
• Maximize reliability/ maintainability

Machined Parts:

The robot has only three machined parts; the weapon blade, one of the inserts for locating the weapon shaft, and a spacer that goes inside of the weapon pulley.

Reliability/ Maintainability

The weapon shaft is supported by two large bushings. This distributes the load from the weapon across a larger area of the frame. The bottom bushing is aluminum because it sees most of the load. The top is 3D printed. The weapon shaft includes a spacer that contacts the inner races of both weapon bearings and allows me to tighten down the shaft without side-loading the bearings.

The weapon motor drives the weapon with a belt, isolating it from shock loads.

The drive base uses brushed motors because they are easy to work with and are unlikely to have motor controller issues.

Foam wheels isolate the drive motor gearboxes from any hits the wheels might take.

The extensive use of 3D printing made the manufacturing of spare parts comparatively simple.

The frame is significantly over-built, allowing it to take direct damage without significant structural implications.

Impulse Response after two design days

I tweaked some minor details after this, but on the whole, things remained the same.

Manufacturing

 Manufacturing Impulse Response was exceptionally easy because almost everything was 3D printed.

I first printed a PLA mockup of the body to make sure everything fit.

PLA test print of main body

 Next I printed the body and some other parts in NylonG and waterjet cut the blade at the Invention Studio. Printing NylonG is interesting. I needed to add glue stick to the PEI printbed to get it to stick and I put the filament inside of a dehydrator beforehand to drive out all of the moisture. I observed some shrinkage in the Z direction with the NylonG, but it was not enough to cause an issue.

NylonG print of main body with quick fit of components

The electrical system was slightly unusual. I prefer driving with a pistol-grip transmitter, but the cheap Hobbyking transmitter does not mix channels for tank drive. To achieve this, I added an Arduino, which did the mixing and also controlled the weapon based on the channel three button on the transmitter. It took killing an Arduino for me to realize that the voltage regulators were not what I thought they were.

Code may be found here: https://github.com/echin98/robotEngineArduino

Rough electrical system fit

 After everything was together, I weighed the bot. It was substantially under weight. In hindsight, I should have waited to machine the weapon until after everything else was done and then adjusted the size to fill up remaining weight.

The robot is very under weight

 Testing

I took advantage of being home for Spring break to test the robot. The weapon immediately proved its power. Its first victim was the old frame from Event Horizon v1.

The corner of my test piece (the frame of EH v1) is machined off by the blade

In the course of testing, my weapon motor became hot enough to melt its PLA mounting block. I am still not entirely sure why it got so hot, though I suspect I just need to add some vent holes somewhere.

Whoops...

The weapon motor became excessively hot and melted its mounting block.

I replaced the PLA block with an aluminum plate. I also replaced the bottom retaining block for the weapon shaft with aluminum after cracking the original plastic piece.

Deeming it difficult to do more extensive testing, I decided Impulse Response was ready to go.

Impulse Response in its completed form

 Competition

Match 1: Loss vs Hypnotic

Hypnotic is a well-built drum spinner by a group of high-schoolers from Alabama. This was the first match ever for both robots, and I was eager to see how Impulse Response would fair. The match got off to a good start and I managed to bend Hypnotic's front floor skids. Shortly thereafter, my weapon embedded itself into the wood bumper in the arena. I was unable to get it unstuck and lost by KO.

Match 2: Win vs Large Hard

Large Hard is a drum spinner from the Rose Hulman robotics team. I was not entirely sure what to expect in this match as Big Hard had been partially disassembled by Hypnotic in its previous match. Unfortunately, my weapon failed to start up at the start of the match and it pretty much devolved into a pushing match. Big Hard lost its drive base partway through the fight and I managed to herd it into a corner where it couldn't use its weapon to get out.
By sheer luck, I won by knockout.

Watch RoboJackets 20th Anniversary - 3lb Combat Bots from RoboJackets on www.twitch.tv

I diagnosed the weapon problem to be a bad motor, though I was not entirely certain. I replaced the motor and added a ramp up to the firmware, which seemed to fix the issue. I also decided that I had limited the acceleration of the drive too severely and I increased that accordingly.

Match 3: Win vs Entropi

This was a quarter finals match.
Entropi is a beater bar style drum spinner from Robojackets. I was quite concerned about this fight because Entropi's beater bar was large and therefore high energy. I also was not 100% confident in my new weapon motor.

The fight started with us trading hits back and forth. I eventually landed a lucky hit on the inside of his weapon frame, causing his beater bar to eject.

Watch RoboJackets 20th Anniversary - 3lb Combat Bots from RoboJackets on www.twitch.tv

I took minimal damage.

Match 4: Loss vs Dynastinai

This was a semi-finals match.

Dynastinai is an undercutter with an extremely large direct drive motor. It was built by another Georgia Tech student and was formerly associated with Robojackets. I planned to try to take out his wheels and possibly damage his weapon motor since I had a reach advantage.

On the second hit Dynastinai flipped Impulse Response over. This proved catastrophic. The motor I used to replace the original weapon motor had a long shaft, which stuck out the top of the robot. This shaft contacted the ground before the intended contact point when the robot was flipped over and because the shaft was behind the center of mass, this left me unable to drive. Dynastinai cut into my top panel and stripped the battery wires, shorting them together. I lost by knockout and the battery went immediately into the sand bucket.

Watch RoboJackets 20th Anniversary - 3lb Combat Bots from RoboJackets on www.twitch.tv

Dynastinai's weapon is made from AR500 steel and this was what it did to my blade, which is made from AR400.

Match 5: Win vs Hypnotic 

This was the third place match.

Hypnotic and Impulse Response both sustained fairly heavy damage in our semifinals matches and we initially agreed to just run without weapons since those weren't working. We both managed to get our weapons working in time, though, so we used them.

I started picking away at Hypnotic's tires and eventually took off enough that it could no longer drive. Watch RoboJackets 20th Anniversary - 3lb Combat Bots from RoboJackets on www.twitch.tv

Impulse Response took third place behind Dynastinai and Mad. 

Post-Mortem

Overall, I am very pleased with the results of this competition.

Positives

• The frame proved to be quite effective even after it had taken damage
• All maintenance tasks were quick and easy to do
• The robot could deal out big hits
• The weapon system could take big hits without taking significant damage
• The drive base was reliable

Negatives

• Weapon electrical/motor system failed during the second match without obvious cause.
• Weapon motor durability is not up to par; I had to replace the weapon motor twice
• Weapon spin up time was too slow
• PETG is not an appropriate combat robot material. I broke all of it.

The positives here are significant and the negatives are (I think) easier fixes. I will likely upgrade the weapon motor and ESC and also change the PETG parts to some other material before the next competition, whenever that might be. I may also upgrade the AR400 blade to AR500.  I feel no need to do a major redesign at this point.

Microbat V2 - the Ultralight Backpacking Battery Pack

In the last two years or so, all of my backpacking electronics have become USB-charged. With this in mind, I wanted an extremely lightweight USB battery pack to replace this monstrosity:

Additionally, I thought it would be convenient to have a pack that allowed its cells to be changed out so that I could carry only as many cells as I expected to need. It turns out that there are not many commercial products that let you do that, so I decided to make one as a PCB design exercise.

Version 1

I threw together a quick design based on the TI BQ24165 battery charge management IC and the TI TPS61026DRCT 5V boost converter.

Note: There are some errors in this schematic.

I then did the layout of the board:

And sent it out to my preferred Chinese board house (JLCPCB at the moment).

I accidentally made the wire-mounting holes a bit smaller than I intended, but other than that, the boards looked good. I assembled one to test:

I connected this power input side of this board to a power supply set at 5V. This should have caused LED2, which is connected to the "Power Good" pin on the BQ24165, to light up, but it did not. I re-checked all of the solder connections and everything looked fine, so I went back to check the schematic against the datasheet for the BQ24165.

Whoops... Some little blue wires fix the problem and the board mostly behaves as I expect it to.

It does require a slightly higher voltage than I expect to decide that the charging supply is "good", which I found concerning, but it properly supplies 5V when a 3.7V supply is connected across the battery terminals. At this point the summer was coming to an end, so I had to go back to school. I fixed the schematic in Eagle in preparation for the next revision and paused development. This was the end of Version 1.

Version 2

I picked up the project again in October and decided that the rectangular board was annoying to incorporate into a compact package. This led me to re-design the PCB with a circular form factor.

Other than correcting the previously discovered errors, the design did not change and the PCB layout was fairly quick.

I chose to make the positive battery terminal a large circular via in the middle of the board with the thought that the positive contact could just be a screw that went through the hole. The battery ground contact was a big rectangular pad to allow a variety of mounting options. Additionally, I put all components on the top of the board to allow it to be mounted flat in some kind of case.

I ordered this board and it showed up eventually after being stuck in customs for an unusually long time.

The astute reader will note that I never actually figured out why the supply pin only "kind of" worked. After assembling one of these boards, I realized that I forgot to connect the PGND pins to ground, which left the voltage divider used to sense the supply voltage poorly referenced because it was connected to the PGND pins. I'm not entirely sure why I connected the schematic that way in the first place, but it was an easy fix and now everything actually works.

That's it for the board, so let's take a look at the case. My first idea was to mount the board in the cap of a twist-lock tube and use EMF gasket material to electrically connect the cap to a ring of copper tape on the main body of the tube. This had the benefit that I could potentially make it waterproof with some O-rings and clever glue application.

I 3D printed this case design when I was home for Thanksgiving.

My EMF gasket idea did not work at all. After I twisted the cap on, it was impossible to remove because the EMF gasket shredded itself by rubbing against the plastic of the tube.

Since the twist-on cap obviously wasn't going to work, I switched to a much simpler design where the board was held in a groove in a U-shaped piece:

This design uses the big via on the board that I originally panned to put a screw through as the positive contact for the battery. This isn't really ideal, but it works well enough for my purposes.

I added a cover so that the big inductor wouldn't decide to run away when inadvertently subjected to shear loads. I also added a little notch that is a tight fit on a USB cable as strain relief and some notches to make the battery easier to remove.

I'm mostly happy with the result. I'll probably build a few more for friends and maybe sell them in small quantities if I get enough interest.