Microsoldering Mistakes: Handbook for Fixes and Accident Prevention

Microsoldering has applications across many fields and is an integral skill for electrical and computer engineers. This handbook is not intended to teach a beginner how to solder. Instead, it’s designed to address some common issues that an intermediate to advanced solderer will encounter. If you are new to soldering and would like an in-depth guide to learning how to solder, we recommend the following tutorial: https://www.instructables.com/How-to-solder/.

There are many different components and tools used in the world of soldering. This section of the handbook will provide you with some baseline knowledge that will be helpful when navigating the rest of this handbook.

The Essentials: Soldering iron, different iron tips, rework station (heat gun), tweezers, solder, flux, sponge, brass coil, solder wick, copper tape, enameled wire, replacement components

Many terms exist to explain different concepts in soldering. These can often be confusing and it’s important to distinguish these terms before using them. This section will acquaint you with some of the terminology used in the handbook.

Soldering can be dangerous in many ways. When operating the iron, it can often get up to 600ºC. Burns are a serious concern and should be avoided by using proper safety etiquette. Further, the fumes that come from soldering are dangerous and should not be inhaled for long periods of time.

Rules of Solder Safety:

1. Always put the soldering iron down onto a stand or holder when it is not in use.
2. Do not ever touch the metal of the iron or rework station with a bare hand, unless the device has been powered off for at least 15 to 20 minutes.
3. Ensure there is proper ventilation while soldering. This can be achieved by soldering under a fume extractor or fume hood.
4. Ensure the cord of the soldering iron is not wrapped around your hand or arm as you solder.
5. Use a vice or helping hands to secure the board before soldering.
6. Try to have a designated hand for holding the iron and use the other one for handling flux, solder, wick, and positioning.

Identification:

Bridging happens when two different leads of a component(s) are accidentally soldered together. Bridging will often appear as a blob of solder that connects two pads/pins of the same component or another component. Even if there is only a little bit of solder bridging two components it is still best practice to remove all of the excess solder to prevent a short of any kind.

Fixes:

There are two good choices for correcting bridged components. The first is using solder wick, which can take longer, but provides an easy and consistent bridge solution. The second is using a bevel tip to pull excess solder away from the pins, which can often be less consistent, but doesn’t require the use of any additional materials, and can be significantly faster.

1. Solder Wick:
2. Apply flux to the bridged connection
3. Place wick over bridged area
4. Press hot soldering iron tip onto the top of the wick
5. Wait until wick absorbs the reflowed solder
6. Lift the iron and wick away from the solder together
7. Repeat until all bridging solder is removed
8. Bevel Tip:
9. Ensure soldering iron tip is beveled and if not swap to a beveled tip
10. Clean soldering iron tip with sponge and brass wire
11. Add flux to the bridged connection
12. Drag the bevel tip across the leg/component that is bridged and the other component that it is bridged to
13. The excess flux should be pulled by the soldering iron away from the two components
14. Repeat, dragging the solder back and forth along the legs, until all excess solder is removed

Identification:

Tombstoning happens when one side of a component is not soldered correctly and the component stands up vertically on the side that is soldered correctly. When a component is tombstoned, current cannot properly flow through it and the system will act as if the component is not present at all. Luckily this issue is one of the easiest to fix.

Fixes:

There are two main ways to fix a tombstoned component (solution 1 is best practice):

1. Completely remove component:
2. Identify tombstoned component by its vertical “tombstoned” orientation on the board (this may take closer examination that expected)
3. Apply flux to the pad/board that the component was properly soldered
4. Place the soldering iron tip onto the end of the component that was correctly soldered
5. Using your other hand, remove the component using tweezers
6. Apply flux to any solder left behind and remove with a solder wick
7. Once excess solder has been removed, resolder the component with new solder
8. Reflow and correct placement:
9. Identify tombstoned component by its vertical “tombstoned” orientation on the board (this may take closer examination that expected)
10. Apply flux to the pad/board where the component was properly soldered
11. Place the soldering iron tip on the correctly soldered side
12. Using tweezers, press down on the side of the component that incorrectly soldered until it is resting as parallel (flat) to the board as possible
13. Apply flux to the pad that was not correctly soldered
14. Place the soldering iron tip onto the incorrectly soldered pad until the solder present is reflowed
15. Using tweezers, press the component the rest of the way down into the reflowed solder and hold until solder has cooled
16. If necessary, add extra solder to the previously bad joint

Identification:

Sometimes a capacitor, resistor, ferrite bead, etc. is broken and needs to be replaced. While a bad component like this is nearly impossible to catch by purely looking at the hardware, it can usually be diagnosed by running some tests on the final system. These tests can be conducted using a voltmeter, diagnostic software, or an oscilloscope. Almost all SMD passive components have two pads which makes utilizing two soldering irons (or a pair of soldering tweezers) the easiest way to remove them.

Fix:

1. Prepare two soldering irons and have one easily accessible for each hand (alternatively use a tweezer soldering iron)
2. Apply flux to both sides of the component
3. Place a soldering iron tip on each end of the component until solder on both ends is reflowed
4. Utilize the two irons as a makeshift pair of tweezers by applying pressure to each end of the component and then lift the component off the pads (be careful to not interfere with other components during this process)
5. Let the solder cool and then remove component off the board completely with a pair of tweezers
6. Apply flux to excess solder left on each pad and remove with solder wick
7. Replace old component with a functioning component and solder

Identification:

Similar to passives with two pads, an integrated circuit (IC) is hard to immediately diagnose as broken without running some tests. It is recommended that you discern the IC’s function prior to running diagnostic tests so that the correct tests can be employed in an efficient manner. Test equipment found in most soldering labs will allow for IC testing and diagnostics. IC’s often have multiple pads, which can make it difficult to remove them with a soldering iron. In this case, a heat gun (or rework station) comes in handy.

Fixes:

1. Place copper tape around components nearby the malfunctioning IC (this copper tape should ideally not touch the components, but rather act as a shield for hot air)
2. Apply a generous amount of flux to the legs of the IC (if legs are unexposed, try to get as much flux on surrounding legs as possible)
3. Set temperature of the heat gun to about 280-300 ºC for leaded solder, or 340-370 ºC for unleaded solder
4. Set airflow to around 50% (this may require some fine tuning to fully melt solder)
5. Move the heat gun around the IC in a circular motion and try to heat each leg (or pad if the legs are unexposed) as evenly as possible
6. The IC should start to move a little after hot air has been applied for a some time, this is your sign to retrieve a pair of tweezers and remove the component
7. Apply flux to the excess solder left on each pad and remove with a solder wick
8. Once the excess solder is removed, fetch a new IC and resolder onto the pads

Identification:

When mechanical stress has been applied to an IC, it can cause the pins to wear or break off. It’s easy to diagnose this problem, as the pins are recognizably destroyed.This may be a missing leg, or a cluster of legs that have broken off.

Fixes:

Depending on the materials you have on hand, there are three main fixes, using either enameled wire, solid core wire, or a donor pin from another IC or salvaged from the existing IC.

1. Cleanup
2. Apply flux to the area of the break. A large amount will help.
3. Remove any pins that might cause shorts or overlap with others. Try to leave as much metal as you can on the actual chip, but if a leg is clearly “hanging on by a thread”, remove it.
4. Enameled Wire Fix
5. If using enameled wire (easiest, but provides a weaker connection, see solid core ). First, put a bit of solder on the exposed metal of the broken off pin.
6. Flow the solder, and put one end of the enameled wire into the ball, and hold it for a few seconds to melt the insulation, then remove the iron.
7. Repeat the previous step on the pad of the PCB where the pin originally sat.
8. Solid Core Wire Fix
9. If using standard solid core wire (ideally around the same thickness as the leg), it helps to solder the end of the wire on the pad as shown in Photo 2.
10. Cut the wire to about the same length as the other pins, and touch it to the metal of the IC. Solder this end to the exposed metal.
11. Pin Graft Fix
12. Much like the solid core wire fix, first solder the pin to the pad of the PCB. Try your best to line it up like the other pins.
13. Next, solder the other end of the pin to the IC. Don’t worry about cutting down the pin if it’s about the same size as the others.

Identification:

While unlikely to occur in the makerspace, a significant and abrupt change in temperature (often >15 ºC per minute) can cause (sometimes microscopic) cracking and make boards unusable. Thermal shock is difficult to fix after it’s already occurred, as the board is likely ruined, and replacement is the only option. It’s hard to identify thermal shock, and the only way to deal with it is to avoid it.

Preventative measures:

1. Always preheat the board with a heat gun to a middling temperature of around 100-125ºC before applying significant heat during reworks.
2. Use plenty of flux. Flux ensures the heat of the soldering iron is focused entirely on the metal by removing invisible oxidation.
3. Gradually press the soldering iron when heating a component to ensure it is not heated too quickly.
4. Use a heat sink to dissipate heat if soldering temperature-sensitive components.