Reflowing a solder joint means reheating it until the solder melts, allowing it to reform a clean electrical connection. It’s one of the most common fixes for cracked or cold solder joints on circuit boards, and with the right approach, it takes only seconds per joint. The key is controlling your heat, using flux, and knowing when to stop.
Why Reflowing Works
Solder joints fail for predictable reasons: thermal cycling cracks them over time, oxidation builds up on the metal surfaces, or the joint was poorly formed in the first place (a “cold joint” with a dull, grainy appearance). Reflowing fixes these problems by melting the solder back to a liquid state so it can re-wet the copper pad and component lead, then solidifying into a fresh, reliable connection.
Flux is what makes this possible. When you heat a joint, flux chemically strips away the oxide layer on the metal surfaces, letting molten solder bond directly to clean copper. Without flux, you’re just melting solder on top of oxidized metal, which produces the same weak joint you started with. Common flux activators begin breaking down oxides at temperatures around 130 to 150°C, well below solder’s melting point, so the surfaces are clean by the time the solder actually flows.
What You Need
Your tool choice depends on the type of joint you’re reflowing.
- Soldering iron: Best for through-hole joints and visible surface-mount connections with accessible leads. A chisel tip transfers heat faster than a conical tip. Temperature-controlled irons let you dial in a specific setting rather than guessing.
- Hot air rework station: Essential for reflowing ball grid array (BGA) chips and other components where you can’t reach the joints with an iron tip. Look for a station with adjustable temperature up to 400°C and variable airflow control.
- Flux: Gel flux is generally the better choice for rework. It stays near the joint rather than wicking under nearby components the way liquid flux tends to. Liquid flux was originally formulated for wave soldering, where capillary action is desirable. On a rework bench, you want the opposite: controlled placement that doesn’t migrate to areas you can’t clean afterward.
- Solder (optional): If the existing joint has enough solder, you may not need to add more. But having thin solder wire on hand lets you add a small amount if the joint looks starved.
Reflowing With a Soldering Iron
Set your iron to around 340 to 370°C for standard lead-free solder, or 300 to 330°C if you’re working with leaded solder. Apply a small amount of gel flux to the joint before you touch it with the iron. The flux needs to be in place before the solder melts so it can do its job cleaning the surfaces.
Place the iron tip so it contacts both the pad and the component lead simultaneously. You want heat flowing into both surfaces, not just one. The solder should melt and visibly flow within two to three seconds. Once you see it turn shiny and liquid, pull the iron away cleanly. A good reflowed joint has a smooth, slightly concave surface with a bright or satin sheen.
If the solder hasn’t melted after four or five seconds, pull the iron away and let the joint cool before trying again. Holding heat on a joint for too long weakens the bond between the copper pad and the board’s substrate. On single-layer boards especially, excessive heat reduces pad adhesion and can lift the pad right off the board. Excessive mechanical force while the pad is hot makes this even more likely, so avoid pressing down hard or prying at components while they’re heated.
Reflowing With Hot Air
Hot air is the standard method for BGA packages and large surface-mount components with solder joints hidden underneath. The process follows a temperature profile with distinct stages, and rushing through them is the fastest way to damage a board.
Preheat
If your station has a bottom-side preheater, set it to 100 to 150°C and let the board warm for one to two minutes. This reduces thermal shock by bringing the entire assembly up to a baseline temperature gradually. The preheat ramp should stay below 2°C per second. Heating too fast causes solder paste to spatter and can create bridges between adjacent pads.
Soak
Set your hot air nozzle to 180 to 200°C and hold it over the target area for 30 to 60 seconds. This stage brings the component and surrounding board to a uniform temperature. Uneven heating is a major cause of warped boards and cracked joints, so take this step seriously even if you’re impatient. Move the nozzle in slow, small circles rather than holding it perfectly still, which creates hot spots.
Reflow
Increase the air temperature to 220 to 250°C. At this point, the solder melts and reforms. You’ll typically need to hold this temperature for 30 to 45 seconds. Keep the nozzle moving. Do not exceed 260°C, as many components are rated for a maximum exposure at or just above that threshold. For the ramp from soak to peak temperature, stay below 3°C per second to avoid thermal shock.
Cool Down
Remove the hot air and let the board cool. Faster cooling produces a finer grain structure in the solder, which generally means a stronger joint. The practical limit is a cooling rate below 6°C per second to avoid stressing the board. Don’t blow cold air directly on the joint or drop the board onto a cold surface. Just letting it sit in room-temperature air is usually sufficient for hand rework.
Common Mistakes That Damage Boards
The most frequent error is applying too much heat for too long. Extended time at high temperature doesn’t just risk lifting pads. It also degrades solder joint fatigue life. Research on lead-free BGA solder balls has shown that fatigue life drops significantly as high-temperature dwell time increases from 15 minutes to 30 and then to 90 minutes. For hand rework, you’re nowhere near those durations, but the principle holds: get in and get out. Every extra second at reflow temperature is accumulated damage with no benefit.
Skipping flux is the second most common mistake. People assume that because solder is already present on the joint, flux isn’t needed. But the whole reason you’re reflowing is that the joint has degraded, and oxidation is almost always part of that degradation. Fresh flux is non-negotiable for a reliable reflow.
Using too much airflow with a hot air station is another pitfall. High air velocity can push small passive components off their pads, create solder bridges, or blow solder balls onto nearby traces. Start with lower airflow and increase only if you’re not reaching temperature. On most rework stations, moderate airflow at a slightly higher temperature setting gives better results than maximum airflow at a lower setting.
How to Tell If the Reflow Worked
A properly reflowed joint looks smooth and slightly shiny for leaded solder, or smooth with a satin finish for lead-free solder. The solder should form a concave fillet between the pad and the component lead. If you see a dull, grainy, or blobby surface, the joint didn’t reach full reflow temperature or lacked adequate flux.
For BGA and other hidden joints, visual inspection isn’t possible. If you have access to a multimeter, check continuity on the affected circuit. In professional settings, X-ray inspection is used to verify BGA reflow quality. For hobbyists, the practical test is whether the device works correctly after the rework. If symptoms persist, the reflow may not have reached all affected joints, or the problem lies elsewhere on the board.
If you’re reflowing multiple joints on the same board, work in one area at a time and let each section cool before moving to the next. Heating the entire board repeatedly accelerates warping and intermetallic growth at joints you’ve already fixed. A systematic approach, starting from one end and working across, gives the best results with the least thermal stress.