What is the reflow profile for Sn63Pb37 solder in a hot air reflow oven?

If you work in electronics manufacturing, you’ve definitely heard of Sn63Pb37 solder—aka eutectic tin-lead, the old reliable that’s still the go-to for so many hot air reflow oven jobs, even with all the lead-free alternatives out there. As someone who sells hot air reflow ovens every single day, I get asked this question nonstop: What’s the right reflow profile for Sn63Pb37 in a hot air reflow oven? A lot of guys I talk to either use a generic profile off a old PCB design or tweak it so much they end up with cold joints, tombstoning, or even burnt components. Let’s break this down like we’re geeking out over a hot oven after a long day on the SMT line—no fancy jargon, just real, actionable stuff that’s worked for hundreds of our customers.

First, let’s get one thing straight: Sn63Pb37 is eutectic, meaning it melts at a single exact temp—356°F (180°C). No slushy semi-molten phase here, which is why it’s been the workhorse for decades. That single melting point is non-negotiable for the profile, because if you don’t heat the board just enough to hit that temp, your solder won’t wet properly, and if you heat it too fast, you’ll damage tiny surface-mount parts (like 0201 resistors or QFP chips) that can’t handle sudden temperature spikes.

Now, the hot air reflow oven is the star here—different oven types (like our six-zone hot air reflow oven from the range at /reflow-soldering/6-zone-reflow-soldering/hw-r612-six-zone-reflow-soldering.html) will change how your profile works. For example, a convection oven with precise zone control (that’s ours) is way easier to nail this profile than an old, outdated reflow setup with uneven heating. I always tell new customers that the oven’s hot air uniformity is half the battle—if one zone is 20°F hotter than another, your profile will be garbage no matter how perfect the numbers are.

Let’s walk through the profile step by step, the way I explain it to our service techs when they train a new manufacturing line operator. Every profile has four main phases, and Sn63Pb37 isn’t messy about these like some lead-free solders are.

1. Preheat Phase (2 to 3 minutes total)
   This is the part a lot of people skip or rush, and that’s where mistakes start. The goal here is to heat the entire PCB and components gradually from room temp up to around 150°C (302°F) to 170°C (338°F) over 2 to 3 minutes. Why? First, you activate the solder flux—if you dump heat on cold parts, the flux will burn off before it can clean the pad and solder ball, leading to poor wetting. Second, you prevent thermal shock to components. I once had a customer test a new oven by cranking preheat to max in 30 seconds, and they had a line of 0402 caps cracking right down the middle. Don’t do that.

The ramp rate here is key—keep it below 2°C per second, and never exceed 3°C per second. Our oven’s zone controls make this super easy because you can dial in each preheat zone independently. If you’re using another type of SMT manufacturing machinery, just make sure you test the ramp rate with a thermocouple placed on the hardest-to-heat part of your board (usually a large IC or a heavy connector)—that’s a trick I learned from an old SMT engineer, and it’s saved so many lines from downtime.

2. Soak Phase (1 to 2 minutes total)
   Wait, isn’t preheat and soak the same? No—preheat gets you close, soak holds you there. This phase is when you keep the board at 150–170°C (302–338°F) for 60 to 120 seconds. Why? It lets all the moisture out of the components (important for things like plastic ICs that can pop if moisture heats too fast) and makes sure every part of the board is at the same temp. If some pads are 160°C and others are 140°C when you hit the reflow peak, you’ll get uneven solder joints.

A lot of new operators think they can skip the soak to speed up production, but I’ve seen that cost more in rework than it saves. One customer of mine tried cutting soak time from 90 seconds to 30 seconds to hit a 10% faster cycle, and ended up with 15% of boards needing rework because the flux didn’t activate properly. Stick to 1–2 minutes here—hot air reflow ovens run efficiently, so adding that time is worth it.

3. Reflow Peak Phase (just enough, not too much)
   This is the make-or-break part for Sn63Pb37. Remember, it’s eutectic, so peak temp has to be just above the melting point of 180°C (356°F). The sweet spot is 183–188°C (361–370°F). That’s not a typo—only 5–10 degrees above the melting point. If you go higher than 190°C (374°F), you start overheating the flux, which leaves residues that are conductive or corrode over time, and you risk damaging components like LEDs or small capacitors.

Hold the peak for only 10 to 30 seconds. Any longer than that, and you’ll have issues: solder joints might become brittle, and components (especially those with plastic casings) can degrade. Our six-zone hot air reflow oven’s peak zone is calibrated to hit that exact range consistently, no hot spots, which is why so many customers switch to us from older ovens that can’t nail this. If you’re using a smaller, entry-level oven, just make sure to monitor the peak with a thermocouple—don’t trust the oven’s built-in display, because that can be off by 10–15 degrees.

4. Cool Down Phase (slow is better)
   Most people forget this phase, but it’s just as important. You need to cool the board gradually down to room temp, or at least below 100°C (212°F), over 1 to 2 minutes. Slow cooling prevents thermal stress on the board and components, and it lets the solder form a uniform, strong joint. If you blast the board with cold air right after reflow, you’ll get quick-cooled solder that’s more brittle and prone to breaking. A lot of hot air reflow ovens (including our line at /reflow-soldering/6-zone-reflow-soldering/hot-air-reflow-oven.html) have dedicated cool zones for this exact reason—no extra work needed, just set it and forget it.

Wait, let’s talk about variables that change this profile—because no two boards are exactly the same. If you have a board with a lot of ground planes or heavy copper, you’ll need to adjust the preheat phase to give those thick areas more time to heat up. If you’re working with tiny parts like 0201s or 01005s, you might want to lower the peak temp a little and shorten the peak hold time to prevent tombstoning. Tombstoning happens when one end of a resistor heats faster than the other, so it pulls up like a tombstone—slow, even heating is your best defense against that, which is why good hot air reflow oven performance matters.

I also get asked all the time about vsn (vent speed) or air flow settings. For Sn63Pb37, you don’t need crazy high air flow—our customers usually run zone air flow at 50–70% for preheat and soak, and up to 80% for the peak zone to make sure heat is evenly distributed. Too high air flow can blow small components off the board, and too low means uneven heating. It’s a balance, and that’s where a well-built oven makes all the difference.

Let’s also bust a common myth: some guys think because Sn63Pb37 is an older solder, you can use any profile for it. Nope—we tested this with a customer a few months ago who used a lead-free profile (which peaks at 217°C, way higher) for Sn63Pb37. They burned out 20% of their line’s ICs in one day. Don’t mix lead-free and eutectic profiles—they’re for different solders, and they’ll cause headaches.

Another pro tip: always verify your profile with a thermocouple. Stick it to the bottom of the board (under a large component, the hardest to heat) and track the temp through every phase. I keep a spare thermocouple in my truck for when customers are testing our ovens, because a lot of entry-level operators don’t realize the oven’s internal temp reading isn’t the same as the board’s actual temp. Hot air reflow oven uniformity is non-negotiable here—if your oven has hot spots, your profile is useless no matter what numbers you use.

At the end of the day, the goal of the Sn63Pb37 reflow profile is simple: activate the flux, heat the board evenly to just above the melting point of the solder, hold it long enough to get good wetting, then cool it slowly. It’s not rocket science, but it’s specific enough that cutting corners will cost you time and money.

If you’re working on upgrading your hot air reflow oven to get consistent profiles like this, or if you need help dialing in a profile for a specific board, our team is here to help. We build manufacturing machinery for micro picking and placing and reflow soldering that’s calibrated for even heating and easy profile adjustments, so you can stop dealing with rework and start shipping good boards on time. Don’t hesitate to reach out if you have questions—we’re not just selling ovens, we’re helping you make better products.

References
IPC-A-610H: Acceptability of Electronic Assemblies
IPC J-STD-001H: Requirements for Soldered Electrical and Electronic Assemblies
Heat Processing in Electronics Manufacturing: A Practical Guide, 2nd Edition, Elsevier