Why is temperature spread a problem in reflow soldering?

A board carries everything from tiny chip parts with little heat capacity to BGAs, shields and connectors with a lot. Under the same hot air they heat at different rates, so small parts can overheat while large parts fail to fully melt their solder. Minimising that spread is the goal of profile design.

How is each part's heating estimated?

Represent each part by one temperature that follows the air with time constant τ = mc/(hA) — heat capacity over surface heat-transfer conductance. In the illustrative example, an 8 s part peaks near 250 °C, a 60 s BGA near 226 °C and a 120 s heavy part near 193 °C, never reaching the 217 °C solder liquidus.

How is the spread reduced?

Lengthening time at high temperature works better than raising the peak. Raising the peak from 250 to 260 °C widened the spread from 57 K to 63 K, whereas extending the peak hold by 30 s narrowed it to 46 K and increased the BGA's time above liquidus from 40 to 76 s. Heavy parts also need attention to placement and preheating.

Reflow Soldering Thermal Analysis

Category: Thermal Analysis | Revised 2026-10-01

CAE visualization for reflow soldering theory - technical simulation diagram

Reflow soldering thermal analysis

Theory: each part heats at its own rate

Overview

🙋

After reflow, only the solder under a large shield can is unmelted. Should I just raise the oven temperature?

🎓

Raise it and the small parts may overheat instead. The same hot air hits the whole board, but parts heat at rates set by their heat capacity relative to surface area. Chip parts catch up with the air in seconds; high-capacity shields and connectors take tens of seconds or minutes. So the problem isn't oven temperature but the difference between parts. Estimating with per-part time constants shows what to change to narrow it.

Per-component time constant

$$ \frac{dT}{dt} = \frac{T_{air}(t) - T}{\tau}, \qquad \tau = \frac{m c}{h A} $$

$m$ is part mass, $c$ specific heat, $h$ the heat transfer coefficient from the hot air and $A$ the heated surface area. Part temperature follows air temperature with lag time constant $\tau$. Heat also flows through the board in reality, but this simple model is enough to gauge differences between parts.

Time above liquidus

$$ t_{AL} = \int \mathbb{1}\left[T(t) > T_{liq}\right] dt, \qquad T_{liq} = 217\,^\circ\mathrm{C}\ (\text{SAC305}) $$

Lead-free SAC305 solder melts at about 217 °C. Good wetting generally needs a time above liquidus (TAL) of several tens of seconds to about a minute and a half, while component temperature limits (often around 260 °C) must not be exceeded. Profile design fits every part within this window.

Coffee Break Trivia

The window that narrowed with lead-free solder

Traditional tin–lead solder melted at 183 °C, leaving over 60 K of margin below component temperature limits. When the EU's RoHS directive restricted lead from 2006, lead-free solders melting around 217 °C became mainstream and the margin shrank to about 30–40 K. On boards mixing parts of very different heat capacity, fitting everything into that narrow window became hard, making in-oven temperature measurement and analysis-based profile design important.

Worked example: an illustrative profile

Air profile: 25 → 180 °C over 0–90 s, 180 °C soak to 180 s, up to 250 °C by 240 s, hold to 280 s, then cool. Part time constants are illustrative.

Part	Time constant	Peak	Time above 217 °C
Small chip	8 s	249.9 ℃	77 s
QFP	25 s	244.8 ℃	71 s
Large BGA	60 s	225.5 ℃	40 s
Heavy shield/connector	120 s	192.5 ℃	0 s

🙋

The heavy part never reaches the solder's melting point.

🎓

Right — a 57 K spread between parts. Let's compare two remedies.

Comparing remedies

Change	Small part peak	BGA above 217 °C	Heavy part peak	Spread
Baseline	249.9 ℃	40 s	192.5 ℃	57 K
Peak raised to 260 °C	259.9 ℃	55 s	197.4 ℃	63 K
Peak hold +30 s	250.0 ℃	76 s	204.5 ℃	46 K

Raising the peak pushes small parts toward their 260 °C limit while the heavy part gains only 5 K, so the spread widens. Extending the hold doesn't heat small parts further — they're already at air temperature — but lets lagging parts catch up. To narrow part-to-part differences, 'longer' beats 'hotter'. If the heavy part still falls short, lengthen the soak to equalise, add bottom heating or change placement.

Analysis and measurement

Procedure

Identify the highest- and lowest-capacity parts on the board and estimate their time constants.
Measure real profiles on a thermocoupled prototype and calibrate heat transfer coefficients and time constants.
Predict temperature distribution with a board-level thermal analysis (board conduction, shadowing by parts, conveyor speed).
Choose a profile that keeps every part within TAL and temperature limits.

Coffee Break Trivia

“Same part, different result by location”

The same BGA placed at two spots on a board showed voids or poor wetting at only one — in the 'shadow' of a tall neighbouring connector that blocked the hot air. A part's time constant depends not only on the part but on its neighbours and the board's copper: directly above copper-heavy power layers, the board draws heat away and the part warms slowly. Such placement effects don't show up in simple per-part models and need board-level thermal analysis and measurement.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
Raising peak only	Small parts overheat, spread widens	Extend time at temperature
Measuring one point only	Unmelted or overheated parts missed	Measure the largest and smallest parts
Ignoring shadowing	Location-dependent defects	Include placement
Ignoring board copper	Cold spots missed	Include the stack-up
Not checking part limits	Component damage	Set limits by the weakest part

🙋

I'd like to compute part temperature histories myself.

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Try the lumped-capacitance tool for time-constant heating and the PCB thermal tool for board thermals. Related topics are covered in reflow simulation and furnace simulation, and solder life in solder-joint reliability and thermal expansion mismatch.

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