Reflow Profile: Reflow Profiling: Measuring What the Solder Sees
Thermal profiling is the measurement that connects a reflow oven’s settings to what the solder actually experiences, and it is the only way to know whether a profile is suitable for a particular assembly. The oven’s display shows the air temperature, not the board’s.
What the Profile Records
A profiler is a small data logger with thermocouples attached to points of interest on a representative assembly. It travels through the oven with the board and records the temperature at each point against time, producing a curve for every attachment.
The attachments matter as much as the logger. A thermocouple that is taped to the surface reads the surface, while one that is inserted into a hole alongside a lead reads closer to the joint. Both are useful, and the difference between them is itself informative.
The Four Zones
A reflow profile has four parts: the preheat ramp, the soak, the reflow spike and the cooling. Each has a purpose, and each has limits that are set by the paste and by the components rather than by preference.
The preheat ramp brings the assembly up without thermal shock. The soak allows the assembly to reach a uniform temperature and activates the flux. The spike melts the solder and forms the joint. The cooling solidifies it with a grain structure that depends on the rate. Our solder joint notes describe how the grain structure relates to reliability.
Ramp Rate and Thermal Shock
The maximum ramp rate is limited by the components, not by the paste. A ceramic capacitor subjected to a rapid temperature change can crack internally, and the crack may not be detectable until the part fails in service.
The limit is expressed as a rate in degrees per second, and it applies to the assembly as a whole rather than to the oven’s air temperature. A small board with little thermal mass will follow the air closely, while a heavy assembly will lag it, and the two need different oven settings.
Soak and Flux Activation
The soak is where the flux becomes active and where the assembly equalises in temperature. A soak that is too short leaves the flux unactivated and produces poor wetting; one that is too long consumes the activator before the solder melts and produces the same result.
The soak also determines how much of the volatile content escapes before the solder becomes liquid. Where a large thermal pad traps volatiles, a longer soak is one of the measures that reduce voiding.
Peak Temperature and Time Above Liquidus
The peak temperature must exceed the liquidus of the alloy by enough to ensure complete melting at every joint, including those connected to large planes that lag behind. The margin is typically twenty to thirty degrees, and it is measured at the coldest point on the assembly rather than the hottest.
The time above liquidus controls the growth of the intermetallic layer. Too little and the joint is incomplete; too much and the intermetallic becomes brittle. The window is set by the paste supplier and should be verified on the actual assembly.
Cooling Rate and Grain Structure
A faster cooling rate produces a finer grain structure, which is generally stronger and more resistant to thermal fatigue. The limit is set by the thermal shock that the components can tolerate, so the two constraints pull in opposite directions.
The cooling rate is influenced by the oven’s last zone and by the rate at which the assembly leaves it. Where a board has a large thermal mass, the cooling may be slower than intended regardless of the setting.
Where the Thermocouples Go
The attachment points should be chosen to represent the extremes: the largest component, the smallest component, a heavy copper area, a component with low thermal mass and a point at the centre and the corner of the board. The profile is acceptable when the coldest attachment is above the minimum and the hottest is below the maximum.
A profile that is acceptable for a bare board is not necessarily acceptable for a populated one, because the components change the thermal mass. The profile should always be measured on an assembly that represents production.
Mixed Assemblies and Two Alloys
Where a board carries both leaded and lead-free parts, the profile has to satisfy the lower melting point while still forming a sound joint at every position. That usually means profiling to the alloy with the lower liquidus and accepting a shorter window for the other.
The alternative is a selective approach, where a different process handles the temperature-sensitive parts. That decision is normally made at the design stage, because it affects the assembly sequence as well as the profile.
Process Control and Verification
On a design of this kind, cooling rate is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Process Control and Verification
On a design of this kind, cooling rate is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
Process Control and Verification
On a design of this kind, cooling rate is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
FAQ
How often should a profile be measured? Whenever anything changes: the paste, the board, the oven, the thermal mass of the assembly or the conveyor speed. In production a periodic verification is used, with a full profile measured on each new product introduction.
Can a profile be copied from a similar product? It can be used as a starting point, and it should then be verified. Two boards with the same outline can have very different thermal behaviour if their copper distribution differs, and the difference is enough to move the coldest point outside the window.
What does gopcb provide with a profile? We provide the measured curve with the attachment points identified, the peak temperature and time above liquidus for each point, and a note on any position that was close to a limit. Where a component has a tighter limit than the paste, we record that constraint with the profile.
