HMI beyond basic pushbuttons (e.g. haptic knobs or touchscreen, even keypads)
High-Level Requirements
A user must be able to reflow solder a one-sided PCB with pre-pasted, pre-placed standard* SMD components.
The user must be able to control the reflow thermal profile for different paste liquidus temperatures.
The user should be able to control the rate of temperature increase to prevent thermal shock.
The user must be able to supply power with a USB-C cable (via USB-PD).
The system must provide the user some visual indication of:
the thermal profile used
the progress, stage, and/or time of a running reflow process.
*Standard SMD components meaning SMD components that have pads on only one-side (Top), lay flat on the PCB, have no special thermal considerations, and no other special reflow/soldering requirements.
Reflow soldering can be divided into several Zones:
Zone / Ramp to Zone
Temperature
Slope
Time
Ramp to Soak
20°..soak
1..3°/s (2°/s)
45s..130s (60s)
Soak
150° (typ.)
-
60s..120s (120s)
Ramp to Reflow
soak..230-250°
1..3°/s (2°/s)
25s..100s (50s)
Reflow
230-250° (Liquidus + 20-40°)
-
30s..75s (45s)
Cooling
230-250°..75°
-2..-10°/s (-4°/s)
15s..90s (60s)
During the reflow process, the most important factors of each zone are:
Temperature: too low and nothing happens, too high and undesirable things happen. For example, during in the thermal soak zone, solder paste volatiles are removed and the flux is activated.
Too high a temperature can cause solder splattering, balling, oxidation of the paste/pads/component terminations.
Too low a temperature and the fluxes are not activated nor are the volatiles removed.
Slope: heating causes thermal stress. Heating too quickly can lead to thermal shock and cracking. This is less important during cooling although it still has an impact on quality.
Time: the effects of each zone take time to occur.
Reflow Process Evaluation
Electronics Manufacturers evaluate their processes using PWI (Process Window Index) with 4 primary metrics: