Soldering in electronics
The same oven that melts the paste well on a light board leaves a heavy board 11 °C colder and barely 43 s above the melting point. The thermal mass of the board is also part of the profile.
01Soft soldering
In electronics, soft soldering is used: an alloy that melts below 450 °C joins the parts without melting them. The solder wets the copper and reacts with it, forming a thin layer of intermetallic compounds, which provides the electrical and mechanical bond; if that layer grows too much, through excess time or temperature, the joint becomes brittle. The flux removes the oxides and protects the surface while soldering.
| Alloy | Melting | Comment |
|---|---|---|
| Sn63Pb37 | 183 °C (eutectic) | The classic; now restricted by the RoHS directive except for exemptions |
| SAC305 (Sn96.5 Ag3 Cu0.5) | 217 to 220 °C | The most widely used lead-free alloy; requires more temperature |
| SnCu0.7 | 227 °C | Cheap, for wave soldering |
| SnBi | 138 °C | Low-temperature, for sensitive components; more brittle |
02The methods
| Method | Use |
|---|---|
| Manual, with a soldering iron | Prototypes, repairs and touch-ups; temperature-controlled tip |
| Wave | Through-hole components: the board passes over a wave of molten solder |
| Selective | A small solder fountain solders through-hole components point by point on mixed boards |
| Reflow | Surface mount: paste applied with a stencil, components placed by machine, and an oven |
| Vapor phase | Reflow in the vapor of a liquid that boils at a fixed temperature; heats evenly |
Plastic components absorb moisture from the environment; in the oven it vaporizes and can crack the package. That is why they are classified by their moisture sensitivity level (MSL), come in sealed bags with desiccant and, if the time out of the bag is exceeded, are baked before soldering.
03The reflow profile
In reflow, the board travels through a multi-zone oven on a belt. Its temperature follows a profile: a preheat ramp, a soak that activates the flux and evens out the temperature of large and small components, a peak above the melting point (liquidus) and a controlled cooling. Paste manufacturers give a window; for SAC305, typical values are:
| Parameter | Typical value |
|---|---|
| Ramp-up rate | no more than 3 °C/s |
| Soak between 150 and 200 °C | 60 to 120 s |
| Time above liquidus (217 °C) | 60 to 90 s |
| Peak temperature | 235 to 250 °C |
| Cooling | no more than 6 °C/s |
The board does not reach the temperature of each zone: it approaches it with a time constant that depends on its thermal mass. With a first-order model the effect can be seen:
An oven with nine 40 cm zones: five preheat zones at 120, 150, 165, 180 and 190 °C, two reflow zones at the chosen temperature and two cooling zones at 40 °C. The curve is the board temperature; the green strip, the liquidus of SAC305.
04Defects and inspection
| Defect | Typical cause |
|---|---|
| Cold or non-wetted joint | Insufficient temperature or time, oxidized surfaces |
| Bridge | Excess paste, misaligned stencil |
| Tombstoning | The two pads of a chip melt at different times and the component stands up |
| Solder balls | Wet paste or preheating too fast |
| Voids in BGA | Trapped flux gases |
The IPC-A-610 standard defines the acceptance criteria for assemblies in three classes: 1, general-purpose products; 2, dedicated-service products; and 3, high-reliability products. Inspection is done by eye or with a magnifier, with automated optical inspection (AOI) and, for joints hidden under a BGA, with X-rays. The long-term reliability of the joints was covered in reliability: thermal fatigue is their main failure mechanism.
05In the lab
Solder through-hole and 0805 SMD components with leaded and lead-free solder, and compare the appearance of the joints with the IPC-A-610 criteria.
With thermocouples and a recorder, measure the profile of a benchtop reflow oven and adjust it to the paste window.
Desolder and replace a SOIC integrated circuit with hot air, protecting the neighboring components.
06Common mistakes
- Using the profile of one board for another. Thermal mass changes the actual profile even if the oven is the same.
- Raising the iron temperature to compensate for a dirty tip. The pads and the components get damaged.
- Leaving moisture-sensitive components out of the bag. The package cracks in the oven.
- Mixing leaded and lead-free alloys. The resulting joints are unreliable.
07Self-assessment
At what temperature does SAC305 melt?
Between 217 and 220 °C.
What is the soak of the profile for?
To activate the flux and let all the components reach an even temperature before melting.
What causes tombstoning?
One pad melts before the other and surface tension stands the component up.
In the lab, how is time above liquidus recovered on a heavy board?
By lowering the belt speed or raising the temperature of the reflow zones.
08Further reading
- Ning-Cheng Lee. Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies. Newnes, 2002. The reflow process, the profile and defects.
- Clyde F. Coombs and Happy T. Holden (eds.). Printed Circuits Handbook. 7th ed., McGraw-Hill, 2016. The chapters on assembly, soldering and inspection.
- Ray P. Prasad. Surface Mount Technology: Principles and Practice. 2nd ed., Springer, 1997. Surface mounting from start to finish.