Hand Soldering Quality: Iron Tips, Heat and Technique

Hand soldering remains the most versatile soldering method and the least reproducible. The difference between a good operator and a poor one is not dexterity but the understanding of where the heat goes.

How Heat Reaches the Joint

The iron tip contacts the joint and conducts heat into the pad, the lead and the solder. The rate depends on the contact area, the thermal conductivity of the tip, the mass of copper attached to the pad and the temperature difference.

A joint on a large plane needs far more heat than a joint on a short trace. The operator can supply it by increasing the tip temperature, by using a larger tip, or by waiting longer.

Increasing the temperature is the wrong answer where a component is sensitive, and waiting longer is the wrong answer where the flux is being consumed. The correct answer is usually a larger tip. Our design release checklist notes where the thermal requirement is recorded.

Tip Selection

A tip that is too small for the joint cannot deliver the heat, and the operator compensates with temperature and time. A tip that is too large transfers heat to adjacent features and increases the risk of damage.

The tip shape matters for access. A chisel tip covers a wide pad evenly, while a conical tip concentrates heat and is better for a small pad in a confined space.

Tip condition matters as much as shape. An oxidised tip does not wet the solder and does not transfer heat, and the operator responds by turning the temperature up. Keeping the tip tinned and clean is the single most effective maintainance habit.

Temperature and Idle Settings

The working temperature is chosen to be high enough to melt the solder and wet the joint quickly and low enough not to damage the component or the pad. The range is narrower than the range of settings available on the station.

An idle temperature lower than the working temperature reduces tip oxidation between joints. A station with a sleep function maintains the tip for longer and reduces the need for frequent replacement.

The tip temperature is not the same as the station setting. A tip with poor thermal coupling to the heater reads much cooler than the display suggests, which is why a tip thermometer is a useful piece of equipment. Our solderability notes describe how the wetting behaviour reflects the actual temperature.

Flux and Its Role

The flux in the solder wire cleans the surfaces as the solder melts and prevents oxidation during the few seconds of the operation. It is consumed as it works, so a long operation ends with no active flux.

Adding flux separately helps on a difficult joint, either as a liquid or as a gel. Adding it before the iron arrives is what makes it useful, since flux applied to an already heated joint acts too late.

The residue must be compatible with the process. Where the assembly will not be cleaned, the flux must be a no-clean type, and its residue must be acceptable on the finished board.

Technique

The iron should contact both the pad and the lead so that both are heated, and the solder should be fed into the joint rather than onto the iron. Feeding onto the iron transfers solder that has already lost its flux.

The dwell time should be short: a joint forms in two or three seconds when the heat supply is adequate. A long dwell indicates that the heat supply is inadequate and that the tip should be changed.

The joint should be left undisturbed while it solidifies. Movement during solidification produces a disturbed joint with a granular appearance and poor strength. Our quality notes describe how a disturbed joint is identified.

Rework and Heat Damage

Every rework adds thermal stress to the pad and to the mask. A pad that has been reworked several times begins to lift, and the lifting is caused by the accumulated heat rather than by the last operation.

Where a component must be removed, the technique is to heat all of the joints together and lift the part rather than to lever it off one joint at a time. Leving a part off while one joint is solid lifts the pad.

A heat sensitive component should be protected with a thermal shunt, a clip that conducts heat away from the body. The shunt is placed before the iron and removes the heat that would otherwise reach the package. Our component reliability notes describe where the shunt is needed.

Training and Verification

Hand soldering is a skill, and it requires training that includes the criteria as well as the technique. An operator who does not know what an acceptable joint looks like cannot produce one consistently.

The verification is the inspection of the joints against the criteria, performed on the first article and on a sample during production. Where the same defect appears repeatedly, the cause is usually the tip or the temperature rather than the operator.

Records of the tip used, the temperature and the solder alloy for each assembly make the process reproducible. Our solder defects notes describe the defects that indicate a technique problem rather than a material problem.

Process Control and Verification

On a design of this kind, rework is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

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, rework is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

FAQ

What temperature should the iron be set to? The setting follows from the thermal mass of the joint and the sensitivity of the part, and it should be verified with a tip thermometer rather than assumed from the display.

Can hand soldering match a machine process? On a well designed joint with the right tip, yes. The variability comes from the operator, which is why the process relies on training and inspection.

What does gopcb provide for hand soldering? We provide joint and tip recommendations for the assembly, temperature verification with a tip thermometer, alloy and flux selection compatible with the rest of the process, trained operators working to written criteria, and records of the parameters for each assembly.