Hand Soldering: Tip Care and Technique

Hand soldering is the last process that still depends almost entirely on the person performing it. The iron is simple, the materials are well understood, and the outcome varies more between operators than between machines.

What makes the difference is not dexterity but an understanding of heat. A joint forms when the pad, the lead and the solder are all above the melting point of the alloy at the same time, and the operator’s job is to make that happen without overheating anything.

What Determines a Hand Soldered Joint

Three variables decide the result: the temperature of the tip, the thermal mass of the joint and the time the iron is in contact. They are not independent, and changing one to compensate for another is the usual source of bad practice.

A joint that is not heating quickly enough invites the operator to extend the contact time, which cooks the flux and the laminate. The correct response is to increase the available heat or to change the tip, not to hold the iron in place for longer.

Tip Temperature and Its Measurement

The tip temperature is usually set between the melting point of the alloy and a value that damages the materials, and for a lead free alloy that window is narrow. The set point on the station is not the temperature at the working surface, because the tip loses heat to the joint as soon as it touches it.

Measuring the tip temperature means using a thermocouple designed for the purpose and holding it against the working face of the tip. The measurement should be made with the tip in the condition it is used in, not freshly tinned, because the plating and the shape of the working face change the reading.

Thermal Mass and the Limits of an Iron

A ground plane, a large pad or a connector shell draws heat away from the joint faster than a small iron can supply it. The result is a joint that never reaches temperature, and the operator responds by holding the iron longer, which heats the surrounding area instead of the joint.

The remedies are a larger tip, a higher power iron or a preheater that raises the whole assembly closer to the melting point. Preheating is often the most effective, because it reduces the temperature difference that the iron has to overcome and it protects the board at the same time.

Tip Geometry and Selection

The shape of the tip determines the contact area and the access. A chisel tip transfers heat well and is used for general work, a conical tip reaches into a tight space, and a blade covers a row of pads for cleaning or for drag soldering.

Selection should follow the joint rather than the habit of the operator. A tip that is too small makes every joint slow and hot; one that is too large cannot reach the joint without touching a neighbouring component. A small set of well chosen tips covers most production work.

Tip Care and Plating Life

Tip care is the practice that determines how long a tip lasts and how consistently it solders. The plating on a tip is a thin iron layer, and it is consumed by oxidation, by abrasion and by the chemistry of the flux.

The routine is simple: keep the working face tinned when the iron is idle, wipe it on a brass wool or a damp sponge rather than filing or scraping it, and never leave it at temperature for long periods without solder on the face. A tip that has lost its plating will not wet, and no amount of flux will restore it.

Solder Wire and Flux Selection

The solder wire carries its own flux core, and the flux type determines the residue and the activity. A no clean wire leaves a residue that is safe to leave, while a more active wire requires cleaning, which is described in the discussion of the cleaning process. The alloy is chosen for the product, as set out in the comparison of lead free and leaded solder.

The diameter follows the joint size. A wire that is too thick delivers too much solder in one touch and forces the operator to remove the excess, and a wire that is too thin takes several feeds to fill a large joint, which keeps the iron in contact for longer than necessary.

Technique: Heat, Feed, Withdraw

The sequence is to place the tip so that it touches both the pad and the lead, add the solder to the joint rather than to the tip, and withdraw the wire before the iron. That order lets the solder melt into the joint and draw the heat with it.

The contact time should be short enough that the flux is not consumed before it has done its work. The visible result is a small concave fillet on both sides of the lead, which indicates that the solder wetted both surfaces rather than sitting on top of them.

Inspection of Hand Soldered Joints

Inspection is visual and it is judged against a written criterion. The features that matter are the wetting angle, the fillet shape, the amount of solder and the absence of disturbance marks, which show as a rough or grainy surface.

A joint that is acceptable in appearance can still be defective, which is why a sample is sometimes checked by a pull test. The criterion should be the same one used for the rest of the assembly, so that a hand soldered joint is not judged by a different standard than a reflowed one.

ESD and Safety at the Bench

Hand soldering brings a person into direct contact with the assembly, which makes the bench an ESD sensitive location. The iron tip should be grounded, and the operator should be wearing a wrist strap, because a charged operator touching a lead is a discharge path into the device.

The other safety issues are the fume from the flux, the temperature of the iron and the disposal of the consumables. Fume extraction at the bench, a stand that does not allow the iron to touch the bench surface and a defined routine for the tip cleaning material cover most of it.

Training and Consistency

Consistency comes from a documented standard, a set of parameters and periodic verification. The station should have a temperature setting, a tip list and a picture of an acceptable joint, and the operator should know why each one exists.

Where several people perform the same operation, the results should be compared as part of the routine quality control. A difference between operators that is larger than the difference between shifts is a training issue rather than a process issue, and it is usually resolved with a shared reference sample.

FAQ

What tip temperature should be used? As high as the materials allow and no higher. The exact value depends on the alloy and the thermal mass of the joint, and it should be verified by measurement.

Can a joint be repaired after a bad solder? Yes, if the pad has not been damaged. The old solder is removed with a wick, the area is cleaned and the joint is remade with the same technique.

How often should the tip be replaced? When the plating has eroded and the tip no longer wets. Abusing the tip is the main cause, and the routine described above prolongs its life considerably.