Manufacturing / Production Technology, Hardware & Services

Lead-free soldering and tools

28 July 2004 Manufacturing / Production Technology, Hardware & Services

In the interests of pollution prevention, starting from 1 July 2006, the European Community laws have mandated use of only lead-free solders for electronic goods. Weller has a long experience with lead-free solder, with customers in Japan and China having successfully worked with Weller soldering equipment and lead-free solder for many years.

Particulars of lead-free solder

Being developed are new alloys and in general, their melting point is increased. Tin (Sn)-copper (Cu) (227°C), tin-silver (Ag)(221°C) or tin-silver-copper (217°C) are used mainly, whereby the eutectic behaviour needs to be kept in mind. Occasionally bismuth (Bi) is added for further reduction of the melting point.

Those alloys are critical, since minor contamination (rest of solder on PCB, lead containing connectors of components) can reduce the melting point dramatically and may cause quality and stability problems. The use of lead-free solder downgrades the process ability because of lower wetting and flow behaviour. This can only be solved by the use of improved fluxes.

Compared to lead-containing solder, lead-free solder joints appear dull and additionally, are temperature-sensitive during hardening. An increase of process temperature should be avoided due to longer cooling times that may cause micro-cracks. Soldering with higher temperatures can result in black layers on the solder tip that make them unwettable and cause early fallout (charred flux, oxidised tin, tin-iron-fusion). Thus, new materials like lead-free alloys demand new tools.

Hand-operated solder tools requirements

Due to higher melting points, working with lead-free solder requires:

* Tools with higher power and optimised temperature control.

* Tools with better thermal conductivity, to bring as much heat to the tip without loss.

* Optimised tip selection. Shorter and thicker tips are preferable to transfer the required heat into the solder joint without increasing the process temperature. Thermal stress on components and PCBs is reduced and the solder tip is prevented from damage (better lifetime, wettability).

* Cost reduction of wear parts.

Due to the higher tin content, lead-free solder causes shorter tip lifetime (due to iron leaching). In the Weller system, since the high-value heater/sensor is separated from the tip, only the tip need be changed for wear. Weller's 'LF' (lead-free) tips additionally have an optimised thicker layer of iron. This results in higher lifetime and still ensures ideal heat transfer.

The Weller soldering irons WMP and WSP are adjustable to every solder task. These adjustments can be calibrated and audited according to ISO standards.

Hints for soldering

* For rework and repair use the same alloy that is used for the production of the boards.

* Select the working temperature as low as possible.

* Choose the largest possible tip shape for your task. Rule of thumb: about the size of the solder pad.

* Note that extensive heat transmission between tip and solder joint through a well-wetted tip.

* Soldering in an inert gas environment increases the flow behaviour and reduces the flux need; so do not use more flux than needed.

* Work with soldering tools that offer high power and a perfect thermal transfer.

* Use an intelligent soldering station with optimised temperature control and setback function to reduce the tip temperature when not in use.

* Preheat the boards with heating plates before repairing to reduce the soldering time.

* It is recommended that sufficient tests be undertaken and a detailed flow chart be set-up to detail the steps to follow in the changeover to lead-free.

Care of the soldering tip

* Clean the tip on a watery swamp (not dry nor wet) and use distilled water.

* Switch off the station during longer working breaks or use the Weller setback function.

* Add sufficient tin to the tip before placing the iron into the stand.

* Unwettable tips can be reactivated with Weller Tip-Activator.

* Use the lowest possible tip temperature.

* Use special tips for applications with drag drop (GW-Gull Wing or KN-Knife tips).

* Apply the tin directly to the solder joint not the tip.

* If possible avoid the use of high activated flux.

* Change the tips with the appropriate changing tool.

* Apply as low a force as possible onto the tip.

* Black layers of oxidised tips can be softly removed with steel or aluminium wool.


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