Introduction
In lead-free soldering we need higher process temperatures. Especially when a selective soldering process is used the solder temperature at small nozzles are often high. However this is in such cases often a necessary demand.
Allowable thermal load
The board soldering specifications of maximum 280°C during 10 seconds are valid for the whole board and mainly specified for wave soldering since that is the process that gives the highest thermal load to the board.
During such a solder process the whole board will be heated very fast due to the good heat transfer of the molten solderwave that touches the board over its full width.
In selective soldering only a small part of the board is touched by the solder. The ‘cool' area that surrounds the soldering area will absorb the heat transmitted to the area that is soldered. So this is quite another situation than in common wave soldering. To be able to transfer sufficient heat to the soldering area the solder needs a higher temperature in order to realize a short dwell time.
This higher temperature at the 'source' does however not mean that the board area is exposed to this high temperature. Because as soon as the solder touches the 'cool' soldering area at the board, the temperature at the source drops as the temperature at the joint area is increasing. The equilibrium temperature at the joint area directly after soldering is at a good process setting much lower than the ‘source' temperature. This is explained in more detail in Information Sheet 064.
A static wave like most of our common MultiWave nozzles has not such an efficient heat transfer as a flowing dynamic wave. For that reason static waves need a higher 'source' temperature.
So dynamic waves have an advantage in this respect. Due to the better heat transfer they can be used at lower temperatures and that can be an advantage. At least for the equipment.
This system has however also drawbacks, such as wear and maintenance of tweetable solder nozzles and such a nozzle needs more free space around the area that is soldered.
It is a matter of what is the best process for the product that should be the decisive factor for what system should be used.
In our equipment we can in principle offer both systems, static waves that does not wear out and allow close tolerances towards surrounding components. On the other hand we can make dynamic waves that give a better heat transfer and can therefore often be used at lower temperatures. If such waves can flow like a well than they need not so much extra process space around the nozzle. For that we need wettable nozzles.
A second benefit of a process that can run with a lower solder temperature is that the flux might perform better, which might result in less solderbridging.
Aspects related to board design in view of lead-free soldering
So far there are to our knowledge yet no 'official' documents that are focused on these aspects.
Within the IPC there are several activities to co-ordinate the needs for new standards in view of lead-free soldering.
There is a need for a new solderability standard since in lead-free soldering the known solderability demands are not realistic for lead-free soldering.
There are a few things to keep in mind in view of lead-free soldering.
The surface tension of the solder is decisive for the amount of solder that will hold on a joint. This surface tension is however not only depending on the alloy composition, but also on the flux that is used and the atmosphere during soldering. In general these factors are more decisive than the effect of the alloy composition on the surface tension.
This means that there are for that reason no arguments for a change of design.
The fact however that the lead-free solders need a higher process temperature can have a major effect for the board design.
The so-called 'thermal' solderability for the joint composition needs to be investigated in view of the new thermal demands. Joint designs that would give joints that wicked up well in a process using tin-lead solder may suffer from complete joint fill when a lead-free soldering process is used on the same joint design.
One has to realize that solder can only wick up into a gap when the solder is liquid. For tin-lead solders this is commonly at a temperature of 183°C. For a SAC alloy lead-free solder this is already 217°C, which is quite a difference.
This temperature has to be reached at the top of the board at the joint area for all joints that are soldered. Some joint designs will make that impossible within the given temperature time process window. Such joints have often lack of thermal solderability.
Also the fluxes used in the soldering process should be able to reduce the oxides that are present and formed during soldering at this higher temperatures. That is where most of the soldering problems in lead-free soldering find their origin. The lack of flux activity throughout the whole process will often result in solderbridging on joints that did not give that trouble when soldered with tin-lead solder.
Another aspect that needs to be mentioned is the thermal load to the components. All components must be able to withstand the soldering process without damage or degradation. The thermal load that a component can withstand is mentioned in the component data sheet. Sometimes a spacer between solderjoint and component body can sufficiently reduce the thermal load during soldering. Such a spacer will in most cases also improve the thermal solderability of the joint, since it gives a greater distance between the solderjoint and the component body.
More specific information about the solderability and the joint design aspects can be found in Information Sheet 035 and 045.
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