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Selective soldering with all its advantages is unbeatable in certain circumstances

Among the many advantages of selective soldering, one stands out very clearly from the rest: greater process security and hence reliable reproducibility of soldering results. These are thanks to the technical advantages of selective soldering. In this way, every individual soldering point can be individually programmed and monitored, depending on its ambient conditions, so as to selectively control flux volumes and soldering time. Definitively higher quality can be achieved at significantly lower cost with selective soldering and this can be done with considerable acceleration of the manufacturing process. This makes selective soldering the technology of choice.

The instances in which selective soldering is unbeatable:

With printed circuit boards with SMD’s mounted on both sides, wave soldering is not always possible for wired components. Then the compromise is manual soldering. In the end, wired components cannot be replaced by SMDs, especially not where there are relatively high mechanical loads (connectors, switches). But power electronics also often requires through-hole mounting of the components on to the printed circuit board. So in production there are individual variants that are due to manual intervention (too low or too high temperature) and therefore quality problems are inherent. But these are eliminated by the use of selective soldering.

How does selective soldering work?

Selective soldering always consists of three stages: fluxing (the application of flux), pre-heating and soldering. Even the programming has been perfectly developed so that even people without any prior knowledge can set up a program for selective soldering within minutes thanks to the NordsonSELECT software.

Fluxing with selective soldering

All NordsonSELECT machines have a MicroDrop jet fluxer that ensures fluxing precisely to the drop. And here the fluxer can process both individual points and entire lines (e.g. connector strips) at once.
The volume of flux in every single drop is adjusted to the requirements of each soldering point. Thanks to selective fluxing of the printed circuit board, contamination is minimised and consumption of flux is drastically reduced.
Monitoring of the jet of flux ensures that flux has actually been applied. All the data on this can be read off and processed in the course of operational data recording.

Pre-heating with selective soldering with NordsonSELECT technology

Infrared top and bottom heating:
The pre-heating of assemblies is becoming more and more important, in particular for multilayer printed circuit boards, assemblies for the high energy sector and when soldering with lead-free alloys. Activation of the flux is crucial to proper wetting of the soldering point and therefore for perfect penetration. To avoid stresses on the printed circuit board during pre-heating and in order to achieve optimum heat distribution, the NordsonSELECT infrared emitters are precisely coordinated for selective soldering. The NordsonSELECT parallel systems heat up two printed circuit boards at once, which halves cycle time per printed circuit board. The modular inline units may be equipped with any desired number of pre-heating modules in order to pre-heat additional printed circuit boards in parallel ready for selective soldering. As a result, pre-heating no longer affects cycle time, which often happens with high-volume applications.

The assembly determines output:
The NordsonSELECT pre-heating is automatically adjusted to suit the size of the assembly. Thus for instance the bottom and top heating in selective soldering is provided by several infrared emitters. Only those emitters located directly below or above the assembly are used for the pre-heating. With continuous operation of this type of selective soldering, this leads to considerable energy savings.

Temperature monitoring with a pyrometer:
Conventional pre-heating units work on a time/output basis. Here the pre-heating is provided with a specific heat output over a specific period. Depending on the printed circuit board, this leads to different temperatures on the assembly. The NordsonSELECT units for selective soldering work with a pyrometer control unit by means of which the temperature is constantly measured directly on the top of the printed circuit board and the infrared emitter is adjusted accordingly. The user can specify a desired pre-heating temperature and the machine heats up the assembly precisely to that. This prevents overheating of the assembly.

Selective soldering itself

All the NordsonSELECT selective soldering machines have a titanium solder pot and can deal with all solder alloys with and without lead. Unlike coated solder pots, titanium ones are

considerably more robust and less susceptible to damage.
In selective soldering, these soldering pots are equipped as standard with NordsonSELECT solutions with a nitrogen coating. The nitrogen atmosphere covers the entire solder bath and prevents the formation of scratches and slag. At the same time, the protective gas flows from the pot via the solder nozzle, thus ensuring an oxidation-free soldering process. Thanks to optimal design, maintenance and cleaning of the solder pot is very easy and with selective soldering it has to be carried out correspondingly less often.

A pump made of titanium ensures that the flow behaviour of the solder in the pump channel is kept even and free from fluctuations. The result is an especially evenly flowing solder wave. The height of the solder wave is set via the pump output. Thanks to wave height monitoring, the actual height of the solder wave is compared with the set value and if necessary automatically corrected. The wave height monitoring ensures constant reproducibility and makes selective soldering particularly attractive.

In particular with the soldering of connector strips in which several pins can be soldered in a line, there is a possibility of building bridges between the last pins. In order to eliminate these bridges, the NordsonSELECT units for selective soldering offer a “pull-off” solution. Before the solder nozzle is moved down away from the soldering point, the machine abruptly reduces the pump output. This pulls excess solder from the pin and only the required volume of solder is left, which contributes to the advantages of selective soldering.