ACW's vapour phase fluid aids solder paste trial

The trial involved production of test boards aimed at comparing through-hole, intrusive reflow solder joints using SN100C lead-free solder paste.

SN100C is a RoHS material with a higher reflow temperature than standard RoHS solder paste.

This type of comparison has been previously conducted with SAC alloys and on a tin/copper/nickel alloy.

However, through-hole reflow can be challenging from a thermal profiling point of view because of the mass of the connector body.

Previous work on convection reflow has shown successful joint formation exceeding the requirements for professional electronics at 245C.

For the trial, Willis approached ACW because its Tonypandy site uses a higher temperature vapour phase liquid for vapour phase reflow than normal.

After successfully reflowing Harting 96-way connectors on multilayer boards using ACW's vapour phase system and 240C boiling solvent, each board was inspected for through-hole fill and soldering quality.

Each board was then x-rayed with a Dage system to examine the level of voiding.

Microsections were also taken from selected joints.

All the samples exceeded the quality levels of IPC 610D with 100 per cent fill and positive solder fillets.

There was little evidence of voiding in the joints and the microsections showed satisfactory wetting to each of the different surface finishes.

Peter Barton, senior process engineer at ACW, said: 'We use 240C temperature vapour phase fluid because some of the assemblies we process have heavy metal fixturing, which is a large thermal mass to heat.

'The extra 10C over the typical temperature used for RoHS soldering helps us to maintain normal profiles.

'We also have a vacuum option on our machine for some soldering applications.

'The higher temperature fluid ensures that when we use the vacuum, the solder is still molten at that stage of the process.

'This extra capability is not at the cost of flexibility.

'Lower temperature processing for non-RoHS applications is also still possible by tailoring the machine heating power and dwell parameters in other reflow programmes.

'This is important to us as we have a number of MIL/Aerospace clients that still require this.'