Comparison of Solder Paste Residues

Emmanuelle Guéné, Anne-Marie Laügt, Céline Puechagut, Aurélie Ducoulombier
Inventec Performance Chemicals
Bry sur Marne, France

Convection soldering remains the most common reflow process in electronic assembly, mostly in air, but sometimes using a nitrogen atmosphere to reduce oxidation. On the other hand, vapor phase soldering remains a niche market: it has been dedicated for years for use on complex boards with heavy mass and/or a large mix of component sizes.

Despite its excellent heat transfer capabilities and high wetting performance, this process suffered from weaknesses that prevented it from being used on a large scale: high fluid consumption (CFCs), high tombstoning effect, long process time, and the inability to integrate it into a production line. In the last few years, vapor phase reflow technology has been improved: New fluids, with boiling temperatures up to 240C, have been developed, the consumptions have been greatly reduced and the control of preheat and peak ramp rate has been improved, lowering the tombstoning effect.

With lead-free implementation, some limitations are observed on complex boards with convection ovens: the thermal reflow process window is reduced due to the higher temperatures required for SAC alloys, the maximum temperature allowed for fragile components, and the wide range of component sizes. Vapor phase reflow might be an option to consider.

However, because of the relatively low peak temperatures and the elimination of oxidation with this process, a question may arise about the reliability of the paste residue. More unburned activators might remain on the board, which could cause some corrosion to develop, even when using no-clean solder pastes.

The purpose of this paper is to assess the reliability of several lead-free, no-clean paste residues after vapor phase reflow in comparison to convection reflow. The authors will use Surface Insulation Resistance (SIR) and Electrochemical Migration (ECM) according to IPC standards, in addition to the Bono test which has been proven to better differentiate the nature of solder pastes residue.