News

Pb-free wave soldering has its own list of common soldering defects. Even the most robust process may, from time to time, experience defects. Following is a list of common problems and descriptions, and some recommendations for their mitigation.

Remember: Successful wave or selective soldering means creating a good solder joint between two surfaces or objects, with both surfaces demonstrating good wettability (also known as solderability). The connection must be designed so that solder will remain liquid during the soldering process and not cool below its melting point during joint formation. This is called thermal solderability. Only when the demands of surface solderability and thermal solderability are met can joints be created for which solder is able to fill completely the holes in a through-hole or mixed-technology board, thus reducing the potential for defects.

Remember also that all parts directly adjacent to the liquid solder in wave soldering will rise in temperature and expand due to the heat introduced into the joint by the liquid solder. The degree of thermal expansion is not the same for all materials, but overall it is higher in Pb-free soldering. For epoxy-glass board materials, this thermal expansion has a variable value, depending on temperature. This expansion will become extreme in the z axis.

The combination of longer contact times and higher solder temperature is dangerous, because it can lead to certain defects.

Blow holes. These result from gases exiting the board material due to high temperatures. These gases escape through the copper barrels into the solder, creating large voids in the solder.

Solder excess. This is due to outgassing on top of the board; the excess solder is clearly visible.

Copper leaching. Copper from the pad dissolves into the solder. Extremely long contact times will result in complete dissolution of the copper if it is too thin.

Secondary reflow. Surface-mount components will re-melt if the process temperatures of the wave exceed the melting point of the paste. The solder will wick away and the component's leads may release from the pad. Occasionally, a small connection between lead and pad will remain, making detection of this defect even more difficult since there is still a current flow. Placing heat sinks on the leads of vulnerable components will prevent re-melting.

Solder wicking. Solder flows up the lead away from the joint area, leaving insufficient solder to form a proper connection.

Component damage. Some components (e.g., MELFs) that are left too long in molten solder wave may crack. In other cases, their adhesive cannot withstand the temperature and the components may drop into the solder.

Solder contamination. Some metals will dissolve into Pb-free solder. Temperature, flow speed of the solder and alloy composition will define how fast this will occur. Continuous monitoring of the solder composition is therefore required.

If the solder pot's construction materials do not have a sufficiently protective layer, the iron in the base material (stainless steel) will dissolve into the solder and form FeSn₂ dendrites. The melting point of these crystals is 510°C; thus they will remain solid in the solder. These crystals usually collect in the corners of the solder pot, where there is less solder flow. However, if they are pumped up with the solder, they may end up in a solder joint, causing bridging.

A number of defects are related to board material quality. The solderability of boards relies on good storage conditions, well-controlled logistics and a qualified board supplier.

With wave soldering, the solder temperature should be as low as possible. This will prevent overheating of components, damaging materials and, most importantly, re-melting of solder paste (secondary reflow).

Lower solder temperatures neutralize the corrosive effects of molten tin on ferrous machine parts - such as the solder pot and impellers - and will eliminate FeSn₂ dendrite formation.

In wave soldering as well as in reflow, many defects are caused by insufficient flux activity. A good flux that is able to withstand high temperatures can prevent bridging and improve through-hole penetration. Many designs of experiments have proven the importance of flux for these processes.

Greater process control will reduce defect levels. Use of SPC and Pareto techniques to monitor process robustness and to improve the design of the assembly is key. Sufficient process control in the reflow process will prevent overheating and reduce the incidence of other defects.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl.vitronics-soltec.com. His column appears monthly.