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Publication Date: 07/1/2009
Archive >  July 2009 Issue >  Special Features: Assembly and Packaging > 

Solving a Solder Wicking Problem
SMT Quattro Peak L Plus (N2) reflow oven.

The complex science of soldering keeps getting even more complex. While the underlying scientific facts are the same for the various soldering processes, each process presents its own unique issues and challenges to the production engineer — in addition to dealing with the inherently common problems. Solder wicking is at the top of the list of these potentially serious problems — presenting a challenging case that was submitted by a demanding automotive customer. This particular wicking situation was ultimately solved by the process engineers at SMT-Wertheim, a German manufacturer of reflow ovens.

Solder wicking is basically defined as solder climbing up on a lead (component, connector, external wire). Wicking is a redistribution of solder away from the intended joint, and this is caused by the solder's surface tension. With leaded packages, excessive wicking can contribute to there being insufficient solder at the lead-to-pad interface. This happens when the pad on the PC board reaches the liquidus temperature later than the lead. The melted solder is therefore tightened completely by the lead (wicking-up phenomenon).
Looking under the hood of themulti-zone reflow oven.

  • The solder alloy will pull to the hotter surface if sufficient wettability exists.
  • Leads will melt faster than rivet solder joint due to the delayed liquefaction of solder paste at the rivet solder joint, and this pulls the leads away from pads.
  • There are non-wetting problems like plating that is too thin or contamination/oxidation that can cause wicking-up.

Testing for the wicking problem called for some special preparations including these wetting parameters:

  • Temperature difference.
  • Wettability of pad.
  • Heat sink at conductor.
  • Gap dimension between pad/lead.
  • Melting time of solder at rivet.

For this test, it was found that the heat sink had to be heated up first, and increased convection heat had to be applied from the bottom.
Preparing the test board with heat sensors.

The oven selected for the test was the well-proven 8-zone (5 pre-heat, 3 peak zones) forced convection reflow oven, SMT Quattro Peak L Plus (N2). The reasons: this oven provides suitable parameters for processing very complex boards, with optimum process stability and excellent reproducibility. This reflow oven model also provides perfect heat transfer, excellent zone separation, and provides high quality cross- and load-profile performance.

Energy-Efficient Oven
The reflow oven's manufacturer is globally active SMT Maschinen und Vertriebs GmbH, Wertheim, Germany. The company is deeply committed to providing environment-friendly production solutions. SMT offers the most energy-efficient thermal equipment in the industry today. The company was selected by the German government to be an active partner in its Energy Efficiency Initiative. The company's ABS process gas cleaning system and the intelligent low consumption nitrogen system are further examples of conserving environmental resources as well as significantly lowering overall costs as well as the customer's cost of ownership.

The test board was carefully prepared by attaching 6 temperature sensors:

  • Through a drilled hole directly below the QFP at a heat sink solder joint;
  • On QFP lead;
  • T1 front;
  • C1 right side;
  • SO8 left side;
  • D1 middle.

Reflow Oven Settings
There are different machine settings for lead as well as lead-free temperature profiles, with considerable temperature difference of up to 30°C (zone 5) for the "Special Lead-Free" profile between top and bottom heaters.
The different machine settings for lead as well as lead-free temperature profiles.

For conventional lead solder paste, the desired time to liquidus is usually achieved much faster than for lead-free, which calls for a higher temperature setting.

The reflow oven used in this test achieved good results for both lead and lead-free solder paste because of its advanced control features.

Pre-Heat Zones
There are 5 pre-heat zones, 3 peak zones and the cooling section for the reflow oven used for this test. The heated process chamber length includes the active convection zones consisting of pre-heat and peak zones. These are perfectly proportioned at a ratio of 3:1.

The reflow oven used for this test provides a highly efficient heat transfer because of its vertically directed airflow. Most other reflow ovens use a laminar airflow, which creates no air circulation on the surface resulting in an isolated laminar air layer, and poor heat transfer.

In addition the laminar airflow causes non-homogeneous temperature settings on the PC board, along with heat "shadows" behind larger components and thus a larger delta T on the PC board.

SMT reflow systems with Power Nozzles use a vertical airflow, which creates an homogeneous air circulation profile on the surface, generating balanced airflow and providing excellent heat transfer. In addition, the vertical airflow is directed in a way that reaches every single spot on the PC board and the heat will be spread out homogeneously. The short distances involved guarantee high efficiency and the lowest possible delta T.

Efficient and optimum heat transfer not only guarantees the best soldering results, it also ensures a positive energy balance. SMT, Wertheim has equipped all its machines with Power Nozzle technology with enhanced geometry to allow an even better process gas delivery. The Power Nozzle is a special system that utilizes high-power fan performance while maintaining low flow speeds.
Details of the melting process of the solder paste.

The Power Nozzle system results in absolute temperature stability in the soldering chamber, and this in turn ensures consistent heating of the modules. Agitating large air masses while at the same time maintaining low flow speeds results in hot air flow directly over the module where it is most effective.

The introduction of Power Nozzle technology has taken the reflow oven to a new level of quality production performance while once more providing absolutely the best energy-saving technology. The soldering results were achieved due to perfect heat transfer as well as bottom-side heaters in all zones and independently controlled fan speed settings. Temperatures on the bottom side were set approx. 20° higher than on the top side.

The fan speed on the bottom side was set at maximum, and set for a minimum on the top side. These settings were chosen to obtain the largest possible difference in heat transfer between top and bottom. The reflow oven used for these tests was fully capable of adjusting the required profiles in such a way that even the heat sink soldering joint, sensor 1, melted 1.5 sec. before the component lead/pad sensor — thus eliminating the potential for any solder wicking to occur.

Contact: SMT North America, Inc., 5905 New Harvard Place, Glen Allen, VA 23059 804-360-0144 E-Mail: Web:

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