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How VPS resolves voids and popcorning: the superior heat transfer to forced convection ensures no overheating.

PART OF THE reason vapor phase soldering technology is increasing in popularity is that Pb-free requirements and product miniaturization drive production issues not thoroughly addressed by conventional convection reflow. Our April 2012 column, "Vapor Phase Heat Transfer," discussed how vapor phase provides solutions for tombstoning. This month we look at void reduction and popcorning/delamination elimination.

Reducing voids. Voids in leadless package solder joints are predominantly created by either trapped gases or entrapped flux. PCB layout, solder paste selection and profiling are the three biggest factors to examine in minimizing voids. In conventional convection reflow, the profiling development steps in this area can be time-consuming, as it is necessary to achieve proper peak temperatures without overheating to the point that fluxes are boiled off or become encapsulated in solder balls. In a temperature-driven vapor phase machine, it is very easy to program a variety of soak profiles, and programming time is minimal. Comparatively, programming a similar number of profiles on a convection reflow machine may take hours.

Today's VPS process addresses this issue by a combination of improved heat transfer properties of inert vapor and controlled immersion into the vapor, which produces a thermal soaking zone.

A fundamental difference between convection reflow and VPS is heat transfer efficiency. While the time required to achieve liquidus in convection reflow can increase peak temperature by as much as 30[degrees]-3.5[degrees]C, in vapor phase the maximum board temperature is physically limited to the temperature of the vapor. Using a Pb-free SnAgCu solder, that would be 230[degrees]C.

From a profiling standpoint, using a controlled immersion process, the PCB would be moved into the vapor. Once the board temperature reaches a preset plateau, it would be moved up to the vapor boundary. After a soaking time of 60 sec. the PCB is moved over dedicated levels downward, which controls the gradient and time to reach solder temperature. In programming the profile, the soldering time (time over liquidus) is preselected and machine-controlled. Since the PCB temperature cannot exceed the vapor temperature, no overheating occurs in the thermal soaking zone.

Popcorning/delamination elimination. Steam pressure in plastics and laminates can cause the board surface to popcorn or delaminate. As shown in FIGURE 1, in convection reflow, PCB temperatures can easily reach 260[degrees]C or higher. With Ph-free soldering, a PCB temperature of at least 250[degrees]C is likely. Comparatively, the recommended maximum temperature for VPS of SAC solder is 230[degrees]C, as the melting point of that solder is 221[degrees]C.

[FIGURE 1 OMITTED]

In vapor phase, heat is transferred to the board both from components above and the PCB below. Comparatively, in convection reflow, the heating process may be impacted by dense components. As a result, assemblies with high-density components such as BGAs may require longer times at peak temperature (FIGURE 2) to ensure that shadowing on the lower side of the BGA doesn't result in unsoldered balls.

[FIGURE 2 OMITTED]

The inert environment created by the vapor drives an even heating process even under the BGA, enabling the solder to achieve liquidus at lower temperatures (FIGURE 3). When vacuum vapor phase equipment is used, voids may be reduced to a level below 2%. This lower soldering temperature combined with the efficiency of heat transfer in a vapor blanket eliminates the popcorning and delamination caused by exposure to higher and potentially uneven temperatures in convection reflow.

[FIGURE 3 OMITTED]

REFERENCE

(1.) H. Leicht and A.Thumm, "Today's Vapor Phase Soldering an Optimized Reflow Technology for Lead Free Soldering," SMTA International Proceedings, August 2008.

ILLUSTRATION OMITTED

JOCHEN LIPP is CEO of IBL Technologies (ibl-tech.com); j.lipp@ibl-tech.com. His column runs bimonthly,
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Title Annotation:VAPOR PHASE SOLDERING
Author:Lipp, Jochen
Publication:Printed Circuit Design & Fab
Date:Dec 1, 2012
Words:616
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