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By Dr. Ning-Cheng Lee, Indium Corporation of America, Clinton, NY [indium.com]

Controlling voiding during solder reflow is an historical industry problem. Careful attention to the causes of voiding, however, can reduce or eliminate the problem in most applications.

Voiding at reflow soldering has been plaguing the industry for decades. Moreover, the trouble has become magnified with the growth of BGA and CSP formats, particularly in the presence of microvias.

Converting to lead-free solder further aggravates the problem. The presence of voids will affect the mechanical properties of joints and will adversely impact the strength, ductility, creep and fatigue life. Voids may also produce spot overheating, which reduces the reliability of joints. Ways to eliminate or reduce voiding are constant challenges to both material scientists and engineers.

Voiding is mainly caused by flux outgassing within the solder joints when the material is in a molten state. During reflow, the outgassing forms bubbles in the molten solder.

These bubbles intermittently form and then pop open when they either grow too large, or when they migrate to the edge of a joint. Upon solidification, the bubbles are frozen as voids. Voiding can be caused by materials, processes and designs.¹ The contributing factors, as well as the critical control parameters, are discussed below.

Figure 1. This chart shows the effect of effect of solderability and flux activity on voiding; both are expressed as wetting time determined by a wetting balance test.

Materials

Fluxes/Outgassing

Selecting a flux material with a low outgassing rate when the solder is in a molten state is the obvious choice. It is the outgassing rate at a temperature above the melting point of solder that dictates the voiding behavior, not the accumulated outgassing quantity.

If the flux can be excluded from the interior of the solder joint, then the outgassing will not cause voiding. When using a solder paste, the flux is in direct contact with the surface to be soldered. Hence, at reflow, any residual oxide may have flux adhering to it.

A higher-activity flux eliminates oxide more efficiently, leaving fewer spots for the flux to adhere to, which results in less voiding, as shown in Figure 1. In lead-free soldering, due to the poor wetting of lead free alloys, the fluxing activity parameter often overrides the outgassing parameter. This dominant, wetting-factor phenomenon can be employed when the solderability of parts is poor.

Solders

Solder Reaction with Base Metal

Solder that is prone to reacting with base metal to form intermetallics accelerates mixing between base metal and solder at the atomic level. This, in turn, facilitates solder wetting,² thereby reducing voiding.

The surface tension of solder produces a dual impact on voiding. Alloys with a lower surface tension offer lower resistance to the joint volume expansion caused by void formation. On the other hand, solders with a lower surface tension spread more easily. This condition excludes the trapped flux from within the solder joint, and consequently reduces voiding.

Experimental results indicate that the effect of surface tension on wetting is the dominant factor. Fluxes with a lower surface tension will also favor the spreading of solder, reducing voiding.

Melting Sequence

At BGA assembly, severe voiding is often observed if the melting temperature of the solder bump is lower than that of solder paste, such as eutectic SnPb BGA assembled onto a PWB with SnAgCu solder paste.

This condition can be attributed to the wrong melting sequence, as shown in Figure 2. If the ball exhibits a melting temperature lower than the paste, the ball will melt first, and the volatiles from the paste can be emitted into the liquid ball until they reach the paste melting temperature. As a result, significant voiding is developed in the ball.

The same phenomenon is also visible in a SnPb system. Significant voiding is observed if a Sn62 BGA is assembled with Sn63 paste, since the former melts at 179°C while the latter melts at 183°C. If the ball melts later than the paste, then voiding will be greatly reduced.

Figure 2. Graphics demonstrate the effect of the melting sequence on voiding.

Oxides

Voiding escalates with increased oxidation of solder or pads. This effect is very pronounced for the 90Pb10Sn bump system, but only barely discernible for the 63Sn37Pb bump system.

This phenomenon is attributable to the effect of the mobility of sphere oxide during reflow. For the low melting point 63Sn37Pb sphere system, the oxide on the sphere surface is mobilized during reflow and can be excluded from the interior of molten solder due to the surface tension.

This will greatly reduce the chance of having some anchored flux entrapped in the molten solder that may contribute to the voiding. However, for the high melting 90Pb10Sn sphere system, the oxide on the sphere surface is immobilized; therefore, any uncleaned sphere oxide will serve as an anchoring spot for the flux and result in more outgassing from the entrapped flux. Oxides on the surface of parts sustain a similar impact.

Solder Paste Metal Content

The void content increases with the increasing metal content of solder paste for typical SMT and BGA assembly. This increase can be attributed partly to an increase in solder powder oxide, resulting in an increase in outgassing due to a greater fluxing reaction.

Solder Powder Size

There is a slight trend showing that voiding increases with decreasing powder size. This relationship can be attributed to the increasing oxide content of the powder associated with the decreasing powder size.

Print Thickness

Our data indicate that a thicker print deposition results in less voiding. This can be explained by the fact that the thicker print provides a higher flux capacity that eliminates the oxides and provides a higher joint standoff that makes it easier for bubbles to escape.

Surface Finishes

The effect of surface finish on voiding is mainly related to wettability, with better wettability resulting in less voiding. In general, the voiding behavior of surface finishes can be roughly ranked as OSP (worst) < non-noble metal < noble metal (best). An improvement in wettability is more effective in reducing voiding than increasing flux activity, as indicated in Figure 1.

Figure 3. This solder joint was formed by reflowing a print of 175µm thickness of 63Sn37Pb solder paste on Ni/Au with 1.63µ Aum.

Intermetallics

Voiding can also be formed if excessive intermetallics are developed at reflow. For instance, if a thick layer of Au or Pd is employed as the surface finish, a large quantity of intermetallic particles, such as AuSn4 or PdSn4, may be formed and scattered within the liquid solder during reflow.

These particles will result in the formation of a viscous liquid and will cause severe voiding due to a sluggish flow of liquid solder. Figure 3 shows a 63Sn37Pb solder joint formed on Ni/Au with 1.63µm Au. This thick layer of Au causes severe voiding of the joint.

Outgassing of Immersion Ag

When soldering onto immersion Ag, occasionally many small voids may be seen on top of the layer of intermetallics. This phenomenon is sometimes referred to as "champagne voiding."

Depending on the chemistry, the immersion Ag may have an organic inclusion up to around 30 percent by volume. When the immersion Ag is plated properly as a thin layer of about 0.2 micron in thickness, Ag leached into solder at soldering, and no organic can remain within solder joints. However, if the immersion Ag layer is thick, it may not dissolve into solder completely, and the organic inclusion in the remaining Ag layer will thermally decompose and outgas during soldering, causing micro-voiding.

Parts

Voiding can also be caused by the outgassing of components or boards. For instance, if the BGA pad is a solder-mask-defined pad, outgassing from components or boards may emit volatiles into the liquid solder joint through the interface between solder mask and pad.

Also, if the through-hole via is not plated properly, pinholes may exist on the copper barrel, and the volatiles from the board may emit into the liquid solder through the pinholes and cause voiding.

Design can serve as a powerful tool in controlling voiding. The following factors are considered crucial in design for non-voiding:

• Large coverage area

  Voiding increases with expanding coverage area under the lead of the joint. This is due to the difficulty in venting the outgas. If a large, flat component termination is to be soldered, designing an efficient venting channel within the solder paste layer or solder layer will be the most effective way of preventing voiding.

  The most effective way of venting is by dividing the large pad into multiple small quadrants. If that is not possible, adding solder mask dividing lines on top of the large pad will also help. Alternatively, modifying the stencil aperture so that the paste will be printed as multiple small quadrants can also be helpful, although less effective. Upon reflow, these divided paste quadrants may become reconnected and seal up the venting channel.

• NSMD pad

  A solder-mask-defined pad offers a greater potential to form voiding caused by the outgassing of parts. Changing the pad design to a non solder-mask-defined pad will help to reduce voiding.

• Microvia

  A microvia is an indispensable link of high density interconnect technology. However, microvias also pose challenges in voiding control, due mainly to the presence of a "dead corner" in the via. Flux entrapped in the dead corner often causes serious voiding by outgassing.

• Size

  Void size at microvia is found to increase with a larger inner microvia diameter. This condition may be attributed to the presence of more dead corners, which offer more opportunity for flux to become entrapped in the via. The shape of microvias can also play a role.

  Certain technologies, such as those employing laser-drills, produce microvias with a mouth wider than the bottom of the via, while plasma etching tends to produce pocket-like microvias. These pocket-like microvias make it more difficult for the flux to escape from the pocket. Therefore, when designing the microvia, it is important not only to design the via with a smaller diameter, but also to select microvia technology that avoids a pocket shape.

  Void size at microvia is found to increase with a larger inner microvia diameter. This condition may be attributed to the presence of more dead corners, which offer more opportunity for flux to become entrapped in the via.

• Location

  The location of microvias produces a significant effect on voiding. By moving the microvias away from the center of the pad, as shown in Figure 4, voiding declines rapidly. Apparently, the further the via move away from the center, the more chances for the volatiles to escape and to eliminate any voids.

Figure 4. Effect of microvia design on voiding (Solectron)

Plug

Since dead corner is the root cause of voiding at microvia, it is logical to eliminate this condition by plugging the hole. There are certain requirements for this plugging approach: First, the plug has to be solderable, and no outgassing source may become buried under the plug, if the plug is to melt at soldering.

Copper plating is a preferred choice, since it can be integrated into the microvia manufacturing process.

Processes

The choice of process is the last opportunity for combating voiding, and often involves conditioning and reflow profiling.

Reflow Profile

Voiding is caused by outgassing within the solder joint when the solder is at its molten stage. It can be reduced by lowering the outgassing or by improving the wetting, or both. The impact of the reflow profile on voiding can be reflected in both directions.

Outgassing Control

Figure 5 shows the typical outgassing behavior of fluxes as heat increases. In general, as heat increases, the outgassing rate of virtually all fluxes increases initially, then decreases gradually after reaching the maximum point.

Minimal outgassing, when the solder is at its molten state, can be realized at either spot 1 or spot 2. Spot 1 represents a minimal heat input, with a short, fast ramp rate and low peak-temperature profile, as exemplified by profile 1. The objective is to complete the reflow process before major outgassing begins.

Spot 2 represents a long, hot soaking and a low peak-temperature profile, as exemplified by profile 2. The objective is to dry out volatiles before the solder melts. A long, hot soaking will favor the elimination of volatiles, and a low peak temperature will favor minimizing further outgassing when the solder is in a molten state.

Figure 5. This illustration shows the relationship between the outgassing rate and the heat input of fluxes, where the heat input is a combined result of time and temperature. Spots 1 and 2 are exemplified by profiles 1 and 2, respectively.

Controlling Wetting

Wetting improves as fluxing reaction increases which in turn, increases with higher temperatures and longer times. Thus, a profile with a high temperature and a long time is desirable. However, the wetting behavior can be complicated by flux loss and oxidation, which occurs when the flux gradually dries out with increasing heat input.

Furthermore, under air, oxidation also increases with higher temperature and longer time. The optimal profile should be a balanced one based on both wetting and outgassing considerations.

Reflow Atmosphere

An inert atmosphere generally reduces voiding by helping wetting. However, this effect may not be significant for fluxes or solder pastes with a high fluxing capacity and good oxidation resistance.

Humidity can affect voiding by interfering with soldering directly or by enhancing the outgassing of components and boards.

At high humidity, the flux outgassing may be aggravated, resulting in more voiding. On the other hand, the moisture within components or boards can also aggravate voiding, mainly for solder-mask-defined pads or other situations where the parts outgassing can be directly emitted into the solder joints.

Summary

Voiding at reflow soldering can be controlled from the aspect of materials, design and process. Materialwise, a solder formula with good wetting, low outgassing and proper melting sequence are desirable for low voiding.

Designwise, adequate venting channels are critical, while processwise, a balanced reflow profile with maximal drying of volatile and maximal wetting is desired.

References

1. N.-C. Lee, Reflow Soldering Processing and Troubleshooting SMT, BGA, CSP, and Flip-Chip Technologies, Newnes, pp. 288, 2001.

2. J. Lau, C.P. Wong, N.-C. Lee and R. Lee, Electronics Manufacturing with Lead-free, Halogen-free, and Conductive Adhesive Materials, McGraw Hill, New York, 2002.

Dr. Lee is vice president of technology at Indium. Prior to joining Indium in 1986, he served with Morton Chemical and SCM. Dr. Lee has more than 20 years of experience in the development of fluxes, solders and solder pastes for the SMT industries. He received his Ph.D. in polymer science from the University of Akron, Ohio, and a bachelor's degree in chemistry from the National Taiwan University. He is the author of Reflow Soldering Processes and Trouble-shooting and co-author of Electronics Manufacturing with Lead-Free, Halogen-Free and Conductive-Adhesive Materials. [nclee@indium.com]