Abstract
As electronic manufacturing continues to develop, LGA packages are being used in an increasing number of electronic products. Because an LGA has a flat structure and no pre-applied solder, large solder-joint voids can easily form after reflow, affecting solder-joint reliability. Voids in LGA solder joints are relatively difficult to control compared with those in other surface-mount components and remain a significant challenge for the SMT industry. This study investigates the effects of three process variables on LGA voiding: solder paste selection, stencil aperture design, and reflow profile. The objective is to identify process optimization methods that can reduce voiding in LGA components.
1. Introduction
Extensive industry research has been conducted on voiding in components with ground or large exposed pads. QFN packages are a typical example. Voiding can generally be reduced through PCB pad design, stencil aperture design, and reflow-profile optimization. Even so, the acceptance criterion for QFN voiding is often more relaxed than that for other components. With an optimized process design, controlling QFN voiding to below 20% is generally achievable.
Driven by the advantages of the QFN package, LGA packages are now increasingly used in electronic products. LGA devices provide excellent integration and support multifunctional designs. However, LGA voiding is among the most difficult types of component voiding to control. Typical maximum void ratios can reach 35% to 45%. Excessive voiding can affect product thermal performance and solder-joint reliability.
2. Experimental Study
2.1 Effect of Solder Paste on LGA Voiding
Many early studies indicated that void formation is strongly related to solder-paste activity. In general, higher flux activity tends to produce smaller voids.
For this experiment, the LGA pad and stencil aperture configurations were designed as shown in Table 1. Three solder pastes, designated A, B, and C, were selected for comparison. One hundred boards were tested with each solder paste. The results showed substantial differences in LGA voiding performance among the three materials.
Table 1 | LGA pad and stencil aperture configuration
| Pad Shape | Pad Size | Stencil Aperture | Stencil Thickness |
|---|---|---|---|
| PAD-1 | 0.75 mm | 0.75 mm round aperture | 0.13 mm |
| PAD-2 | 1.00 mm | 1.00 mm round aperture | |
| PAD-3 | 1.25 mm | 1.25 mm round aperture |
Solder paste A produced the lowest void ratio. Most voids were smaller than 10%, and the maximum void ratio was only 17%. Solder paste B produced a maximum void ratio of 43%, while the smallest observed void was approximately 23%. With solder paste C, approximately half of the LGA samples had void ratios greater than 25%.
Table 2 | Void statistics after soldering with three solder pastes
| Solder Paste | Sample Quantity | Samples With Voids >25% | Maximum Void | Minimum Void |
|---|---|---|---|---|
| A | 100 boards | 0 boards | 17% | 0 |
| B | 100 boards | 86 boards | 43% | 23% |
| C | 100 boards | 42 boards | 36% | 9% |
The void images for the three solder pastes are shown in Figure 1. The results clearly demonstrate that solder-paste selection has a major effect on LGA voiding.
Figure 1 | Void conditions produced by three solder pastes
Thermogravimetric analysis was used to compare the flux volatilization behavior of the three solder pastes. Thermal mass loss began at approximately 150 °C for all three pastes, and the rate of mass loss increased rapidly at approximately 200 °C. Solder paste C exhibited the lowest thermal mass loss and the greatest amount of residual flux.
Thermal mass loss in solder paste primarily reflects the volatilization and reaction of the flux. Understanding flux composition helps explain the soldering behavior. The principal flux components generally include rosin, activators, solvents, and rheology modifiers.
Rosin is a distilled pine-resin product consisting of medium- and high-molecular-weight organic materials. It contributes flux activity and tackiness while also helping prevent oxidation. Rosin alone is usually insufficient to provide effective wetting, so activators are added to increase flux activity. Activators commonly include dicarboxylic acids and other carboxylic-acid compounds. In practice, a flux normally contains a combination of activators rather than a single material. Because the individual materials have different melting points, they also exhibit different volatilization behaviors.
Activators with different melting points affect the rate at which flux gases are released. The TGA data also show that the volatilization rate of different flux systems varies at the same temperature. A smaller residual-flux quantity generally indicates that more of the flux has been consumed or volatilized during reflow.
Flux escape is mainly influenced by two factors. First, low-melting-point flux materials can volatilize before the metal becomes fully molten. Second, highly active flux wets the surfaces quickly, helping force the flux out as the solder coalesces.
Solvent is another important solder-paste component. Its primary function is to dissolve and balance the metal powder, rosin, and activators so that the paste has suitable printing characteristics. Solvent itself provides limited direct assistance during soldering. Solder pastes often contain multiple solvents with different melting points. If low-melting-point solvents are used, storage stability and stencil life may be affected, and the paste can dry out more easily. If the solvent melting point is too high, a large amount of gas may be generated in the high-temperature zone, increasing the risk of void formation.
Therefore, the melting points, volatilization rates, and activity levels of the materials in a solder paste all affect LGA voiding. When the solder paste is changed, the reflow profile should also be adjusted according to the paste's characteristics.
2.2 Effect of Stencil Aperture Design on Voiding
Void formation has multiple causes. LGA components often contain pads of different sizes and use a flat package structure. During soldering, the flat geometry makes it difficult for flux vapors to escape. A significant amount of flux can become trapped beneath the component, resulting in large voids.
The industry has accumulated considerable experience in reducing voiding through stencil optimization. This experiment evaluated which stencil aperture designs could effectively reduce voiding in LGA components.
The LGA component used in the experiment is shown in Figure 2. Its central circular pad has a diameter of 1.4 mm.
Figure 2 | LGA pad layout
Several stencil designs were used during production. Two representative aperture designs are shown in following figure. The first uses a 1:1 aperture pattern. The second uses a combination of bridge and inverted-triangle apertures. The purpose of the second design is to create additional escape paths for flux gases. The total aperture-area ratio was 100%.
Figure 3 | Images of different stencil aperture designs after printing
A long-duration heating profile was used so that flux volatiles had sufficient time to escape. The two stencil designs were then compared using X-ray inspection.
Stencil A used the 1:1 aperture design. Some solder joints exhibited large voids because the gases inside the joint could not escape effectively through the molten solder. Stencil B used multiple aperture geometries. The voids produced by the central bridge design were noticeably smaller than those produced by the inverted-triangle design. The bridge provided a more effective gas-escape path.
However, the inverted-triangle design deposited solder paste over a smaller effective area of the pad. As a result, the paste could not be pulled back onto the pad effectively, leaving some material outside the pad and producing solder beads.
Figure 4 | Soldering results for different stencil aperture designs
2.3 Effect of the Reflow Profile on Voiding
Extensive historical process data indicate that the reflow temperature profile is one of the dominant factors affecting voiding. Considerable work has been performed on SAC305 profiles, and different component types often require different thermal profiles to optimize void performance. Before adjusting the reflow profile, it is necessary to understand the void-formation mechanism of the LGA component and the characteristics and active temperature range of the solder paste.
Because an LGA uses a flat package structure, the gap between the component and the PCB is determined by the thickness of the solder paste deposited by the stencil. The stencil thickness is typically 0.08 mm to 0.12 mm. After accounting for paste slump, the gap between the pad and component can become even smaller. Under these conditions, flux gases have very limited escape paths during reflow. A large amount of flux can accumulate beneath the component and form large voids.
Two general approaches can be used to reduce voiding through the reflow profile. The first is to pass the assembly through the oven rapidly, reducing the amount of gas generated by the flux. In other words, soldering is completed before the flux has fully reacted. The second approach is to extend the heating or soak period. Before the metal becomes molten in this temperature range, surface tension remains relatively low, allowing gases to escape from the solder more easily.
In this experiment, the soak time was adjusted to evaluate its effect on voiding. The LGA contained 57 pads with a diameter of 2.0 mm and 76 pads with a diameter of 1.6 mm. The stencil used a 1:1 aperture design, as shown in Figure 5.
Figure 5 | LGA pad layout
With the linear profile and rapid oven passage, the LGA void ratio ranged from 25% to 45%, as shown on the left side of Figure 10. With the profile using a longer high-temperature soak, the void ratio was more consistent and stable, ranging from 6% to 14%, as shown on the right side of Figure 6.
Figure 6 | Soldering results produced by different reflow profiles
3. Conclusions
The experiments demonstrate that LGA voiding is not caused by a single factor. Under typical conditions, the LGA void ratio is approximately 25% to 35%. By optimizing the solder paste, stencil apertures, and reflow profile, the void ratio can be reduced to below 20% in the tested process.
A highly active, low-residue solder paste indicates that the flux can volatilize quickly during soldering without compromising solderability.
When the pad size is relatively large, a bridge-type aperture design can help flux escape from the solder more quickly. However, the solder-paste volume should not be reduced excessively. Insufficient solder can itself cause voiding or open solder joints.
A longer high-temperature soak allows flux gases to escape before the solder develops high surface tension. However, if the product includes fine-pitch CSP components, an extended high-temperature zone may cause component warpage, open solder joints, or head-in-pillow defects.
Irregular LGA solder-joint shapes were also observed after reflow, as shown in Figure 7. This condition may be related to LGA deformation or warpage, poor coplanarity, or uneven solder-mask topography. When irregular solder joints occur, bridge-type stencil apertures should be avoided because they may further aggravate post-reflow solder-joint irregularity.
Figure 7 | LGA soldering results after reflow
As LGA packages become more widely used, their flat structure places stringent requirements on pad coplanarity. LGA footprints commonly contain pads of different sizes, so solder volume must be controlled carefully during stencil-aperture design. Otherwise, variations in solder volume may further aggravate coplanarity problems and increase the risk of voiding or irregular solder joints.
