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Fill the Void II: An Investigation into Methods of Reducing Voiding



Fill the Void II: An Investigation into Methods of Reducing Voiding
This paper is a continuation of previous work on voiding. The voiding results are summarized and recommendations made for reduction of voiding.
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Authored By:


Tony Lentz
FCT Assembly

Patty Chonis and JB Byers
A-Tek Systems

Summary


Voids in solder joints plague many electronics manufacturers. Do you have voids in your life? We have good news for you, there are many excellent ways to "Fill the Void." This paper is a continuation of previous work on voiding in which the following variables were studied: water soluble lead-free solder pastes, a variety of stencil designs, and reflow profiles. Quad Flat No-Lead (QFN) component thermal pads were used as the test vehicle. The voiding results were summarized and recommendations were made for reduction of voiding.

In this work several new variables and their effects on voiding were studied. No clean lead-free solder pastes were tested and compared to water soluble lead-free solder pastes. Water soluble solder pastes tend to create more voiding than no clean solder pastes. This is due to the relatively higher volatile content in water soluble solder pastes, and also due to the hygroscopic nature of water soluble solder pastes. The particle size of the solder powder was studied; using IPC type 3, IPC type 4 and IPC type 5 powders.

The oxide content of the solder powder increases with decreasing particle size and higher oxide content tends to produce higher voiding levels. Different manufacturers of solder powder were also studied. Solder powder from one manufacturer might lead to higher voiding than from another manufacturer. Finally, the effects of convection reflow were compared to vapor phase reflow with and without vacuum.

Convection reflow is commonly used and voiding results from this type of reflow are well documented. Vapor phase reflow is conducted in an oxygen free environment which tends to reduce voiding. Vapor phase systems also lend themselves well to the use of vacuum because the equipment is sealed and vapor tight. Integrating vacuum creates differential pressure between the void and the surrounding atmosphere during the liquid stage which facilitates the escape of the trapped gases. The lowering of the gas pressure outside the solder joints will aid in reduction of voiding.

Reworking solder joints with voids is not an easy task. This typically involves removing the affected components and re-soldering them with the hope that voiding might be reduced. This is a very labor intensive process which can thermally stress nearby components. The possibility of using a vapor phase reflow system with vacuum to rework solder joints with voids was investigated.

In many cases voiding will be reduced only if a combination of mitigation strategies are used. Recommendations for combinations of solder paste, stencil design, reflow profile, and type of reflow are given. The aim of this paper is to help the reader to "Fill the Void."

Conclusions


Voiding in solder joints is affected by many factors. Solder paste has a dramatic effect on voiding. In general, no clean lead free solder pastes generate lower voiding than water soluble lead free solder pastes. Solder powder size was shown to have an effect on voiding. Voiding tends to decrease with decreasing solder powder size. Solder powder from different manufacturers also has an effect on voiding due to differences in the manufacturing process. Stencil design has an impact on voiding, although this impact can be slight when compared to other factors.

ENIG and OSP surface finishes had little affect on the voiding levels with the solder paste used in this evaluation. Reflow profile has a large effect on voiding, and the reflow profile must be tuned to minimize voiding with each solder paste. Vapor phase reflow and convection reflow with nitrogen reduced voiding as compared to convection reflow in air. The use of vapor phase reflows with vacuum gave extremely low voiding levels. It was also shown that vapor phase reflow with vacuum can be used to rework and reduce existing voids in soldered components.

Only some of the factors that influence voiding were studied in this work. There is much more testing to be done. Due to the commonplace use of bottom terminated components, it is clear that voiding will be an issue that many must address. The authors will continue to study factors that influence voiding in an effort to help the reader to "Fill the Void."

Initially Published in the IPC Proceedings

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