Authored By:
Li Baocai, Wang Xiaoan, Sun Xiaoyan, Zhu Ailan, Tu Yunhua, Renzhe Zhao
Huawei Technologies Co., Ltd.
Shenzhen, Guangdong, China
Summary
In the past decade, Quad Flatpack No Lead (QFN) components were widely used in a variety of electronic products and their long term reliability was highly concerned by the industry.
Compared with other package type components, such as Quad Flat Pack (QFP), the solder joints of QFN are under its body, and the standoff between component and Printed Circuit Board (PCB) is very low, and it will hinder the escape of solder paste flux at reflow stage. After reflow process, evaporation of ingredients of flux is not fully completed and the flux residue underneath QFNs, especially for large size ones, is always "gooey", and the stage of this flux residue is quite different from that reflowed in open air, whose Surface Insulation Resistance (SIR) reliability is usually evaluated according to IPC standard.
In this paper, the factors that affect SIR reliability of "gooey" flux residue underneath QFNs will be discussed. A variety of dummy QFNs that made of PCBs are assembled on PCBs whose surface finish is OSP by reflow in air, then they are tested under 5V bias voltage and 85, 85%RH test condition for 168 hours. After the test, the dendrites of tested QFNs are checked and measured by X-ray. Further, two solder paste fluxes from different vendors are chosen, their SIR performances of "gooey" flux residue and the properties that may affect their SIR performances are studied.
Conclusions
Under extreme design levels and harsh test condition, the SIR reliability issue of QFNs should be concerned. In order to avoid the failure of QFNs when they are used, the full cooperation in industry is needed.
When a QFN component is designed, if the pads always have bias voltage when the component is in normal operation, the size of these pads should be as small as possible while the airgap should be as big as possible. When a QFN component is assembled on PCB, under the premise of ensuring assembly quality and solder joint reliability, the "gooey" flux residue between the pads those always have bias voltage should be reduced as less as possible by properly optimizing the apertures of stencil and reflow profile.
In addition, before a QFN is assembled on PCB, a solder paste that has better SIR reliability of "gooey" flux residue should be chosen. Of course, the premise is to establish a standard method and corresponding test vehicle that could effectively evaluate the SIR reliability of "gooey" flux residue, such as IPC-TM-650 method 2.6.3.7 and IPC-B-24 test vehicle for dry flux residue. In this test, a test method is optimized but it is not fully mature yet and should be optimized further.
As to the paste flux itself, the SIR reliability of "gooey" flux residue may be related to its conductivity and the content of activators after reflow.
Initially Published in the SMTA Proceedings
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