Several new stencil designs on QFN thermal pads were tested. Data was gathered and statistical analysis performed to compare and contrast the voiding data. What was learned from this work is summarized below:
  • Solder powder particle size affects voiding for some solder pastes. Voiding tends to decrease as solder powder size becomes smaller.
  • Solder alloy affects voiding and seems to follow a trend with melting range. Wider melting ranges tend to generate lower voiding levels.
  • Organic solderability preservative (OSP) surface finish gave higher voiding than electroless nickel immersion gold (ENIG) surface finish for a no clean lead free solder paste that was tested. Two different water soluble lead free solder pastes showed similar voiding levels for OSP and ENIG finishes.
  • New “low voiding” solder pastes are becoming more common in our industry. In this work, low voiding solder paste F showed lower voiding than an industry standard solder paste C.
  • Stencil design has an impact on voiding. In this study, area of printed solder paste coverage was varied. Voiding levels were much higher for 50 to 60% solder paste coverage than for 70 to 80% solder paste coverage. Based on the results of this work, the following recommendations were made to reduce voiding.
  • Use of smaller solder powder sizes can reduce voiding with certain types of solder paste.
  • Use of specific solder alloys may help mitigate voiding.
  • Use a solder paste that works well with the surface finish to minimize voiding.
  • Use “low voiding” solder pastes to reduce the overall potential for voiding.
  • Optimize the stencil design for the components to allow for complete wetting and gas escape routes. Applying these recommendations may create issues on the printed circuit board assembly (PCBA). For example, use of smaller solder powder particle sizes may lead to increased random solder balling and/or graping. Increasing the volume of printed solder paste may lead to bridging for micro BGA and ultra-fine pitch components. Reduction of voiding may be important enough to risk these potential issues. This is left to the reader's judgement. It is the intent of this work to analyze general trends in voiding and give recommendations to help the reader to "Fill the Void."">

    Journal of SMT Article

    Fill The Void III

    Author: Tony Lentz
    Company: FCT Assembly
    Date Published: 6/30/2018   Volume: 31-2

    Abstract: This study is part three in a series of papers on voiding in solder joints and methods for mitigation of voids. In this study several new variables were tested and compared to previous data on voiding. A new circuit board design was used which is different than the circuit board used in previous studies. The new circuit board design includes two sizes of QFNs (Quad Flat No Lead), BGA's (Ball Grid Array), and LGA (Land Grid Array) components which are susceptible to voiding.

    The following variables were evaluated with respect to voiding:

  • Solder powder size was varied using a no-clean lead-free solder paste and IPC Type 3, Type 4 and Type 5 SAC305 solder powders.
  • Solder alloy was studied using SAC305 alloy, SN100C® alloy, SN100CV® alloy and a mixture of SAC305/SN100C alloys.
  • Surface finish on the circuit board is thought to have an impact on voiding. Organic solderability preservative (OSP) surface finish is more difficult to wet with solder than other surface finishes and in theory should produce higher voiding levels, and this was evaluated with multiple solder pastes.
  • The performance of a new "low voiding" no clean solder paste was compared to other industry standard no clean solder pastes.
  • Several new stencil designs on QFN thermal pads were tested.

    Data was gathered and statistical analysis performed to compare and contrast the voiding data. What was learned from this work is summarized below:

  • Solder powder particle size affects voiding for some solder pastes. Voiding tends to decrease as solder powder size becomes smaller.
  • Solder alloy affects voiding and seems to follow a trend with melting range. Wider melting ranges tend to generate lower voiding levels.
  • Organic solderability preservative (OSP) surface finish gave higher voiding than electroless nickel immersion gold (ENIG) surface finish for a no clean lead free solder paste that was tested. Two different water soluble lead free solder pastes showed similar voiding levels for OSP and ENIG finishes.
  • New “low voiding” solder pastes are becoming more common in our industry. In this work, low voiding solder paste F showed lower voiding than an industry standard solder paste C.
  • Stencil design has an impact on voiding. In this study, area of printed solder paste coverage was varied. Voiding levels were much higher for 50 to 60% solder paste coverage than for 70 to 80% solder paste coverage.

    Based on the results of this work, the following recommendations were made to reduce voiding.

  • Use of smaller solder powder sizes can reduce voiding with certain types of solder paste.
  • Use of specific solder alloys may help mitigate voiding.
  • Use a solder paste that works well with the surface finish to minimize voiding.
  • Use “low voiding” solder pastes to reduce the overall potential for voiding.
  • Optimize the stencil design for the components to allow for complete wetting and gas escape routes.

    Applying these recommendations may create issues on the printed circuit board assembly (PCBA). For example, use of smaller solder powder particle sizes may lead to increased random solder balling and/or graping. Increasing the volume of printed solder paste may lead to bridging for micro BGA and ultra-fine pitch components. Reduction of voiding may be important enough to risk these potential issues. This is left to the reader's judgement. It is the intent of this work to analyze general trends in voiding and give recommendations to help the reader to "Fill the Void."

    Keywords: 

    voids, solder joint, solder paste, solder powder size, solder alloy, surface finish, stencil design, QFN



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