SLDA021B March   2014  – February 2020 AM3892 , AM3894

 

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2.1.1 Non-Solder-Mask-Defined (NSMD) Land

With NSMD-configured pads, there is a gap between the solder mask and the circular contact pad (refer to Figure 1). With this configuration, the solder flows over the top surface and the sides of the contact pad.

NSMD lands have these advantages:

  • The additional NSMD soldering area results in a stronger mechanical bond
  • NSMD pads are smaller than SMD pads, allowing more room for escape trace routing

A disadvantage of the NSMD land is that surrounding traces can be exposed if not properly dimensioned, providing potential for short circuits during ball attach and reflow.

Table 1 shows optimum solder mask opening land pad diameters for the package and the PCB for a flip chip BGA with 0.65-mm pitch. The solder mask on NSMD lands is considered a PCB fabrication process defect.

Table 1. Recommended PCB Land Pattern Design Guidelines(1)

Ball Pitch Ball Size(2) Solder Mask Type PCB Design Stencil Design Area Aspect Ratio(3)(4)
SMO Pad Size Thickness Aperture
0.65 0.4 NSMD 0.45 0.35 0.10–0.125 0.35 0.69
(1) All measurement are in mm.
(2) Ball size, SMO(solder mask opening), Pad Size, and Aperture are shown as diameters.
(3) Area Aspect Ratio = Area of Aperture / Area of Aperture Wall
(4) For optimal release of solder paste, it is recommended the Area Aspect Ratio ≥ 0.66.