Taking on the 0.3 mm ultra-fine pitch device challenge in PCB design
It becomes obvious that the stage is set for creating innumerable solder defects, resulting in a rather dismal final yield. PCB designers use solder-mask defined pads when following the 0.5 mm pitch design guidelines.
There is a +/- 3-mil tolerance for the solder-mask layer aperture and very small pitch 11.8 mil (0.3 mm) as shown in Figure 3 below. Therefore, the very small solder-mask (less than 4 – 5 mil) cannot securely attach and exist between the two pads.
Figure 3 – Bottom view and cross-section of an example of two-layer CSP. The 0.3 mm pitch means that the distance is 11.8 mil between the center of solder-ball and the center of next solder-ball. (Source: Actel Corporation application note)
Distance is 11.8 mils between the center of a pad and the center of next pad. This is a 40 percent pitch reduction. Lost here are 7.1 mils to accommodate the mask tolerance and the gap to securely holding the mask-layer to prevent a solder bridging defect. Traditional design guidelines require at least 6-mil solder mask gaps between the two nearby solder-pads to achieve solder-joint quality and reliability.
Now, when it comes to the new 0.3 mm ultra-fine pitch technology, PCB design engineers aren’t able to find sufficient room for a 6- mil solder mask gap. There is no room for dog-bone style pad layout; and no room for non-solder mask covered clearance between the copper and solder mask.
It is too risky to allow solder-paste above the SMD mask layer for the SMD pad layout and to allow solder-paste above the non-mask areas for NSMD layout. Both SMD land pad and NSMD land pad have very high defect tendency for solder-bridging, solder-splashing, insufficient solder and open solder defects. Basically, the DFM requirement is in conflict with the layout requirement from the 0.3 mm pitch semiconductor manufacturer’s spec.
Current PCB fabrication limitations cause a lot of problems for 0.3 mm pitch technology. For example, there is +/- 3-mil mask-layer aperture tolerance and a +/- 1-mil copper-pad tolerance. Standard mask technology cannot securely apply a mask-layer within the very small areas (less than 4 – 5 mil).
What is needed is at least 6 mils for the solder mask attachment. The +/- 3-mil mask-layer aperture error will cause the pick-and-placement machine (P&P) (with even with best machine vision alignment capability) to miss the fab ball-pads. The fab ball-pad of the CSP is too small (4 – 6 mil per 20% reduction rule) for the P&P machine. The resulting solder-joints will have open solder and insufficient solder defects due to standard fabrication technology limitations.
The 0.3 mm pitch assembly process also has a numbers of issues associated with stencil limitations, which greatly reduces the final assembly yield. One is solder-paste release from the stencil is not effective due to the small stencil aperture and mask/pad tolerance.
There are some of paste on the top of mask-layer because the mask-layer placement error, because the Gerber-defined stencil aperture positions (which are very accurate) are not the mask-defined pad positions (which are usually +/- 3 mil off from the Gerber-defined X-Y positions).
As a result of the extremely tight 0.3 mm pitch, solder-paste can easily form solder-bridging defects between solder-ball joints. Distance between the two joints is reduced by 40 percent (Compared to 0.5 mm pitch technology, 0.3 mm pitch is 0.2 mm less than 0.5 mm.
That means 0.3 / 0.5 = 60%). Moreover, the missing or un-secured solder-mask between the two joints makes things worse, which is another root cause of solder bridging.
In effect, a series of issues at the PCB design layout when using 0.3 mm pitch devices trigger a domino effect at the fabrication and assembly stages. Those include highly probable pitfalls relating to stencils, solder paste, and pick and place.
Most pick and place machines today can only handle 0.4 mm pitch components and not 0.3 mm pitch micro CSPs, thus creating such reflow problems as insufficient solder at the joints and solder bridging. Today, there are no effective measures to prevent mis-placement on pads with a +/- 3-mil error.
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