Taking on the 0.3 mm ultra-fine pitch device challenge in PCB design
A significant majority of the electronics OEM market is rapidly moving toward increasingly greater product miniaturization along with higher levels of functional integration.
The best examples are the new crops of smart phones and mobile/wireless devices. And whether the industry is ready or not, 0.3 millimeter (mm) ultra-fine pitch micro CSPs (chipscale packages), micro BGAs (ball grid arrays), and other active devices are quickly being ushered in to lead the designs of newer generations of thinner, faster, and sleeker portable, handheld electronics and communications devices.
Consequently, printed circuit board (PCB) assembly – with or without the benefit of proven 0.3 mm pitch technology know how -- is at the top of this totem pole with the burden placed on contract manufacturers (CMs) and electronic manufacturing service (EMS) providers to resolve related issues.
Or, you can think of it this way. Numbers of naïve electronics sub-assembly houses and OEMs are inadvertently forging ahead using earlier generation layout rules, but remaining innocent of the consequences.
Let’s start out by saying that currently there are no formal design guidelines or layout rules specifically tailored at supporting 0.3 mm pitch CSPs. The electronics industry hasn’t come up with the highly important specifications and expertise to perform 0.3 mm pitch design and layout projects.
As a result, today, many PCB design and layout engineers largely rely on traditional 0.5 mm pitch IPC design guidelines and layout rules to develop new 0.3 mm pitch devices-based designs.
For example, the older design guidelines allow PCB design and layout engineers to design a solder-ball-joint pad with the diameter of 20 percent less than the diameter of a BGA/CSP solder-ball. Figure 1 below shows a solder-ball-joint pad with the diameter of 20 percent less than the diameter of a CSP/BGA solder-ball (A = B = 80% x ball-diameter).
Figure 1. A solder-ball-joint pad with the diameter of 20 percent less than the diameter of a CSP/BGA solder-ball (A = B = 80% x Ball-Diameter). (Source: Agilent application note)
As show in Figures 2a through 2c below, the solder-ball-joint pad with a 20 percent less diameter of a CSP/BGA solder ball allows PCB designers to design a dog-bone style layout, NSMD non-solder-mask-defined (NSMD) pads with non-solder mask covered clearance between the copper and solder mask, and to design solder-mask defined (SMD) pads with low-accuracy solder mask defined pad dimensions and low-accuracy pad center position.
Fig. 2a - A dog-bone style layout for CSP/BGA pad. (Source: Actel Corporation application note)
Fig. 2b – Side view of NSMD CSP/BGA land pads and SMD CSP/BGA land pads (Source: Altera Corporation application note).
Fig. 2c – Side view of NSMD CSP/BGA solder joints and NSMD CSP/BGA solder joints (Source: Altera Corporation application note)
Now, consider that the solder-ball dimension for 0.3 mm pitch mini-CSP components is extremely small – less than 8.2 mil. After a 20 percent reduction as used in 0.5 mm pitch design guidelines, the CSP pad dimension is considerably smaller.