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Conquer 0.3mm ultra-fine pitch device challenge in PCB design

Posted: 20 Oct 2011     Print Version  Bookmark and Share

Keywords:ball grid arrays  chipscale packages  printed circuit board 

A significant majority of the electronics OEM market is rapidly moving towards increasingly greater product miniaturisation 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, 0.3mm ultra-fine pitch micro chipscale packages (CSPs), micro ball grid arrays (BGAs), 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.

Challenges

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 per cent less than the diameter of a BGA/CSP solder-ball. Figure 1 shows a solder-ball-joint pad with the diameter of 20 per cent less than the diameter of a CSP/BGA solder-ball (A = B = 80% x ball-diameter).

Figure 1: Here's a solder-ball-joint pad with the diameter of 20 per cent 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, the solder-ball-joint pad with a 20 per cent 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 centre position.

Figure 2a: Here's a dog-bone style layout for CSP/BGA pad. (Source: Actel Corp. application note).

Figure 2b: Here's a side view of NSMD CSP/BGA land pads and SMD CSP/BGA land pads (Source: Altera Corp. application note).

Figure 2c: Here's a side view of NSMD CSP/BGA solder joints and NSMD CSP/BGA solder joints (Source: Altera Corp. application note).


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