Ultra-fine pitch devices pose new PCB design issues

Ball-grid array (BGA) packages come in a variety of pitches and sizes. As device complexity increases and OEMs continue their drive toward smaller components, ball pitches of 0.5 mm and lower are becoming more popular.

Pitch is defined as the space between the center of one BGA ball to the center of the next one. Today, you find 0.4 mm pitch BGAs in virtually every smartphone, and 0.3 mm ultra-fine pitch BGAs are the next generation. Currently, the major transition taking place with a move from 0.5mm to 0.4m, initially in ASICs (application specific integrated circuits) and WLCSPs (wafer level chip scale packages).

The next step is to increase functionality within the same real estate. This is the next generation where 0.3mm pitch has been selected. Early OEM adopters are venturing into this unfamiliar and uncharted territory. Most are unknowingly relying on previous 0.5 mm pitch design guidelines since the IPC (Association of Connecting Electronics Industries) has not as of this writing set forth design standards for 0.3mm pitch. Unfortunately, these OEMs will soon confront certain particular issues by placing their faith on 0.5mm pitch guidelines.

The most important and difficult aspect of printed circuit board (PCB) designs using fine pitch BGAs is the layout of their “lands” and fan outs. PCB lands are where device balls sit and get soldered to. Fan outs are traces from the device lands to adjacent via. A via is required to distribute the I/Os, powers and grounds from the device to the peripherals and typically there is one via per land, as shown in Figure 1 below.

Figure 1. A via distributes I/O, power and ground signals from the devices to the peripherals.

Design missteps incurred at PCB layout when the designer innocently takes the 0.5mm pitch design route can have adverse effects on board fabrication and assembly. A wide range of issues arises in those areas to include problems associated with stencils, solder paste, and pick and place, among others. In particular, the designer must be aware of the current manufacturing capabilities in addition to the PCB’s electrical requirements.

As the industry makes greater inroads into 0.3mm ultra-fine pitch BGAs, the industry is already pushing current manufacturing capabilities to the limits. In other words, there’s concern focused specifically on fabrication registration tolerances, as well as on the device mounting tolerances of pick and place machines on the assembly floor.

Typical tolerances for both have been +/- 3mils. But recent advances in manufacturing technology and equipment have reduced them to +/-1 mils. Additional equipment is also required for such fine pitches for manufacturing validation and rework. This means 3D X-ray may be necessary for validation, and advanced BGA placement/rework stations are required for rework.

Hence, the layout must not in any way add to the problems to the already difficult tasks of board fabrication and assembly. If land size and solder mask openings are not discussed and finalized with the manufacturing group in the initial stages of the layout, it may be the difference between a working product and one that fails during testing or inhibits intermittent failures in the field.

Consequently, it behooves the PCB designer to show diligence while laying out with fine pitch BGAs and things like type of solder paste used, equipment tolerances, stencil thicknesses and type of under-fill used, should be taken into account.