How to succeed the first time with ultra-small QFN packages

Mobile device designers see anything small as positive so desinging with small packages doesn't require much of a "sell" for them.

On the other hand, many industrial device designers, unaccustomed to real-estate restrictions, see small as a disadvantage. The QFN (quad flat pack, no leads) along with the micro-BGA (ball grid array) seem to be the package of choice for new RF and control devices.

With many of these new parts, the opinion of the designer is moot. Small is the only option. Despite the difficulties involved in moving down to parts with .5mm lead pitch that can't be hand assembled for the prototype, there are a number of advantages relating to ground path and thermal management.

Like it or not, designers using the new RF standards, such as 802.15.4 and the upcoming 802.11n will have to learn how to effectively utilize these small packages.

ZigBee example
Take a scenario that involves adding wireless into the redesign of a distributed process monitor and control system. The design pairs a Texas Instruments MSP430F1232 microcontroller (MCU) and a Freescale MC13192 based ZigBee radio into each remote device for more computing power, lower power consumption and wireless communications capability.

The MCU is available in several packages, including a QFN-32 but the radio part only comes in a 5mm x 5mm QFN-32 package. A PG2012TK L-band GaAs FET switch is also required and that part only comes in a 1.1mm x 1.5mm leadless minimold package.

The big volumes today are in the mobile world so chip companies are catering to that market first by introducing products in increasingly smaller packages and in many cases, never releasing in larger form factors. By using such small parts, the MCU and radio were placed very close to each other and both were placed at an optimum distance from the antenna.

Most QFN packages have an exposed metal pad covering most of the underside of the component. Generally, the metal pad is used for grounding and heat management. This puts the ground plane within as little as a millimeter away from the power connection as well as all signal pads.

The silicon is directly bonded to that center metal pad which greatly helps to conduct heat away. Even if the size advantage isn't important, the ability to locate capacitors so close to the signals, exercise control over the physical distance of the part from the antenna and so effectively draw heat away from the chip die gives the QFN package a distinct appeal in high-speed and RF designs.

QFN Float
At a recent trade show, I received a sample part in a 3 x 3 mm QFN package. While I haven't tried it yet, I'm pretty sure that, like a water bug, the part is light enough to float on a water surface because it doesn't have enough weight to break the surface tension. But, that's not what I'm talking about.

Like many high-speed QFN packaged devices, the middle of the part has an exposed metal contact pad covering most of the underside. The float that I'm talking about happens when there is too much solder paste on the pcb for that center pad. Like the water bug, the QFN doesn't have enough mass to sink down to the board.

On any surface mount device, to a small extent, the height of the solder paste deposit is proportional to the aperture in the solder stencil opening (bigger opening = taller deposit). With most parts, that isn't a problem because either all of the pads are big enough so that that ratio doesn't have a first order impact, or because all of the pads are the same size and will be equally impacted.

Since the QFN center pad is a much larger opening in the stencil than the signal pad openings, and the signal pad openings are in the 10 - 20 mil or less range, this deposit height-to-width ratio will have a first order impact.

When the opening for the center pad on the QFN is too large, the solder paste deposit in the center may be taller than the deposits on the small signal pin pads. The part high-centers and never gets the opportunity to contact the signal pads. In some cases, the part will tilt a little sideways and contact some of the signal pads but not all.

Figure 1. Difference between a QFM package that is floating up due to excessive solder in the center pad area and a second QFN that is properly assembled.