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Leading Edge Technology Doesn’t Need To Translate To Bleeding Edge Prototypes

Tue, 12/02/2008 - 5:59am

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By Duane Benson, Marketing Manager, Screaming Circuits

Miniaturization of larger form factor power and RF components is one recent trend that has made the pcb layout job even more challenging. However, use of this leading edge technology doesn’t need to equal bleeding edge prototypes. 

QFN with small passives nearby

The electronics industry’s mantra has long been smaller, faster, cheaper. The engineering team at prototype assembly house Screaming Circuits in Canby, OR often sees that philosophy pushed to the limit.

Miniaturization of larger form factor power and RF components is one recent trend that has made the pcb layout job even more challenging. However, use of this leading edge technology doesn’t need to equal bleeding edge prototypes. 

This article focuses on trends in this area and key issues to watch when using smaller power and RF components.

Cost reduction and the exploding market for hand-held devices are the major drivers for miniaturization. In the RF realm, Bluetooth, wi-fi and Zig-Bee components are often only available in QFN or microBGA packages. Power components are now being produced as QFNs or chip scale BGAs. 

Texas Instruments TPS62601, 1mm x 1.5mm step-down, bottom side next to dime

Technology Drivers Cost reduction and the exploding market for hand-held devices are the major drivers for miniaturization. In the RF realm, Bluetooth, wi-fi and Zig-Bee components are often only available in QFN or microBGA packages. Power components are now being produced as QFNs or chip scale BGAs. 

Some amazing next generation components include Allegro’s A3901 dual motor control that will drive 800 mA of current through a 3mm x 3mm DFN, and TI’s chip scale TPS62601, 1.8 volt, 500mA step-down converter in a 1mm x 1.5mm, .4mm ball pitch BGA.

Issues to Watch in the Design Phase

Unique Component Specifications CAD software parts libraries often lack land patterns for newer components, such as QFNs. Picking a land pattern that is close is not a good solution. The better option is to create a custom part for the component library. This is particularly important when specialized solder paste openings or copper patterns are required. 

For instance, too much solder paste on the flag pad of a QFN can cause the part to float, preventing a reliable connection. It is better to segment the solder paste layer in the custom component library to provide 50% solder paste coverage.

Another area where customization is important is high frequency chips. These components may require specialized copper and solder paste patterns in the ground area. It is important to check both the manufacturer’s data sheet and relevant application notes to determine if a specific pattern is required.

Many of the smaller power and RF packages are only available in RoHS-compatible versions. The positive is these newer components are designed for RoHS-processes and may be more robust than older component options. The negative is that if the design requires some leaded components, process incompatibilities may exist.

Texas Instruments TPS62601, 1mm x 1.5mm step-down, top side next to dime

Compatible Component Availability Many of the smaller power and RF packages are only available in RoHS-compatible versions. The positive is these newer components are designed for RoHS-processes and may be more robust than older component options. The negative is that if the design requires some leaded components, process incompatibilities may exist.

In the case of older BGAs, microBGAs and CSPs it is unacceptable to mix lead and no-lead. If lead and no-lead must be mixed, it is easier to put a lead-free BGA on a leaded board than the converse. The end result is not a reliable board, but may be acceptable for prototyping. The converse situation may overheat the solder balls and the component.

Thermal Issues QFNs may need to have thermal vias in the pad and a thermal pad on the back side of the board. Open vias are unacceptable, but QFN vias can sometimes be capped with solder mask. Chip scale, high power, miniature BGAs may require thermal or escape vias in the pads and those have to be filled and plated with metal. 

Larger components such as metal can capacitors or large inductors next to small high power devices can cause thermal issues during assembly. It is important to analyze airflow not just for operation, but also for reflow.

Chip scale BGAs and QFNs may drive the selection of small passives which are more susceptible to tombstoning. For example, when a small trace is going to one pad and a large trace is going to another pad, the large trace will act as a heat sink.

ENIG surfaced PCB with passives nearby

Chip scale BGAs and QFNs may drive the selection of small passives which are more susceptible to tombstoning. For example, when a small trace is going to one pad and a large trace is going to another pad, the large trace will act as a heat sink.

The smaller pad melts first and surface tension causes the large trace’s side to pop up. Inner copper layers under one pad, but not another, can cause the same effect. Larger thermal mass components can also cause this. Another common cause of tombstoning is a thick solder mask. Good mask registration will help.

Using a board vendor that has tight tolerances and delivers a thin flat mask surface will also help. In some cases, with the smallest parts, you may want to keep the soldermask off the pads by using non-solder mask defined (NSMD) pads.

Conclusion Design teams will continue to be challenged to adapt leading edge technology to keep their companies at the head of the pack. Considering likely challenges in PCB and PCBA assembly during the design phase speeds product development time, saves money and improves product quality.

The Screaming Circuits blog http://blog.screamingcircuits.com/ regularly discusses the issues listed here in greater detail. Feel free to visit and learn more.

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