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24/02/2009
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Electronics engineers have an ever increasing range of components from which to choose for use in their next designs
 . But a common trend amongst all those components in increasing complexity.
Not only are significant numbers of parts being supplied in high pin count packages, there’s a range of voltages appearing – and the trend for supply voltages is ever lower.
While the functionality provided by these latest devices enables innovative products to be developed, spare a thought for the poor old pcb layout engineer. There are some high complexity boards being designed which are not only getting bigger, but which also require a range of power distribution networks to be included.
John Isaac, director of market development for Mentor Graphics’ systems design division, put the moves in context. “Today’s business drivers are product differentiation, cost reduction and time to market. Any company that doesn’t come out with a better product at lower cost won’t get the sales.”
Isaac pointed out that, until recently, the majority of components used on pcbs required a 5V supply. “Today,” he continued, “to keep power consumption in check, most ics are moving to lower voltages – but there’s a mix of voltages.”
The effect is felt downstream at the pcb design stage. “When everything required a 5V supply,” Isaac observed, “you could create one power plane and life was good. Today, in the more complex boards being developed, you can have up to 30 power distribution networks. But you can’t put each network on a separate plane, because that’s a 30 layer board without anything else.
“The power networks in modern boards need to be designed carefully,” he contended, “so good voltage and current levels can be provided to all parts on the board.” Incorrect design creates major problems, including ground bounce issues. “Companies typically design a distribution network and hope it delivers,” he continued. “And, to cope with noise issues, they add lots of decoupling capacitors and hope it all works.”
Getting the power network wrong, of course, means respins – and that has time to market and product cost implications. Isaac pointed out one example. “We have customers – Alcatel-Lucent is one – that have had to respin boards two or three times. But more and more customers are realising that up front investment [in the appropriate tools] means fewer respins. That cuts time to market.”
In the absence of the appropriate tools, engineers tend to fall back on that old favourite – over design. “These will be conservative designs,” Isaac believed, “that might not get optimal performance, the costs may be increased and manufacturing time might be longer.”
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Author
Graham Pitcher
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