Sun Microsystems' long-time chip chief David Yen has left the building for an undisclosed role at Juniper Networks.

Yen most recently served as the EVP in charge of Sun Microelectronics. In the past, he has been responsible for Sun's storage division, UltraSPARC-based servers and microprocessor efforts. Sun has tapped Technology Fellow Mike Splain as the new head of the Microelectronics group.

Juniper refused to tell us what role Yen will hold, saying that it will disclose this information on April 7. Apparently, Yen's departure from Sun is not official until April 4.

Yen enjoyed a distinguished, almost 20-year career at Sun, leading the company's silicon battles against the likes of Intel, IBM and HP. Recently, however, his track record has been so-so.

Sun, for example, was forced to cancel or delay a number of UltraSPARC projects over the past few years. In addition, Yen failed to improve Sun's fortunes in the storage game during his time as head of the division.

We were once told that Yen threatened to leave Sun a couple of years back only to have the company's top brass offer him a lucrative "feel better" package. McNealy and Co. feared losing the man who knew where all the UltraSPARC bodies were buried and the inner-workings of such a complex business.

[Update: Yen denies that the above ever took place. You'll find his rebuttal in the comments section.]

It's intriguing to postulate about what Juniper might be up to with Yen. Juniper seems to be on an expansion kick, having entered the Ethernet switch game in Jan. Yen could work on networking silicon or perhaps some efforts around new types of hardware. ®