Attorneys who once represented Qualcomm Inc. in its ill-fated federal patent case against Broadcom Corp. have asked a judge to pierce their client’s privileged communications.

With the threat of formal sanctions bearing down on them, lawyers at Heller Ehrman and Day Casebeer Madrid & Batchelder — Qualcomm’s former litigation counsel — are asking for a rare exception to privilege so they can explain to the judge how their side failed to produce hundreds of thousands of relevant documents during discovery in the San Diego case.