Indeed, the Code of Conduct for Article III Judges of the United States has specific provisions that would seem to absolutely prohibit a judge from handling a case where a litigant is a former client. Some may argue, perhaps correctly, that at some point any conflict requiring disqualification that might have existed as the result of an attorney-client relationship would subside, if not completely wash away. But after what length of time? Certainly, the answer is not after 18 months.
As was first reported by Steve Brachmann, Administrative Patent Judge Matt Clements represented Apple as patent infringement defense counsel up to his appointment as an APJ in March 2013. So far we have identified 17 final written decisions where Apple was the petitioner and Clements sat on the PTAB panel assigned to the case, with Clements authoring 4 of those decisions (see below for a full list and links). The earliest decisions we have found so far where Clements participated in Apple petitions are a series of CBM initiation decisions made in September 2014, just 18 months after he left private practice and joined the Patent Trial and Appeal Board (PTAB). These Apple CBM proceedings with September 2014 institution decisions were filed in April 2014, and obviously assigned to Clements well before the institution decisions were made public. The institution decision in CBM2014-00108 was authored by Clements. See also the institution decisions in CBM2014-00106 and CBM2014-00112.

Code of Conduct for Federal Judges

An appearance of impropriety occurs when reasonable minds, with knowledge of all the relevant circumstances disclosed by a reasonable inquiry, would conclude that the judge’s honesty, integrity, impartiality, temperament, or fitness to serve as a judge is impaired. Public confidence in the judiciary is eroded by irresponsible or improper conduct by judges. A judge must avoid all impropriety and appearance of impropriety. This prohibition applies to both professional and personal conduct. A judge must expect to be the subject of constant public scrutiny and accept freely and willingly restrictions that might be viewed as burdensome by the ordinary citizen.
The judge may participate in the proceeding if, after that disclosure, the parties and their lawyers have an opportunity to confer outside the presence of the judge, all agree in writing or on the record that the judge should not be disqualified, and the judge is then willing to participate. The agreement should be incorporated in the record of the proceeding.

Real Party in Interest

Unlike federal judges who have a specific Code of Conduct that is available for everyone to see, there seem to be no similar public Code of Conduct rules or regulations that govern Administrative Patent Judges at the PTAB. I understand that there is an internal set of guidelines that govern conflicts of interest, and there is a two-year bar under those internal USPTO conflict guidelines. Assuming that is true and there is a two-year bar, Clements participating in at least several of Apple’s petitions seems to have violated even an exceptionally lenient internal USPTO conflict guideline by participating in cases involving Apple less than 2 years after he was representing them as defense counsel.