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                                                                     EXHIBIT 5.1


                                  July 9, 2001


PDF Solutions, Inc.
333 West San Carlos Street
Suite 700
San Jose, CA 95110


         Re: PDF Solutions, Inc.
             Registration Statement on Form S-1

Ladies and Gentlemen:

         At your request, we are rendering this opinion in connection with a
proposed sale by PDF Solutions, Inc., a Delaware corporation (the "Company") of
up to 5,175,000 shares of common stock, $0.00015 par value (the "Common Stock").

         We have examined instruments, documents, and records which we deemed
relevant and necessary for the basis of our opinion hereinafter expressed. In
such examination, we have assumed the following: (a) the authenticity of
original documents and the genuineness of all signatures; (b) the conformity to
the originals of all documents submitted to us as copies; and (c) the truth,
accuracy, and completeness of the information, representations, and warranties
contained in the records, documents, instruments, and certificates we have
reviewed.

         Based on such examination, we are of the opinion that the 5,175,000
shares of Common Stock to be issued and sold by the Company (of which up to
675,000 shares are to be issued to cover over-allotments, if any), are duly
authorized and will be, when issued against payment of the purchase price
therefor, legally issued, fully paid and nonassessable.

         We hereby consent to the filing of this opinion as an exhibit to the
above-referenced Registration Statement and to the use of our name wherever it
appears in said Registration Statement, including the Prospectus constituting a
part thereof, as originally filed or as subsequently amended or supplemented. In
giving such consent, we do not consider that we are "experts" within the meaning
of such term as used in the Securities Act of 1933, as amended, or the rules and
regulations of the Securities and Exchange Commission issued thereunder, with
respect to any part of the Registration Statement, including this opinion as an
exhibit or otherwise.

                                      Very truly yours,

                                      /s/ ORRICK, HERRINGTON & SUTCLIFFE LLP