UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. 6)*

PDF SOLUTIONS INC

(Name of Issuer)

COMMON STOCK

(Title of Class of Securities)

693282105

(CUSIP Number)

December 31, 2009

(Date of Event which Requires Filing of Statement)

Check the appropriate box to designate the Rule pursuant to which
this Schedule is filed:

[x] Rule 13d - 1(b)

Rule 13d - 1(c)

Rule 13d - 1(d)

*The remainder of this cover page shall be filled out for a
reporting person's initial filing on this form with respect to the
subject class of securities, and for any subsequent amendment
containing information which would alter disclosures provided in a
prior cover page.

The information required on the remainder of this page shall not be
deemed to be "filed" for the purpose of Section 18 of the
Securities Exchange Act of 1934 ("Act") or otherwise subject to the
liabilities of that section of the Act but shall be subject to all
other provisions of the Act (however, see the Notes.)

(Continued on following page(s)
Page 1 of 6 Pages

CUSIP NO. 693282105           13G             Page 2 of 6 Pages

1  Name of Reporting Person
S.S. or I.R.S. Identification No. of Above Person

T. ROWE PRICE ASSOCIATES, INC.
52-0556948

2  Check the Appropriate Box if a Member of a Group*
(a) ____
NOT APPLICABLE                                     (b) ____

3  SEC Use Only

____________________________________

4  Citizenship or Place of Organization

MARYLAND

Number of      5  Sole Voting Power
**
Shares            239,400

Beneficially   6  Shared Voting Power
**
Owned By Each     -0-

Reporting      7  Sole Dispositive Power
**
Person            2,679,101

With           8  Shared Dispositive Power

-0-

9  Aggregate Amount Beneficially Owned by Each Reporting Person

2,679,101

10 Check Box if the Aggregate Amount in Row (9) Excludes Certain
Shares*

NOT APPLICABLE

11 Percent of Class Represented by Amount in Row 9

10.0%

12 Type of Reporting Person*

IA
*SEE INSTRUCTION BEFORE FILLING OUT!
**Any shares reported in Items 5 and 6 are also
reported in Item 7.
CUSIP NO. 693282105             13G       Page 3 of 6 Pages

1  Name of Reporting Person
S.S. or I.R.S. Identification No. of Above Person

T. ROWE PRICE SMALL CAP STOCK FUND, INC.
23-1622210

2  Check the Appropriate Box if a Member of a Group*
(a) ____
NOT APPLICABLE                                     (b) ____

3  SEC Use Only
______________________________

4  Citizenship or Place of Organization

Maryland

Number of      5  Sole Voting Power
**
Shares            1,350,000

Beneficially   6  Shared Voting Power
**
Owned By Each     NONE

Reporting      7  Sole Dispositive Power
**
Person            NONE

With           8  Shared Dispositive Power

NONE

9  Aggregate Amount Beneficially Owned by Each Reporting Person

1,350,000

10 Check Box if the Aggregate Amount in Row (9) Excludes Certain
Shares*

NOT APPLICABLE

11 Percent of Class Represented by Amount in Row 9

5.0%

12 Type of Reporting Person*

IV
*SEE INSTRUCTION BEFORE FILLING OUT!
**The aggregate amount reported on this page is also included in
the aggregate amount reported by T. Rowe Price Associates, Inc. on
page 2 of this Schedule 13G.
SCHEDULE 13G
PAGE 4 OF 6

Item 1(a) Name of Issuer:

Reference is made to page 1 of this Schedule 13G

Item 1(b) Address of Issuer's Principal Executive Offices:

333 WEST SAN CARLOS ST, STE 700, SAN JOSE, CA  95110

Item 2(a) Name of Person(s) Filing:

(1)  T. Rowe Price Associates, Inc. ("Price Associates")

(2) T. Rowe Price Small Cap Stock Fund, Inc.

X       Attached as Exhibit A is a copy of an agreement between
the Persons Filing (as specified hereinabove) that this
Schedule 13G is being filed on behalf of each of them.

Item 2(b) Address of Principal Business Office:

100 E. Pratt Street, Baltimore, Maryland 21202

Item 2(c) Citizenship or Place of Organization:

(1) Maryland

(2) Maryland

Item 2(d) Title of Class of Securities:

Reference is made to page 1 of this Schedule 13G

Item 2(e) CUSIP Number: 693282105

Item 3    The person filing this Schedule 13G is an:

X       Investment Adviser registered under Section 203 of the
Investment Advisers Act of 1940

X       Investment Company registered under Section 8 of the
Investment Company Act of 1940

SCHEDULE 13G
PAGE 5 OF 6

Item 5 Ownership of Five Percent or Less of a Class.

X    Not Applicable.

This statement is being filed to report the fact that, as
of the date of this report, the reporting person(s) has
(have) ceased to be the beneficial owner of more than five
percent of the class of securities.

Item 6 Ownership of More than Five Percent on Behalf of Another
Person

(1) Price Associates does not serve as custodian of the
assets of any of its clients; accordingly, in each
instance only the client or the client's custodian or
trustee bank has the right to receive dividends paid
with respect to, and proceeds from the sale of, such
securities.

The ultimate power to direct the receipt of dividends
paid with respect to, and the proceeds from the sale
of, such securities, is vested in the individual and
institutional clients which Price Associates serves as
investment adviser.  Any and all discretionary
authority which has been delegated to Price Associates
may be revoked in whole or in part at any time.

Except as may be indicated if this is a joint filing
with one of the registered investment companies
sponsored by Price Associates which it also serves as
investment adviser ("T. Rowe Price Funds"), not more
than 5% of the class of such securities is owned by any
one client subject to the investment advice of Price
Associates.

(2) With respect to securities owned by any one of the T.
Rowe Price Funds, only State Street Bank and Trust
Company, as custodian for each of such Funds, has the
right to receive dividends paid with respect to, and
proceeds from the sale of, such securities.  No other
person is known to have such right, except that the
shareholders of each such Fund participate
proportionately in any dividends and distributions so
paid.

Item 7 Identification and Classification of the Subsidiary Which
Acquired the Security Being Reported on By the Parent
Holding Company.

Not Applicable.

Item 8 Identification and Classification of Members of the Group.

Not Applicable.
SCHEDULE 13G
PAGE 6 OF 6

Item 9   Notice of Dissolution of Group.

Not Applicable.

Item 10  Certification.

By signing below I (we) certify that, to the best of my
(our) knowledge and belief, the securities referred to
above were acquired in the ordinary course of business
and were not acquired for the purpose of and do not have
the effect of changing or influencing the control of the
issuer of such securities and were not acquired in
connection with or as a participant in any transaction
having such purpose or effect.  T. Rowe Price Associates,
Inc. hereby declares and affirms that the filing of
Schedule 13G shall not be construed as an admission that
Price Associates is the beneficial owner of the
securities referred to, which beneficial ownership is
expressly denied.

Signature.

After reasonable inquiry and to the best of my (our)
knowledge and belief, I (we) certify that the information
set forth in this statement is true, complete and
correct.

Dated: February 12, 2010           Dated: February 12, 2010

T. ROWE PRICE SMALL CAP            T. ROWE PRICE ASSOCIATES, INC.
STOCK FUND, INC.

By: /s/ David Oestreicher          By: /s/ David Oestreicher
David Oestreicher,                David Oestreicher,
Vice President                    Vice President

Note:    This Schedule 13G, including all exhibits, must be filed
with the Securities and Exchange Commission, and a copy
hereof must be sent to the issuer by registered or
certified mail not later than February 14th following the
calendar year covered by the statement or within the time
specified in Rule 13d-1(b)(2), if applicable.

12/31/2009
EXHIBIT A

AGREEMENT

JOINT FILING OF SCHEDULE 13G

T. Rowe Price Associates, Inc. (an investment adviser
registered under the Investment Advisers Act of 1940) and T. Rowe
Price Small Cap Stock Fund, Inc., a Maryland corporation, hereby
agree to file jointly the statement on Schedule 13G to which this
Agreement is attached, and any amendments thereto which may be
deemed necessary, pursuant to Regulation 13D-G under the
Securities Exchange Act of 1934.

It is understood and agreed that each of the parties hereto
is responsible for the timely filing of such statement and any
amendments thereto, and for the completeness and accuracy of the
information concerning such party contained therein, but such
party is not responsible for the completeness or accuracy of
information concerning the other party unless such party knows or
has reason to believe that such information is inaccurate.

It is understood and agreed that a copy of this Agreement
shall be attached as an exhibit to the statement on Schedule 13G,
and any amendments hereto, filed on behalf of each of the parties
hereto.

Dated: February 12, 2010           Dated:  February 12, 2010

T. ROWE PRICE SMALL CAP            T. ROWE PRICE ASSOCIATES, INC.
STOCK FUND, INC.

By: /s/ David Oestreicher          By: /s/ David Oestreicher
David Oestreicher,                David Oestreicher,
Vice President                    Vice President