<PAGE>
                                                                    Exhibit 4.21



                                                                  EXECUTION COPY


THIS SHARE PLEDGE AGREEMENT (the "Agreement") is entered as of this 26th day of
December, 2003 by and among:

          (i) MR. SE-HO CHUN, a shareholder of Simmtech Co. Ltd (the "Company")
          holding 39.9% of the issued and outstanding shares of the Company,
          with resident card number 560515-1005415 (the "Pledgor"); and

          (ii) ST ASSEMBLY TEST SERVICES LTD., a company duly incorporated and
          existing under the law of Singapore and having a place of business at
          5 Yishun Street 23, Singapore 768442 (the "Pledgee").


                                    RECITALS

WHEREAS:

A.   By the Loan Agreement dated as of December 26, 2003 (the "Loan Agreement")
     made by and between the Company, the Pledgor and the Pledgee, the Pledgee
     has agreed, subject to and upon the terms and conditions contained therein,
     to provide a loan to the Company in the amount of USD 15 million (USD
     15,000,000);

B.   The Pledgor is the legal and beneficial owner of the Securities (as defined
     below); and

C.   For the purposes of securing the Secured Obligations (as defined below),
     the Pledgor hereby agrees to, inter alia, deposit with and pledge to the
     Pledgee the Securities (as defined below) pursuant to terms and conditions
     herein.

NOW, THEREFORE, it is agreed as follows:


SECTION 1. INTERPRETATION

Words and expressions defined in the Loan Agreement shall, unless otherwise
defined herein or the context otherwise requires, have the same meaning when
used in this Agreement. References to any agreement or document shall be
construed as references to such agreement or document as varied, amended,
novated or supplemented from time to time. In addition thereto, as used in this
Agreement:

1.1  "Securities" shall mean the securities described in Schedule I hereto.

1.2. "Secured Obligations" shall mean (i) any and all obligations, liabilities
     and indebtedness of the Company owing to the Pledgee, presently existing or
     to be incurred hereinafter under or with respect to the Loan Agreement,
     (ii) moneys,

<PAGE>
     obligations and liabilities owing and payable by the Company to the Pledgee
     under the Yangdo Tambo Agreement dated December 26, 2003 between the
     Company and the Pledgee, (iii) moneys, obligations and liabilities owing
     and payable by the Company to the Pledgee under the Factory Kun-Mortgage
     Agreement dated December 26, 2003 between the Company and the Pledgee and
     (iv) moneys, obligations and liabilities owing and payable by the Pledgor
     to the Pledgee under this Agreement.

1.3. "Proceeds" means all proceeds of, and all other profits, income or
     receipts, in whatever form and whatever currency, arising from the
     ownership, collection, sale, exchange, assignment or other disposition of,
     or realization of any of the Securities, and all interest, dividends (cash,
     stock or otherwise) and other payments and distributions on or with respect
     to such Securities or in exchange for such Securities.


SECTION 2. PLEDGE

2.1. The Pledgor hereby agrees to pledge the Securities to the Pledgee and
     hereby agrees to grant in favor of the Pledgee a first priority security
     interest (jilkwon) in the Securities and the Pledgee shall accept the
     security interest in the Securities for the due and punctual payment,
     performance and discharge of the Secured Obligations. The Pledgor hereby
     agrees that any Proceeds of any Securities on which a pledge under this
     Agreement has been established and perfected shall be subject to the pledge
     (jilkwon) on such Securities pursuant to this Agreement and that if any
     such Proceeds may not be subject to such pledge, the Pledgor shall pledge
     the Proceeds for the benefit of the Pledgee in the manner applicable to the
     nature of it and perform any and all actions necessary or appropriate in
     the opinion of the Pledgee for the establishment and effectuation of such
     pledge under the laws of Korea.

2.2  Simultaneous with, or prior to the provision of the Loan on the Loan Date,
     the Pledgor shall have the Pledgee's name and address recorded in the
     shareholders' registry of the Company (maintained with the Korea Securities
     Depository) as a holder of the security interest (jilkwon) in the
     Securities and have the Pledgee's name recorded on the share certificates
     representing the Securities.

2.3  Immediately upon performance of Section 2.2 hereof and at least [one (1)]
     business day prior to the provision of the Loan on the Loan Date, the
     Pledgor shall deliver or cause to be delivered the share certificates
     representing the Securities in physical form to the Pledgee or to the agent
     of the Pledgee, as requested by the Pledgee together with a certified copy
     of the shareholders' registry of the Company and any documents necessary or
     appropriate in the opinion of the Pledgee to constitute a legally valid
     delivery of the certificates.

<PAGE>
SECTION 3. REPRESENTATIONS AND WARRANTIES

3.1  The Pledgor hereby represents, warrants and covenants to the Pledgee that:

     (a)  the execution, delivery and performance by the Pledgor of this
          Agreement are not in conflict with any provision of any applicable
          laws or regulations or the articles of incorporation of the Company;

     (b)  the execution, delivery and performance by the Pledgor of this
          Agreement does not violate or cause any default under any indenture,
          agreement or undertaking by which the Pledgor is bound;

     (c)  all authorizations required from any government or governmental agency
          in connection with this Agreement have been obtained and are in full
          force and effect;

     (d)  this Agreement constitutes valid obligations of the Pledgor, legally
          binding upon it and enforceable in accordance with its terms, subject,
          as to enforceability, to laws relating to bankruptcy, insolvency,
          liquidation, reorganization, court schemes, moratoria, administration
          and other laws generally affecting the rights of creditors;

     (e)  the Pledgor has full right, title and interest over the Securities;

     (f)  the Pledgor has not agreed to sell, assign, transfer, pledge or create
          any security, interest, lien or other encumbrance in or over all or
          any part of the Securities other than in favor of the Pledgee
          hereunder;

     (g)  no person (other than the Pledgor and the Pledgee) is exercising, has
          exercised or attempted to exercise any ownership interest or other
          lien over any of the Securities;

     (h)  to the knowledge of the Pledgor, there are no circumstances that may
          diminish the value of the Securities;

     (i)  the Pledgor has not taken any action nor have any other steps been
          taken or legal proceedings been started by him or any third party for
          bankruptcy, workout, or similar proceedings over the Pledgor or his
          assets, and there occurs or continues no cause/event that may lead to
          such action or steps; and

     (j)  the Pledgor has acquired the Securities in accordance with any
          provision of any applicable laws or the articles of incorporation of
          the Company and has been, and during the term of the Loan Agreement
          will be, permitted to have right, title and interest over the
          Securities under such laws and articles of incorporation.

3.2  Each of the warranties and representations contained in the preceding
     sub-clause shall survive and continue to have full force and effect until
     the termination of this Agreement.


SECTION 4. COVENANTS OF THE PLEDGOR

The Pledgor covenants and undertakes the following with the Pledgee:

<PAGE>
     (a)  duly and punctually to observe and perform all the conditions and
          obligations imposed on it by this Agreement;

     (b)  not, without the prior written consent of the Pledgee, to attempt to
          sell, assign, transfer, create any security, interest or lien over, or
          otherwise dispose of or encumber any of the Securities to or in favor
          of any person other than the Pledgee;

     (c)  to do or permit to be done each and every act or thing the Pledgee may
          from time to time require to be done for the purpose of enforcing the
          Pledgee's rights under this Agreement or in respect of any of the
          Securities and to allow its name to be used as and when required by
          the Pledgee for such purpose;

     (d)  to make all registrations, filings and recordings, and take all other
          actions as are necessary or determined by the Pledgee to be advisable
          to ensure the continued legality, validity, enforceability, priority
          and admissibility in evidence of this Agreement;

     (e)  to be responsible for the resolution of any disputes with a third
          party in connection with the Securities, including any disputes
          involving claims against the Securities, and in connection therewith,
          the Pledgor agrees not to take any action which might adversely affect
          the enforcement of the Pledgee's rights or Pledgee's interests in the
          Securities and to affirmatively protect and defend such rights and
          interests in accordance with the instructions of the Pledgee;

     (f)  to pay all taxes, assessments and other governmental charges of any
          kind imposed on or in respect of his income or any of his assets when
          due;

     (g)  to immediately notify the Pledgee of any and all notices that the
          Pledgor has received from the Company in his capacity as a shareholder
          upon the receipt of such notice;

     (h)  if and when it is required to or entitled to vote, give any consent or
          exercise any other rights with respect to the Securities, to first
          notify such situation to the Pledgee immediately and act in accordance
          with the instruction of the Pledgee;

     (i)  not to make or cause to make any action, procedure or event that may
          diminish the value of the Securities; and

     (j)  to notify the Pledgee of any event that would result in or lead to a
          breach of any representation and warranty provided under Section 3
          immediately upon becoming aware of such event.

<PAGE>
SECTION 5. ADMINISTRATION OF SECURITIES

5.1  Until there shall have occurred an Event of Default or a Potential Event of
     Default, the Pledgor shall be entitled to vote, give any consent or
     exercise any other rights with respect to the Securities in any manner not
     inconsistent with this Agreement and subject to applicable law.

5.2  Unless an Event of Default or a Potential Event of Default has occurred,
     and without prejudice to Section 2 hereof, the Pledgor shall receive or
     shall become entitled to receive, any dividend, interest or any other
     distribution whether in securities or property (including cash) with
     respect to the Securities by way of stock-split, spin-off, split-up or
     reclassification, combination of shares or the like. For the avoidance of
     doubt, any distributions (whether in cash or in shares or other property)
     received by the Pledgor shall be deemed to be the Proceeds and subject to
     the pledge created under this Agreement, and if any such Proceeds may not
     be subject to such pledge, the Pledgor shall pledge the Proceeds for the
     benefit of the Pledgee in the appropriate manner and perform any and all
     actions necessary or appropriate in the opinion of the Pledgee for the
     establishment and effectuation of such pledge under the laws of Korea.


SECTION 6. ATTORNEY-IN-FACT

The Pledgor shall be deemed to irrevocably appoint the Pledgee as its true and
lawful attorney-in-fact with full power to require, demand and receive any and
all moneys and claims for money due and to become due under or with respect to
the Securities to the extent pledged hereunder and to take any action or execute
any instrument which the Pledgee may deem necessary or appropriate to accomplish
the purpose hereof.


SECTION 7. TERMINATION AND REASSIGNMENT

The term of this Agreement shall begin on the date of this Agreement and end on
the date on which all Secured Obligations shall have been unconditionally and
irrevocably paid and discharged. Upon termination of this Agreement, the Pledgee
shall release the security interest in the Securities held by the Pledgee,
return the share certificates representing the Securities to the Pledgor at the
cost of the Pledgor and take any actions as may be reasonably requested by the
Pledgor to effectuate such release.


SECTION 8. ENFORCEMENT BY PLEDGEE

8.1  If any of the Secured Obligations becomes due and payable, the Pledgee
     shall be entitled, as and when it may see fit, to put into force and to
     exercise all or any of the rights and power possessed by it as pledgee of
     the Securities under the applicable laws and this Agreement in or towards
     satisfaction of the Secured Obligations, including without limitation, the
     right and power to:

<PAGE>
     (a)  sell, transfer or otherwise dispose of the Securities at such times
          and at such prices as are generally recognized as appropriate in
          custom and practice, without the necessity of any legal process or
          procedure;

     (b)  acquire title to the Securities in lieu of foreclosure of the
          Securities in any manner in its discretion without notice to the
          Pledgor; and

     (c)  demand, sue for, collect or receive, in the name of the Pledgor, any
          money or property at any time payable or receivable on account of or
          in exchange for any of the Securities, but shall be under no
          obligation to do so.

8.2  Upon the exercise of its rights pursuant to Section 8.1. hereof, the
     Pledgee shall apply the Proceeds, after deducting the costs of such
     exercise of rights, against the Secured Obligations. Any excess amount
     shall be paid to the Pledgor and any Secured Obligations not fully covered
     by the Proceeds shall remain due and owing and shall promptly be paid by
     the Pledgor. In the case of acquisition of the title to the Securities
     pursuant to Section 8.1.(b) hereof, the Pledgee shall value the Securities
     based on commercially reasonable standards and return any excess amount
     (net of any and all costs and taxes involving transfer of the ownership of
     the Securities) to the Pledgor and any Secured Obligations not fully
     covered by the value of the Securities shall remain due and owing and shall
     promptly be paid by the Pledgor.

8.3  The Pledgor hereby consents to the procedures set out in Sections 8.1 and
     8.2 above and waives any and all objections it may have thereto.

8.4  The Pledgee shall incur no liability as a result of the sale, acquisition
     or any other disposition of the Securities or any part thereof by the
     Pledgee pursuant to Section 8.1 hereof conducted in a commercially
     reasonable manner. The Pledgor hereby waives any claims against the Pledgee
     arising by reason of the fact that the price at which the Securities has
     been sold at any private sale by the Pledgee may be less than the price at
     which it could have been sold if the Pledgee had not accepted the first
     offer received or had offered the Securities to more than one offeree.

8.5  The application of the Proceeds shall be determined by the Pledgee in its
     absolute discretion, subject only to any other agreements between the
     Pledgee and the Pledgor relating thereto and to the requirements of any
     applicable law.

8.6  In case where the Pledgor or a third party provides collateral (other than
     the Securities) to the Pledgee to secure all or any part of the Secured
     Obligations, the Pledgee shall in its sole discretion have the authority to
     determine the priority and extent of enforcement between/among its rights
     under this Agreement and such other collateral arrangements, without the
     necessity of any legal process or consultation with the Pledgor.


SECTION 9. ASSIGNMENT

This Agreement and the security interest created hereunder shall be binding upon
and inure to the benefit of the Pledgor and the Pledgee and their respective
successors and assigns. The Pledgee may, in accordance with the Loan Agreement
and applicable laws, at any time

<PAGE>
assign all or any part of its rights or obligations hereunder to any party (each
an "Assignee"). The parties hereto agree that to the extent of any transfer, the
Assignee shall be deemed to have the same rights and benefits under this
Agreement as it would have had if it were a Pledgee signatory hereunder. The
Pledgor may not assign any of its rights or obligations hereunder without the
prior written consent of the Pledgee.


SECTION 10. FURTHER ASSURANCE

The Pledgor shall do all such acts as may be necessary or appropriate in the
opinion of the Pledgee including, but not limited to, the execution and delivery
of all further instruments, notices and documents and all further action (whilst
acting in a commercially reasonable manner) that may be necessary or appropriate
in the opinion of the Pledgee in order to perfect and/or protect any lien
granted or purported to be granted hereby or to enable the Pledgee to exercise
and enforce its rights and remedies hereunder with respect to the Securities and
the Proceeds.


SECTION 11. EXPENSES, TAXES AND INDEMNIFICATION

The Pledgor shall be liable for and shall indemnify the Pledgee on demand
against, and shall pay, (a) the reasonable costs, expenses, taxes and other
liabilities (including legal fees on a full indemnity basis) incurred by the
Pledgee to protect or enforce the Pledgee's interest in the Securities and the
Pledgee's rights against the Pledgor under this Agreement, provided that any
such enforcement shall be in a commercially reasonable manner; (b) all
withholding, excise, stamp, registration and other taxes, fees and duties
payable in connection with the entry into, performance, enforcement or
admissibility in evidence of this Agreement, and all reasonable costs, expenses
and other liabilities, incurred by the Pledgee in connection with, or otherwise
attributable to, receipt by the Pledgee of, or payment by Pledgor of, any
reasonable costs, expenses and other liabilities and indemnities provided for in
this Section or any other provision of this Agreement, other than taxes imposed
on overall net income of the Pledgee.


SECTION 12. MISCELLANEOUS

12.1 Notices. All notices, requests and demands to or upon the respective
     parties hereto to be effective, shall be in writing and shall be personally
     delivered or sent by facsimile (with subsequent written confirmation) or by
     registered or certified first class mail, postage prepaid, return receipt
     requested, or by overnight (or next business day) courier service shall be
     deemed to have been duly given or made when delivered by hand, on the day
     that such facsimile is transmitted, or, if by first class mail, five days
     following the date on which such writing is deposited with the postal
     service, or the day after the date when deposited with an overnight (or
     next business day) courier service, addressed as follows, or to such other
     address as either party hereto may hereafter specify in writing to the
     other party:

<PAGE>
     To:               ST Assembly Test Services Ltd
                       5 Yishun Street 23
                       Singapore 768442
     Attention:        Director, Legal
     DID:              65 6824 1948
     Fax:              65 822 7837

     To:               Mr. S.H. Chun
                       SimmTech Co. Ltd.
                       70-5 Songjung-dong
                       Huengduk-ku
                       Choungju, Chungchongbuk-Do
                       Korea
     Attention:        Chief Marketing Officer
     DID:              82-43-269 9280
     Fax:              82-43-269 9048

12.2 Severability. If any of the provisions of this Agreement shall contravene
     any law or regulation or be held invalid, this Agreement shall be construed
     as if not containing those provisions, and the rights and obligations of
     the parties hereto shall be construed and enforced accordingly.

12.3 Amendments, Changes and Modifications. This Agreement shall not be amended,
     changed, modified, altered or terminated, unless the prior written approval
     of each of the Pledgor and the Pledgee is obtained. This Agreement shall
     not be amended by an oral agreement.

12.4 Counterparts. This Agreement may be executed in multiple counterparts, each
     of which, when executed, shall constitute an original but all of which
     together shall constitute one and the same instrument.

12.5 Heading. Headings and titles herein are for convenience only and shall not
     affect the construction or interpretation of this Agreement.

12.6 Entire Agreement. This Agreement is intended by the parties as the written
     final expression of each party's obligations and rights in connection with
     the Securities and supersedes all prior and contemporaneous understandings
     or agreements concerning the subject matter hereof.

12.7 Conflict. In the case of a conflict between the provisions of this
     Agreement and the provisions of the Loan Agreement, the Loan Agreement
     shall prevail.

12.8 No Waiver. The Pledgee shall not, by any act, delay, indulgence, omission
     or otherwise, except by an express written instrument clearly indicating an
     intention to waive, be deemed to have waived any right or remedy hereunder
     or to have acquiesced in any Event of Default. No failure to exercise, nor
     any delay in exercising on the part of the Pledgee, any right, power or
     privilege hereunder shall operate as a waiver thereof. No single or partial
     exercise of any right, power, privilege hereunder shall preclude any other
     or further exercise thereof or the exercise of any other right, power or
     privilege.

<PAGE>
12.9  Remedies Cumulative. The rights and remedies provided herein are
      cumulative and may be exercised individually or concurrently, and are not
      exclusive of any other rights or remedies provided by law.

12.10 Currencies. All Secured Obligations under this Agreement are payable in
      United States dollars ("the Required Currency"). If the Pledgee receives
      or recovers any moneys in any other currency, whether upon enforcement of
      its rights under this Agreement or otherwise, the Pledgor shall indemnify
      the Pledgee against the difference (if any) between the amount received by
      the Pledgee when it converts amounts received in currencies other than the
      Required Currency to the Required Currency and the amount due in the
      Required Currency. The Pledgor shall assist the Pledgee at all times to
      obtain any exchange conversion approval required by the Pledgee in
      connection with this Agreement or the Secured Obligations.

12.11 Governing Law and Jurisdiction. This Agreement and the security created
      pursuant hereto shall be governed by the laws of Korea in all respects,
      including matters of construction, validity and performance. The parties
      hereto agree to submit to the non-exclusive jurisdiction of the Seoul
      District Court for the purpose of this Agreement.





















                            SIGNATURE PAGE TO FOLLOW

<PAGE>
IN WITNESS WHEREOF, the parties hereto have caused this Pledge Agreement to be
duly executed as of the day and year first above written.


                  PLEDGOR: /s/ Se-Ho Chun
                           -----------------------------



                           In the presence of: /s/ Young Goo Kim
                                               -----------------------------





                  PLEDGEE: ST ASSEMBLY TEST SERVICES LTD



                           By  /s/ Suh Tae Suk
                              --------------------------
                              Name:  Mr. Tan Lay Koon
                              Title: President and CEO



                           In the presence of: /s/ Juliana Ho
                                               ----------------------
<PAGE>
                                   SCHEDULE I


Description of Securities

2,400,000 shares of common stock of the Company in denominations as the
following:

<TABLE>
<CAPTION>
Serial Number           Denomination              Number of share certificate(s)
-------------           ------------              ------------------------------
<S>                     <C>                       <C>
2601~2636               10,000 shares                           36
2397~2446               10,000 shares                           50
2447~2496               10,000 shares                           50
2497~2546               10,000 shares                           50
2547~2596               10,000 shares                           50
2597~2600               10,000 shares                           4
</TABLE>