how to use updatepanel in gridview footer row

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Ajax

hi
i am using updatepanel in gridview and i footer row i kept one button.when click on button updatepanel is not woking i,e entire page is postbacking.how can i resolve this issues please anyone tell me.here is my code

<asp:UpdatePanel ID="UpdateDetails" runat="server" ChildrenAsTriggers="true"
                                UpdateMode="Conditional">
                                <ContentTemplate>
  <asp:GridView ID="GridDetails" runat="server" AutoGenerateColumns="false" ShowFooter="true"
         Width="100%"
         >
     <Columns>
       <asp:TemplateField HeaderText="ReAssignTo" HeaderStyle-HorizontalAlign="Left">
        <ItemTemplate>
        <asp:DropDownList ID="ddl" runat="server"></asp:DropDownList>
        </ItemTemplate>
        <FooterTemplate>
         <asp:Button ID="btnupdate" runat="server" Text="Update" OnClick="btnupdate_Click" OnClientClick="javascript:return validateCombobox();" CausesValidation="false"/>
        </FooterTemplate>
       </asp:TemplateField>
       </Columns>
  </asp:GridView>
  </ContentTemplate>
  </asp:UpdatePanel>

thanks
Regards
pradeep