Place the button into separate UpdatePanel and set the PostBack trigger there
Ex:
<asp:updatepanel id=""up1"" runat=""server"><br" mode="hold" xmlns:asp="#unknown" /><ContentTemplate>
<table><tr> <td>
<asp:Button ID="btnSubmit" runat="server" CssClass="tbutton" OnClick="btnSubmit_Click"Text="Submit" ToolTip="Click here to Submit" ValidationGroup="Save" /> </td >
</tr>
</table>
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="btnSubmit" />
</Triggers>
</asp:UpdatePanel>