Pull Testing - Cold Bump Pull

This ground breaking technique was developed by Dage in 1998 and has now gained acceptance in the industry with a JEDEC standard procedure (pending early 2007).

Solder bumps are grasped by tiny jaws in a load cell, which then pulls the bump from the device. The force to failure is measured and of course the failure mode is also easily recorded.

The jaws which grip the solder ball have been carefully designed with a slight cavity, and different sizes are used according to the diameter of the ball. Both features ensure that maximum force is applied to the bond. During the procedure, the jaws grip the solder ball with a defined closing force and reshape or reform it into a mushroom or rivet shape. If the correct size of jaws is chosen, there is no encroachment of the jaws over the bond area.

The method has several advantages listed below, and particularly lends itself to the use in rapid screening procedures for the evaluation of bond strength in terms of solder, pad finish, etc.

Two technical papers on CBP are available here:

  1. A general article on the testing of bumped devices
  2. A paper devoted to pull testing on BGAs and CSPs

Advantages of pull over shear testing

  1. Some solder balls are mounted in cavities where they gain support from the side walls. Such support might actually cause a bad bond to pass a shear test.

  1. During a shear test at the point of maximum load there is some deformation of the ball above the bond site, but this is minimised during a pull test.

 

  1. Surface irregularities at the bond interface may cause mechanical bonding even though there is no chemical adhesion. Such bonding may manifest itself in a shear test by a higher test load. In a pull test this type of bonding will have far less influence.

 

  1. Additionally, a pull test applies a simple tensile load to the bond, rather than the complex load of a shear test which comprises a triplet of shear, compressive and tensile forces. It is possible that the latter might complicate analysis of test results.

 

  1. The test is rapid and options are available for automation.

Applications

Dage provides...

Automated Cold Bump Pull on 12” wafers (link to 4300 machine)

Automated Wafer Handling (link to 4000W machine)

High speed CBP (see also 4000HS page)

Recently, you will have seen the launch of DAGE’s new bondtester, the 4000HS. Research over the past few years has shown that applying a high strain rate during testing facilitates transfer of the load to the bond. In turn, this enables enhanced detection of brittle fracture failures and pad-cratering in bondtesting. At conventional speeds, it was clear that the majority of the time, only solder strength was being probed.

Indeed there is a shift in emphasis in high speed bondtesting away from precise measurement of failure force to evaluation by failure mode. With the advent of lead-free materials, bond strengths have often increased, but joints are nonetheless failing.

Developed first for solder bump shear, high speed testing has been extended to CBP.

As in the case of high speed shear for solder bumps, developing the machine for CBP has required considerable innovation and some reconfiguration of the test method. At the point of test, the sample must be travelling at constant velocity. In order to achieve this, as in the shear test, the sample must be accelerated. One crucial difference from the shear test is that the jaws must be fitted around the test ball before the acceleration phase. In the DAGE system, the sample is driven to the target velocity by an air operated piston. When the sample has reached the chosen speed, the work holder hits a hard stop and the actual pull test takes place.

Hot bump pull

Hot bump pull is an older technique in which a probe attached to the load cell is essentially soldered to the ball. This enables the application of high loads during the test and is therefore a stringent test of the intermetallic bond, pad finish and pad construction. However the test involves dedicated equipment, some skill in execution, and is very slow (up to 2 minutes, versus around 15 seconds) compared with CBP.

If you think that Hot Bump Pull is necessary for your application, Dage can provide all the necessary equipment for modifying your existing 4000 bondtester or setting up a new machine. Contact us to find out more.

 

© 2008 Dage Holdings Limited