Creep- and Shear-Tester

A non-microscopy product for research and production-line quality control

The Task - Creep test / Shear test

If electronic boards fail, severe consequences may occur in the operation of complex systems. Therefore, quality tests are a must for the manufacturer of such boards. This page shows a test instrument system for mechanical testing of printed circuit boards (pcb´s). The “Multi-Tester” allows quality control groups to do spot checks - for instance - of the inferior points of lead-free soldering of the elements mounted on the boards.

The Multi-Tester System 

The Concept:

Have a look at the photos below, showing a variety of elements mounted to the "Europe Standard Electronic Boards". The images show boards made in large quantities for the automotive industry. The elements are attached by state of the art lead free soldering techniques. Dimensions range from less than a millimeter size up to beyond a centimeter.

Test objects: Europe Standardized Electronic Boards with elements such as CR01005 –CR1206, etc. (mounted in various directions). Different boards may be used too.

Board with components

Board side view

The boards come in quite different dimensions, some of them individually designed for the user's own requirements, and sometimes they are made with elements on both faces.

Most mechanical tests simulate failures in the mounting quality of the soldered objects on the board, e.g. if they were disturbed by mechanical touching, a warping movement of the board, or a simulation of thermal disturbances that may result in malfunction of the solder joints. The main goal is a testing procedure with a measurement in the sub-µm-range. The early design phase assumes a "tool" that will apply the load to the element under test horizontally, and very slowly, in one direction only (cyclic loading will be part of the next phase). During the actual in-touch time, the load will be kept constant by the system software. If this tool movement is done rapidly, then the procedure changes to become a "shear-test". Our prototype is equipped with a 500 N load cell. Changes of the load range are easy to do.

It seems that during the later phase, it will be quite advantageous to run the creep test alternatively with the shear test, both in one instrument.

Various Factors Can Lead to Failures of the Soldered Components.

The rapid shear test alone will not concisely reveal all phenomena. If the testing apparatus is sufficiently rigid, then slow creeping will bring about additional information, and so far unknown detail:

solder_joint1. Rupture of the main (ceramics) body of the element.
2. The metallic layer at the border forms fissures and begins to tear.
3. The interface between the element and the solder begins to cave in.
4. The solder volume fails
5. The solder begins to loose contact with the "pad" surface below.
6. The board itself cracks, and the element finally breaks off.

Fig. 3: The main elements of a solder joint on a board.

Mechanical testers performing the "Shear test" are well established. Unfortunately often other makers’ shear testers are too flexible to show ALL relevant data. Therefore the creep tester in its actual design is made in a way that the measurement does not record the intrinsic flexure of the apparatus, and certainly not the warping movement of the board under test. Boards with elements on both sides will be held in place by a new method, to keep flexing movements at bay.

Please see the results shown in following diagrams..

Shear Test and Creep Test Mode Diagram:

A Typical Shear Test Recording

Note, that the movement of the tool (red line in this diagram) does not show the slightest disturbance or "shake" the moment the tip touches the element under test. It keeps moving absolutely smooth. Assuming a rigid instrument, the shear test is the simplest mode among these test methods. It is done in minutes.

The blue curve displays the "Load" (in N), the red one shows the tool movement (in µm/sec.).Shear test

The shear test shows very clearly, that there is no intrinsic flexure in this instrument, that would affect the measurement accuracy neither in the shear test nor in the creep test mode. It is this rigidity of the instrument that makes creep-tests possible in the first place.

The results are almost self-explanatory. We will be glad to answer your inquiries for more detail about this technique.

The Creep Test

By mouse click, the software will change from the "Shear Test Mode" to the "Creep Test Mode". The basic function is, to apply a very stable "constant load" to the object under test to make the object “creep at its own speed”. This will cause a "plateau", during the time while the applied load keeps its selected value.The larger the difference between the maximum load detected during the shear test, and the selected "constant load" for the creep test, the longer the time will be, during which this plateau will stretch out (Fig. 10). Therefore, a creep test may very well take quite some time, depending upon what you wish to test.

Creep test 1. approach

First Approach – Creep Test Diagram "Load versus Displacement".

As these two displays (9 and 10) show, there is much more complexity in the real, undisturbed creep test. First, these diagrams need a bit of an explanation, before they are obvious and easy to read.

If we select "load versus displacement" for the horizontal axis (First Approach), then the advancing will become very, very slow - almost seems to stand still - during the time when the object under test strongly resists the tool. This could be expected, but it does not really tell much about the object's creeping properties.

Here again: The blue curve displays the "load" (in N), the red one shows the tool movement (in µm/sec.).

The above diagrams are examples only. Creep tests can run for minutes, hours, days, or even weeks.

Creep test 2. approach

Second Approach – Creep Test Diagram Load versus Time  
(Note the green Curve).

The green line shows the creep movement versus time (during which the load is being applied!). At the beginning, the load increased with 0.5N/sec (this value was entered by the operator). We now see the advancing of the object under test, as the solder slowly gives way. Rigorous tests on bulky specimens showed, that the flexibility of the creep tester was below detection limit; therefore the instrument is a number of magnitudes stiffer than the test objects.

The blue recording represents the load (in N), the red one represents the advancing movement (in µm/sec.), and the green line shows the creeping movement of the object under test (in µm/sec). It is quite interesting to observe, how slowly and evenly this very ductile soldering breaks off.

The above diagrams are examples only. Creep tests can run for minutes, hours, days, or even weeks.

Specifications:

Dimensions [in mm]
~ 800 x 500 x 580 (W x L x H)
Weight [in kg]:
~ 150
Travel range [in mm]:
X~170; Y~200; Z~20; R~±20°
Movement resolution [in µm]:
0,01
Speed range [in µm/s]:
0,01 - 3000
Nominal force[in N]
500 (other load cell on request)
Force accuracy [in N]:
0,01% of full range
Power supply:
Single Phase Grounded
110 / 230 (Vac); 60 / 50 (Hz)
Certification:
CE, EN 610

Specifications subject to change without prior notice.

Topics if this article:
rheology, creep, shear, testing, pcb, boards, solder joints, instrument, Tester

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