This extensive how-to consists of 9 paragraphs and 6 pages. Please use the navigation on the right.
Introduction #
This manual is a guide advising what to consider and how to best perform a tweezer pull test. Also described is tweezer peel testing. More on Cold Bump Pull testing, a type of tweezer testing on solder balls or bumps, can be found in a separate how-to.
Tweezer pull and peel tests are normally not difficult to do, if you own the right equipment. This how-to covers a variety of subjects. Follow it to obtain consistent results and the highest degree of quality assurance.
What are tweezer pull and peel? #
Tweezer pull is simply a pull test performed by a tweezer. During the test an upward force is applied to a wire, a ribbon or some other feature on your sample after being gripped by the tweezers. In many cases, the sample has to be prepared for testing first.
A peel test is essentially the same, but apart from an upward movement, the XY stage also does a controlled move. Normally, the purpose is to maintain a perpendicular force on the ribbon or wire being pulled off the substrate.
Tweezer design #
Manual and automatic #
There are two fundamental types of tweezers; those which open and close manually and those with automatic opening and closing.
Types of jaws #
There are many different types of jaws to suit a wide range of applications. The most common are plain, serrated, hook and cavity type as shown in the illustration.
Closing function #
There are three types of closing function. They are selected to suit the application;
- Increasing grip force
- Constant grip force
- Closing distance
Increasing grip force as the pull force increases is typically used when gripping hard and parallel parts. This way the grip is maintained as the pull force increases. A constant grip force regardless of the increasing pull force is useful when gripping soft parts, where the sample can be deformed to obtain a grip but squashed too much if the grip force gets too high. A fixed closing distance (or: closing position) is used when the part already has a feature that the jaws can “hook” under. You can use the following chart to select the type of hook your application requires.
Material #
Jaws are normally made from a tough steel. Tool steel is a good example. When the jaws require small detail and precise tolerances, hard materials are required because smaller detail can be machined into these materials. The disadvantage of harder materials is that it makes the jaws brittle and fragile.
Quality #
As with other tools, if the jaws are damaged or worn they may not work correctly. The amount of acceptable damage depends on the application. Any jaw damage is acceptable if;
- You get the failure mode of interest
- You get the same highest possible test force as with undamaged jaws
It is also important that the tips of the jaws align closely with each other when closed. The same guidelines for acceptability exist for jaw alignment as for jaw damage. However, as a very general guide, misalignment should not be larger than 5% of the jaw width.
Force control #
Depending on the type of sample it is often highly important to have precise control over the grip force of the tweezers. This is not only vital in order to do a correct test on one bond tester, but also to maintain correlation between tests on multiple machines. Especially with pneumatic tweezers, it is almost impossible to obtain consistent settings between multiple systems. Electric tweezers with intelligent jaws and built-in closed loop force control solve this issue.
Handle tips with care #
Handle tips with care Tweezer jaws (tips) can be fragile, depending on the application. To avoid damage, it is important to handle them with care and always store them in their cases when not installed in a bond tester. Preferably, use tweezers with quick, safe and easy methods of tips exchanging.
Xyztec USB tweezer tips ship to you in a safe case and already mounted in a bracket. This bracket later enables the operator to place the tips in the USB Tweezer and tighten the fixation screw without ever touching the fragile tips by hand.
When contaminated, use extreme care to clean the tips. Many companies that do Cold Bump Pull use the contactless CBP Jaw Cleaner, which virtually eliminates the risk of damage when cleaning the cavities.
Alignment #
For best results, generally, the tweezer jaws should be aligned centrally to the sample. Jaw features like serrations, hooks or cavities must be correctly engaged in the vertical axis as well. A sufficiently advanced bond tester can in many cases assist the operator by aligning automatically. This works by vision technology or the detection of surfaces.
Gold Wire Tweezer Pull Failure Modes #
Here we list the most common failure modes for gold wire tweezer pull. We also refer to our grading library, where you can download result codes.
Ribbon pull #
Ribbon pull can often work effectively with a flat hook and/or an offset hook to keep the pull force on the center of the bond. However, hook type tweezers are also a good solution for ribbon pull, sometimes better. It always distributes the load under the bond evenly.
Ribbon loop heights are often very low. Special hooks can help but the tweezer solution has the highest pull force, so it may be the preferred solution.
Peel testing #
Perpendicular #
Peel tests are normally done by tweezers. The principle of a peel test is to move two axes at the same time. This is typically the Z axis and the X or Y axis. The following schematic outlines the way this works when the objective is to keep the angle of the pull force perpendicular to the sample.
Angled peels #
Less common are angled peels. Here, the movement shown in the X is not equal to the upward movement. Backward peeling decreases the lifting force on the sample and can be useful testing thin substrates that tend to bend upwards. Forward peeling has no known advantages.
Results #
The results from a peel test often require more analysis than the results from a simple pull or shear test. The useful data in the force displacement graph can consist of the maximum (peak) force, the average force, the minimum force or the energy. A bond tester with a good SPC package can give you all these results directly after the test is completed.
Copper pillar #
Solder to copper #
Copper pillars can be tested in multiple ways. The best method depends on your failure mode of interest and the construction of your sample. If you are interested in the interconnect between the solder and the copper, it depends on the shape of the construction whether a Cold Bump Pull (CBP) type test is feasible. If not, a shear may be the only effective test to qualify your process.
The test sequence for the CBP-approach is as follows:
Whether to use a closing distance or constant closing force on the tweezer jaws depends on which method produces the most failure modes of interest or highest force. The failure modes are;
Copper to substrate breaks are a common failure mode. When this happens the bond strength and failure mode of the copper to the substrate pad is known. Copper solder break is not so common and probably indicates a non-wet. Copper extrusions indicate strong bonds.
The test sequence for the shear-approach is as follows:
The rationale behind the top landing and the rest of this test sequence is further explained on our copper pillar test type page. In this how-to we focus on tweezer pull tests only.
Copper to pad #
The failure mode of interest between the copper and the pad can be produced by either a pull or a shear test and the measured bond strength used for your process control. In many other applications pull testing is typically preferred because the bond is subjected to a simple tensile load, distributed over the bond area. The bond separation is clean making failure mode analysis of the surfaces relatively easy. Unlike solder bumps, copper is relatively hard and gripping it therefore easier.
Solder balls require precise reforming in order to be able to apply a meaningful test load on to the bond. Copper also has to be reformed in order to be able to grip it but this takes the form of well know gripping methods such as plain surfaces and friction, a few serrations that slightly reform the copper in order to get a mechanical grip or a slight tapper.
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Stud Pull / Pick and Place #
The stud pull test is based on the principle that an adhesive connection is made with the carrier (or object to be tested) by using a glue that is stronger than the bond that needs to be tested.
The Sigma can be provided with a micro dispenser that can dispense very small drops of glue (micro liter amounts) very accurately and is cured with UV light. Combined with a USB Tweezer and special tips that look like the inverse of a mushroom, a glass stud can be taken from a magazine fully automatically and placed with a predefined pressure on the surface with the glue.
There are many other approaches to stud pull.
