Semiconductor Test Contactors/Sockets

Contactors/Sockets

The contactor is the test accessory that makes the actual physical contact with the device under test (DUT) to establish the necessary electrical connection between the ATE and the DUT so that the former can electrically test the latter. The contactor assembly is generally a part of the test handler. Thus, before production testing can begin, the test handler must first be fitted with the test contactor assembly or contactor block suitable for the device to be tested.

Fig. 1. Examples of contactors

The proper selection of contactors for electrical testing has a great impact on test yields, device grading, repeatability and reproducibility of testing, and productivity. A poor choice of contactor can eventually lead to contact problems that cause invalid failures or test miscorrelations, which in turn can result in unwarranted machine downtimes, unexplained yield problems, and even customer returns.

A contactor has a set of contact elements that are usually in the form of metal fingers (also known as 'contact fingers') or spring-loaded pins. These contact elements are the ones that come into contact with the leads or solder balls of the DUT during electrical testing. Contact elements are commonly composed of a beryllium-copper base metal with gold-plating on the surface.

The profile of a contact element is critical to contact integrity and life prolongation. Traditional test sockets are designed and manufactured with contact elements that act as cantilevers on which the leads of the DUT can rest. These cantilevers are given an inherent amount of ‘spring’ in them so that they’ll tend to oppose the forces exerted unto them by the leads of the DUT, ensuring good electrical contact. This contact force is a function of the flexing and compression properties of the contact elements.

New contactor designs use an ‘S’ structure for the contact elements which are believed to be more reliable and robust than those using the cantilever design. The contact force provided by these new designs depend on the flexing properties of the elastomeric elements supporting the contact elements, and not the properties of the contact elements themselves. As such, they are more immune to contact issues caused by contact element deformation.

The motion of the contact element as it comes into contact with the lead is another factor in selecting a good contactor. Old contactor designs simply push the contact elements against the leads to make the contact. New contactor designs give the contact elements a ‘wiping’ action, so that contaminants, oxides, and residues on the DUT lead will first be removed before the final contact is made.

Fig. 2. More examples of contactors

Ease of maintaining, replacing, or configuring a contactor is another consideration when looking for a good contactor. Contact elements degrade with usage, and should therefore be monitored and maintained regularly. It should be promptly replaced once it has exhausted its useful life, since poor contact caused by worn-out contact elements can result in a lot of test issues. Well-designed contactors should be able to do several hundreds of thousands of contact cycles within its lifetime.

The electrical characteristics of the contactor is also critical in ensuring the integrity of the test process. Contact resistance, stray capacitance, and stray inductance, as well as capacitive and inductive coupling must all be minimized. The dimensions, shapes, and material of the various features of a contactor as well as the contact area and tip topography of the contact element all affect the electrical performance of a contactor.

Fig. 3. Examples of sockets

The term 'contactor' is also used to refer to sockets employed to get convenient electrical access to the DUT, especially if the DUT is a fine-pitch, high-pin count complex device. Sockets are often used for hand-testing DUT's or in failure analysis. The fine-pitched device is simply inserted into the socket, which has leads that correspond to those of the device but more widely-spaced for convenient electrical access. In some cases, the contactor used in the contactor block may also serve as a socket on its own.

Examples of manufacturers of contactors are Johnstech International, Dimensions Consulting Inc. (DCI) and Kulicke and Soffa (K&S).

Table 1. DCI's Descriptions of their Contactor Manufacturing Capabilities

DCI's Description of their Product Offerings:

We offer test socket and contactor solutions for CSP, BGA, PGA and QFP devices in the semiconductor test industry... and a broad product range-from sockets for RF micro lead frame packages to high-speed ASICs with more than 1,000 pins.... We utilize all current contact technologies such as low inductance micro pogo pins, elastomers and etched microstrip, and can adapt the contact technology most suited to our customers' needs.

DCI's Description of their Design Process:

Our socket design process covers several rigorous phases. We use state-of-the-art computer-aided design (CAD) tools, including 3D mechanical modeling. After customer approval of designs, the product goes into computer-aided manufacturing (CAM). Finally, we conduct a range of stringent performance verification tests-from a simple point-to-point continuity test to a complete characterization of the socket's resistance, capacitance, inductance, bandwidth and crosstalk.

DCI's Description of their Advantages:

- Highest first pass yield - solid reliable contact

- Lowest cost of test - sockets typically last beyond 200,000 insertions and are easily maintained

- Quick-turn design - 3D Modeling software with standardized configurations

- 10 years of contactor design and tester interface experience

- Reduced time to production - same socket for characterization and final test

DCI's Description of their Capabilities:

- Fine pitch sockets to 0.3mm

- RF sockets with center ground contacts; DC to >10GHz, <0.1nH self-inductance

- Strip contactors for testing prior to part singulation

- Multi-site sockets for parallel testing to 64 devices

- E-Beam sockets for Schlumberger IDS systems

- Handler specific contactors for Delta, Seiko-Epson, Daymark

- Characterization sockets for evaluation boards

Table 2. K&S's Descriptions of their Contactor Manufacturing Capabilities

K&S's Description of their Product Offerings:

K&S is a leading designer and producer of high performance test sockets and contactors. We have designed and produced DUT (Device Under Test) test sockets for the smallest chip scale packages, with less than 100 contacts, and for the largest microprocessors, with over 1,000 contacts. And our test sockets handle pitches as tight as 0.4 mm.

K&S's Description of their Design Process/Capabilities:

The test socket design process is highly automated and progresses from computer-aided design (CAD), which may include 3D electrical and mechanical modeling, to computer-aided manufacturing (CAM). Performance verification testing ranges from a simple point-to-point continuity test to a complete characterization of the socket's resistance, capacitance, inductance, bandwidth and crosstalk....

We use a combination of design techniques - including spring-loaded probe contacts, a floating base, and package specific inserts - to gently guide DUT package contacts into position while socket contacts and other mechanical seating devices adjust and follow. This prevents damage to the IC package during manual or automated actuation, and ensures that contact force is distributed uniformly....

We design a variety of spring-loaded probe contacts to meet different performance parameters. And because our contacts are separate and independent from each other, low-inductance contacts that deliver high-speed signals without degrading edge rates (rise and fall times) can be placed along side higher-inductance contacts for power signals. Separate, independent contacts also decrease crosstalk.

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