Good Contact Design Improves Test Performance in BGA/CSP Applications

The expanding range of new package types requires new production process controls and contact methodology for electrical test interface solutions.

By Randy Knudsen, Johnstech International Corp., Minneapolis, Minn.

Semiconductor manufacturers are continuously facing new challenges in developing test interfacing for the diversified and rapidly-changing world of semiconductor packaging technology. For years, the majority of packages manufactured were dual-in-line package styles. Later, surface mount technology changed IC packaging to QFP, PLCC and many other formats.

With tens of billions of these SMT packages produced each year, manufacturers have learned how to design test interfaces for the various lead pitches and lead counts. The recent introduction of area array SMT packaging, such as BGA/CSP, reduces the mounting area of the package, making it suitable for today's high density, compact products. This size reduction and the use of solder balls has produced a new set of challenges.

Figure 1. The Short Contact BGA/CSP socket.

The expanding range of new formats requires package manufacturers to develop production process controls and contact methodologies for electrical test interface solutions. Test interface solutions have to be designed for engineering characterization and production performance requirements. The manufacturer's goal is to provide a single solution that can be transported from the engineering lab directly to the production floor.

Challenges
There are currently dozens of different BGA/CSP packaging technologies being promoted. Each manufacturer has its own package design, manufacturing process controls and tolerance issues. Due to the wide variability of manufacturing tolerances, building a test interface to encompass them all can introduce a number of interface challenges.

One interface problem that many test socket manufacturers face is the potential of damaging the device during contact. While industry opinion varies on how much damage the solder balls can take during testing without affecting final test and production, it's easy to remove this variable with good contact design. In BGA/CSP applications, test socket and handler manufacturers must take great care to ensure that the surface of the solder ball is not seriously disturbed or damaged.

Contact Design
By incorporating elements of good contact design, test socket manufacturers can contribute to the customer's overall performance and profitability by reducing retest rates, increasing first pass yields and eliminating false "good parts" readings. Test sockets are often taken for granted or regarded as commodity items. This can be a costly oversight, since the performance characteristics of a test socket have a direct impact on testing costs, manufacturing quality, production yields and corporate profits. The test socket and its contacts are an integral part of the total test interface, and critical elements of design can affect performance. Test socket manufacturers must follow elements of good contact design that include: profile, contact material and plating, motion, biasing and contact force. An examination of these important elements follows.

The contact profile is critically important to achieving high bandpass, low contact resistance, long life and high reliability. Spring-style or vertical (Pogo®) style contacts are the most common type of interface element, but may not be appropriate in all applications, including BGA/CSP device testing. Instead, a novel contact, based on a modified "S" shape called, the Short Contact™ (Figure 1 ) may be used. (Figure 2 shows a detailed view of the contact and the BGA test socket.) The S-shaped, short, rigid contact provides a shorter electrical path, as well as lrywer resistance, capacitance nnd inductance. This contact type also offers increased rigidity and much greater immunity to breakage. For BGA/CSP applications, in particular, the Short Contact requires less force and presents a less damaging contact to the device under test.

In BGA/CSP applications, damage to the lower hemisphere of the solder ball may cause flux entrapment and co-planarity error. Due to the new contact's profile and motion, a "no damage zone" (NDZ) on the lower half of the solder ball remains. (Figure 3 shows a representation of the NDZ.) By contacting the side of the ball at approximately 45° from the equator with minimal force (Figure 3), no contact is made to the bottom surface of the ball (the NDZ) and only a witness mark is visible on the side of the ball.

Figure 2. This BGA/CSP test socket design is based on a Short Contact and wiping motion.

Material Selection
Contact designers must be careful when selecting alloys for use in contacts to ensure that the proper material for good electrical performance and mechanical life is selected. A beryllium-copper alloy enhances the contact's tensile strength and spring characteristics. Beryllium-copper also has the advantage of decreasing contact resistance, which allows a greater current-carrying capacity for the same cross-section of metal than other alloys.

Contacts must be plated to achieve low-resistance connections over a wide temperature range and frequency spectrum. Plating also prevents oxidation of the base contact material and helps prevent solder build-up. At high operating frequencies, most current travels near the contact surface. This is a phenomenon known as the "skin effect." For this reason, the plating thickness should be 2-3X the penetration of the current or "skin depth." Beryllium-copper, plated with 200 micro-inches of electroless nickel, followed by 50-100 microinches of hard gold, provides outstanding electrical performance and long contact life.

Motion
The contact tips develop complex horizontal and rotational wiping actions against the solder balls of the device under test. The Short Contact has an advantage over spring-style contacts that generate wipe through the compression of the contact itself. This compression can lead to solder ball damage or solder build-up in BGA/CSP packages.

The Short Contact's wiping action is controlled through the supporting element, or elastomer, made from a proprietary silicone rubber compound. The amount of wipe with the new contact is between 0.006 and 0.010 inch. Too little wipe may result in solder buildup and frequent cleaning of the contacts; too much wipe allows solder to accumulate in the test site area, decreasing reliability. Excessive wiping force may also damage solder balls.

Figure 3.By lightly touching the side the solder ball, the Short Contact leaves the lower hemisphere untouched, reducing the potential for damage to the solder ball.

Biasing
In traditional test sockets, the amount of "spring" built into the spring-style contact determines the biasing force that enables consistent, reliable contact with the device under test. Accidental bending, expansion or contraction caused by temperature changes and metal fatigue may cause these forces to change, resulting in misalignment. This change, in turn, results in lower contact accuracy, which decreases reliability and may damage the solder ball in BGA/CSP applications.

Conventional sockets also have the contacts set into a plastic housing for retention. Constant use in production testing can cause the contacts to be unseated, causing the contact tips to lose their co-planarity. When this occurs, overcompression, or greater force, is required to contact the lower points and achieve continuity. This may result in damage to the solder balls in BGA/CSP applications.

In the Short Contact, however, the metal contacts are rigid and housed in Torlon® 5030. This material performs well at high and low temperatures, with excellent rigidity and wear resistance. It also closely matches the thermal expansion rate of BGA/CSP packages. Biasing forces are determined by the compression and tensile properties of the elastomer, removing the point of force from the solder ball to the elastomer, allowing for a "soft contact" to the ball itself. The elastomer is engineered to offer predictable, long-term performance over a broad, temperature range and for hundreds of thousands of actuations.

Contact Force
Ideally, the downward force applied to the conventional contact tip determines the amount of deflection. However, sideways flexing of the contact can cause unpredictable deflection if the contact is made of a tensile metal, has a top-heavy profile, or is supported by a narrow stem. The Short Contact requires less force and is less likely to flex; therefore, it is able to provide a softer contact to the solder ball in BGA/CSP applications.

Additional Benefits of the Short Contact
The S-shaped contact, by its shape and inherent wiping motion, provides these added benefits for BGA/CSP device testing:

• Minimal Marking and Reduced Solder Build-up
• Missing-Ball Detection
• Repeatable Results
• Replaceable Contacts
• Improved Electrical Performance
• Handler Interfacing

If a sufficiently large mark, dent or pocket is formed on the bottom of the solder ball, flux may be trapped, leading to a poor interconnection during the reflow process. In addition, any surface disturbance to the bottom of the solder ball may increase the overall co-planarity error of the device, which is critical to device placement. The Short Contact eliminates this possibility.

Missing Ball DetectionB
Inherent in this design is the detection of missing balls. Because this method of contacting makes contact off the axis to the ball and land area of the device, missing balls are detected because the contact is not directly under the area of the ball. A vertical style contact against a 0.75 mm ball, typical on a 1.27 mm pitch BGA, may require that the compression (or overtravel) be so great that the contact reaches the land area where a ball may be missing. This compression results in false "good parts" readings and possible damage to the higher solder balls (Figure 4).

Figure 4.Pogo pin test solutions may give false "good parts" readings by pushing to the bottom of the board in the absence of a solder ball