ABSTRACT Ball densities of chip-scale packages are increasing as the CSP form grows in popularity. Printed circuit board input/output densities for test and burn-in applications, however, are not keeping up, and the increasing cost of meeting burn-in requirements is pushing everyone's budget. An immediate solution to this density problem is the use of surface-mount sockets on high-density interconnect PC boards. While surface-mount sockets achieve denser boards at lower cost, successful implementation requires close attention to design rules to assure success. By Ariane Loranger, Loranger International Corp., Warren, Pa. Future chip-scale packages will be produced with finer pitches and increased pad counts on PC boards (with smaller pads spaced closer together). These changes are creating challenges to the design of present and future burn-in sockets that must remain reliable for multiple insertions over many years of use. (Typical surface-mount burn-in sockets for chip-scale packages are shown in Figure 1.)
Gold-plated beryllium-copper (BeCu) contacts in the surface-mount sockets are compression-style, as represented in Figure 2. The physical and mechanical attributes of the Be-Cu contact are controlled closely to ensure optimum force of the contact against the surface-mount pads and solder balls of the package.
Meeting the Challenge Surface-mount burn-in sockets are meeting the challenge posed by the decreasing pitch in CSPs. Sockets designed with compression-style contacts allow the surface-mount sockets to adapt to the decreasing pitch of solder balls more easily than through-hole-style sockets. The larger diameter plated-through holes consume more real estate on inner layers (compared with vias) and therefore deny more escapes from inner rows of pads. As the ball pitch drops to 0.4 mm, only compression-style contacts will be easily able to connect with the solder ball, because these contacts require less room than other styles. With tweezer-style contacts, there is insufficient room in the greater densities for contact to be made by grabbing the solder ball at the center from above. Larger ball counts for future chip-scale packages will place increasing demands on through-hole socket-lead alignment during installation, an issue that surface-mount sockets will not have to face. Package cost is dropping, as depicted in the table, which shows a reduction of greater than 18% in cost over the next four years. Burn-in costs must also drop. The lower costs needed for fabricating burn-in boards can be achieved with surface-mount sockets. The ease of replacement and repair of surface-mount sockets and boards will also greatly contribute to lower maintenance costs.
The Advantages Advantages of surface-mount sockets over through-hole types include: (a) Easier installation of sockets on burn-in boards-it's not necessary to align hundreds of fine leads with their holes, as is necessary for through-hole sockets. (b) The smaller footprint of surface-mount sockets requires less area for contacts, permitting an increased number of sockets on burn-in boards, thereby reducing cost-per-position. (c) Easier replacement of surface-mount burn-in sockets for repair or maintenance, since the sockets are fastened to the burn-in board by four screws, instead of multiple soldered leads equal to the I/O count of the package (d) Easier removal and replacement of contacts, resulting in a longer socket life (e) Reduced layers on burn-in boards-escape vias on the inner layers of surface-mount socket boards are smaller in diameter than through-hole boards, so traces can be routed more easily. For example, outer rows can be routed straight out to meet the schematic requirements of the board without requiring plated-through holes. (f) Wider traces can be used on the burn-in board permitting lower impedance. (g) Fewer plated-through-holes must be drilled into the burn-in board to interconnect copper layers. The top layer of the surface-mount socket board does not require vias for the outer peripheral balls (pads). However, the top layer of these same balls in a through-hole board does require plated-through holes. Liabilities Potential liabilities of surface-mount sockets over through-hole types yields a much shorter list. Things to watch out for include: (a) A possibly higher contact resistance. This higher resistance can occur because the connection between the contact and surface-mount pad is mechanical rather than electrical (soldered). However, after many years of field use, response from our customers shows no difference between through-hole and surface-mount sockets (b) More care must be directed at maintaining a clean surface on the burn-in board to ensure that the mechanical contact makes a good electrical connection Summary Surface-mount vs. through-hole burn-in sockets reveal many advantages in the performance and cost of the former. Surface-mount burn-in sockets require less on-board real estate, which reduces layers of board construction, resulting in reduced cost. Future trends of shrinking dimensional attributes in packages and PC boards favor the more compact and reliable design of surface-mount burn-in sockets. References 1. SEMATECH, International Technology Roadmap for Semiconductor Assembly and Packaging, Austin, Texas, 1999. 2. Roadmap `97, The National Technology Roadmap for Electronic Interconnections.
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