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By Herbert Endres, Molex
Traditional techniques of applying solder have become impractical for today's high-density mezzanine connector market. To stay relevant, connector manufacturers have designed new methods for attaching surface mounted connectors. A new solder charge method has proven to provide good fatigue strength and lower applied costs compared to traditional SMT mounting methods.
Traditional methods of applying solder for applications with high-pin-count devices on mezzanine cards or module PCBs are being stretched to the limit. The solder grid footprint is reduced to as small as 1.27 × 1.27 mm, as well as the corresponding paste printing area, which now is approximately 1 × 1 mm. For most uses, this small amount of space does not allow enough paste coverage to make a good solder joint.
One common way of solving this problem is to use the components as a carrier for the solder to the joint. This attachment method, otherwise known as BGA, was readily adopted in the industry; achieving excellent density benefits. BGA attach technology does come with drawbacks.
One of the biggest hurdles connector manufacturers encounter with BGA is solder wicking during reflow. Also referred to as solder creepage, this phenomenon occurs when the solder wets the contact rather than the board. Solder wetting control techniques have been developed for BGA connector products; however, each has its own limits. Other common BGA failure modes include missing connections, coplanarity, solder bridges, voids, open connections, mis-regulation of contact lead on the PCB pad, and poor intermetallic bonds.
To combat these issues, an alternative method of mounting was developed for standard SMT attachments. The solder charge optimizes the density benefits already realized by a BGA tail and improves processing and manufacturing aspects to achieve a cheaper, stronger, more reliable PCB interface.
The solder charge concept is fairly simple. It consists of a blanked solder mass that is extruded through a hole near the tip of a terminal until it fills a volume on the opposite side of the pin. The chamfer on the solder charge side of the hole provides retention and prevents the solder from backing out of the terminal. A laser ablasion process ensures that there is no solder creepage during reflow and the solder mass goes fully into the solder joint.
While a BGA, when manufactured, has a ball of solder manually placed and then heated to adhere to the J lead on the bottom of a connector, the solder charge method uses stamping equipment for adhesion to the terminal. This allows precision placement with high accuracy.Solder charges use standard reflow processing, so a reflow profile is available for every product using this feature. The solder charge extends slightly beyond the tip of the terminal to seat within the solder paste on the pad. The terminal contains an extended tip, or nipple, to provide a pivot point to the PCB and avoid the undesired characteristics of a butt joint. The terminals then lower onto the pad during reflow.
Figure 2. The fillets created by solder charges are more robust than those seen with BGAs.
When heated, the solder transforms into a rectangular sloping solder fillet (Figure 2). This bugle shape is a more robust structure than ball-shaped solder fillets. The solder provides a 360° coating around the terminal and reaches through the hole in the pin for added retention strength. A solder charge provides a significant amount of tolerance to compensate for variability in the PCB flatness, giving added insurance that PCB adhesion will work effectively. In-house tests, where both solder charge and BGA products were processed on a PCB and then pulled off of the board, have shown the solder-charge joint to be three times stronger than that of a BGA.
ConclusionA solder charge, since it can adjust easily to inconsistencies in the flatness of the PCB, facilitates consistent soldering processes and counteracts problems that may occur at the solder interface from other design variables. Because the solder charge extends beyond the tip of the terminal, the terminals will lower when processed in a reflow oven. Solder will form around the nipple to accommodate for inconsistencies in board flatness and add retention to the resulting solder joint. Lower-cost board designs can be used without sacrificing product integrity. The solder charge technology, which can be leveraged in a number of products that currently use the BGA mounting method, has been adopted quickly by major OEMs and contract manufactures (CMs) to enhance yields and increase reliability over typical BGA attach methods.
Herbert Endres is a qualified engineer and technology marketing director at Molex Incorporated. Since 1978, he has led sales, marketing, and product management activities for connectors and interconnect systems at TRW, Labinal, and Molex. From 1971, he worked in project planning and service for electronic industrial controls, then product manager for passive components. Endres studied at Nuremberg's Ohm Polytechnic.