By Gary Eastman
With the rapid transition to environmentally friendly manufacturing, creating a new reflow profile to accommodate everything from lead-free component materials to solder paste changes can be quite a challenge. As electronic OEMs move toward compliance with the Waste Electronic and Electrical Equipment (WEEE) and the Restriction of Hazardous Substances (RoHS) Directives in Europe, as well as other worldwide environmental initiatives, changes in reflow profiles and manufacturing processes are resulting in a new set of process-related issues. Knowing what to consider when ball grid array (BGA) sockets are part of the mix can help you avoid misconceptions and down-time.
Start With Basics
When everything is changing, accuracy and control are essential. Since it is nearly impossible to visually distinguish traditional tin/lead solder balls from their lead-free counterparts, controlling component identification including part numbers and lot numbers will ensure that only lead-free surface mount sockets make it into your new reflow process. (See Fig. 1) 
In addition to being RoHS-compliant, meaning free of all banned substances including lead, sockets must be selected which are RoHS-compatible, i.e. capable of withstanding the elevated temperatures present in lead-free reflow profiles. To achieve both compliance and compatibility, typical BGA sockets have changed in two major ways. First, the banned substance lead (Pb) was removed from the BGA socket’s solder ball terminals, making them compliant to the RoHS Directive for material content. Suitable replacements such as tin/silver/copper (Sn/Ag/Cu) solder balls, however, require higher processing temperatures for proper reflow (liquidus at 218¬?C [424¬?F] vs. 183¬?C [361¬?F] for eutectic tin/lead terminals). This caused a domino effect, sometimes requiring insulator materials to change, not to eliminate banned substances but for compatibility with the new high temperature processing requirements.
In addition to temperature considerations, new lead-free BGA sockets must be compatible with different PC board surface finishes, reflow methods and paste media. All combinations of finishes and solder paste are not universally compatible.
Avoiding Defects
A typical concern in any reflow application is open solder joints. Determining the source of reflow discontinuity when utilizing surface mount sockets can be a challenge. Communication and detailed information (see sidebar) are critical to identifying the root cause and providing an effective resolution.
Agreeing ahead of time on coplanarity parameters and measurement techniques will ensure that process engineers and socket manufacturers are on the same page when reviewing results and analyzing defects. Coplanarity is the measurement of flatness. Typically a minimum of twenty readings are taken with precision electronic measuring equipment such as a SmartScope by Optical Gaging Products Inc. of Rochester, NY. After recording the twenty readings, the technician takes the lowest and the highest reading and the difference is the coplanarity. This method can apply to sockets, devices and PC boards.
Open solder joints are easily identified through electrical test. Recreating the scenario in a controlled environment is often not so easy. To effectively perform failure analysis, the entire PC board must be reviewed. The first step is to identify all the different components and their mass. Equating the impact of the thermal profile on the different components can uncover previously hidden problems. For example, as the reflow profile temperature increases, solder paste and flux experience thermal transfer first, reaching their melting point before the solder ball on the BGA socket. In this scenario, the flux will vaporize and the paste will reflow to the pad on the PC board. As the thermal transfer continues, the lead-free solder ball, often made of Sn/Ag/Cu, will reach its melting point, but without the flux present so there is no reflow with the paste to the PC board.
This phenomenon, called the ‘head in pillow’ effect, occurs when the second item to reflow (in this case, the solder ball) forms itself around the paste which has already reflowed. The absence of the flux does not allow proper solder joint formation (see Fig. 2). 
The solution lies with timing of soak time and the thermal transfer so that all components reach melting temperature at or near the same time. Too long of a soak will diminish the effectiveness of the flux. In a phrase, process discovery leads to process control.
Other potential causes of reflow discontinuity are contamination, insufficient solder paste or incompatibility of the paste with the solder ball composition. These problems can be diminished by process disciplines such as storing components in vacuum-sealed packaging, or at minimum, in sealed containers, as well as process inspection to prevent clogged stencils. A review of the chemistry, profile, paste, component and PC board finishes with your solder paste supplier can also help prevent down-time and process-related defects. Solder paste manufacturers are a reliable resource for guidance on process recommendations and compatibility issues.
Conclusion
Experienced socket manufacturers will provide application assistance to help ensure a smooth transition to lead-free manufacturing. Communication is important throughout the design and manufacturing processes from socket selection, to setting up a new profile, processing and final inspection. Partnering with an experienced interconnect supplier will help you avoid processing issues, ensuring a smooth transition to lead-free manufacturing.
Technical Support
When lead-free processing problems arise, the following information should be gathered and communicated to the socket manufacturer to enable a complete analysis:
– Type of defect
– Method of defect identification
– Photos, cross-sections, and/or x-ray images
– Location of defect on PC board assembly
– Soldering surface finish
– Pad size
– Solder paste type
– Stencil type, aperture style and thickness
– Hand or machined placed
– Measurements of coplanarity for the PC board
– Thermal profiles for the soldering cycle
– Data pack thermal measurements
– Product/component identification
Gary Eastman is the Application Engineering Manager at Advanced Interconnections Corp. in West Warwick, RI USA.
advintcorp.com
