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transition to Pb-free soldering due to the novelty of materials and questions of compatibility with higher reflow process temperatures. 47.3.6 Cooling The fourth and final stage is cooling. It is here that the board s temperature is lowered beyond that of solder liquidus prior to exiting the oven. Once again, recommendations for device heatingand-cooling ramp rates should be heeded. Most electronic packaging and board materials are reluctant to shed heat quickly. If the board is thick, the laminate, a relatively poor thermal conductor, will remain hot. It must be cooled to below solder liquidus prior to exiting the oven to preclude dislodging of any soldered components or disturbing the solder-joint solidification process. On the other hand, with the trends toward thinner circuit boards, slimmer components, and hot, fast-soldering profiles (to keep up with high-volume product demand), it is wise to ensure that the negative thermal ramp requirements are considered for this step also. As mentioned previously, the solder paste manufacturer will provide recommended profile guidelines, but those are only a starting point. The process engineer should optimize the reflow oven profile with product profile boards and analysis based on actual product runs. Board profiling will be covered in a subsequent section. Older textbooks and industrial references cite completeness of fillet formation and solder reflectivity as typical hallmarks of a good reflow process, but these are subjective measures that may not be true indicators of solder-joint quality. As mentioned previously, overheated joints will be brittle although they may appear to be well wetted. SAC and other Pb-free solder alloys have a naturally grainy-looking solid structure as compared to that of Sn-Pb, so a dull solder joint is not necessarily indicative of a poor solder joint. Tests such as tensile pullpeel, shock and vibration, thermal cycling, x-ray for solder void content, and metallographic cross-sectioning are much better means of assessing solder-joint quality. There is yet another implication with the move to SAC solders. If not quenched quickly, this alloy is known to result in some constituent separation. Silver may precipitate out as platelets or needles. This can be minimized by using a fast-cooling ramp. Check with component manufacturer s specifications so as not to jeopardize component integrity. 47.3.7 Successful Oven Reflow Successful soldering is dependent on several factors including suitable, well-maintained, and controlled reflow equipment; good-quality solderable parts; a thermally balanced board designed for the reflow process; a well-tested and reliable solder paste; a proven time-temperature profile; and good thermometry techniques. 47.3.7.1 Adequately Maintained and Controlled Reflow Equipment. The oven should have small thermal differentials across the width of the tunnel and should be capable of adequate heating and cooling ramps as well as adequate zone separation characteristics. It should be stable for a given zone setting, not varying by more than a couple of degrees centigrade. Similarly, the conveyor speed should be stable, not varying by more than 1 cm to 2 cm/min. 47.3.7.2 Good-Quality Solderable Parts. The best reflow equipment, profiles, and solder paste will not make up for inadequacies in part quality or restore solderability. Good solderable parts and boards are a must for successful reflow. 47.3.7.3 Coplanarity. Coplanarity of component leads, BGA balls, or CCGA columns is a must.All component leads should share a common seating plane within the specified tolerances associated with the component vendor s specification. The coplanarity requirement also depends on the dispensed solder paste height and paste deposit uniformity, as the solder paste accommodates some small differences in lead-to-lead coplanarity both as a paste and also as the solder assumes the characteristic dome shape upon melting.
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47.3.7.4 Component Storage. Extended storage should be avoided and parts should be kept cool and dry prior to use. Sufficient quality control methods should be established to ensure that the solder and components are solderable. Plastic molded parts such as plastic quad flat packs (PQFPs) and plastic ball grid arrays (PBGAs) can absorb moisture from the atmosphere over time. When moisture-laden plastic packages are subjected to reflow soldering, the entrapped vapor may expand to the point where the package fractures, often damaging the silicon die and associated wires internal to the package. This phenomenon is commonly known as popcorning throughout the industry. Ensure that plastic packaged components are stored in unopened bags as supplied by the component vendor until time of use. Once the components are removed from the manufacturer s packaging, follow IPC/JEDEC guidelines1 for useful shelf-life in the unbaked state and any bake-out requirements prior to soldering. Parts must be suited for the temperature required by the reflow process. Lastly, ensure that component contact or surface finish is compatible with the solder being used, be it leaded or lead-free. 47.3.7.5 Board Design for Oven Soldering. Thermal balance of the board design is important for reliable and adequate reflow. Ensure that a design for manufacturing (DFM) review of the board has been done and that thermally massive components are not relegated to one portion of the board. Also, it is desirable to distribute smaller components rather than to create component fields to permit uniform thermal balance across the board. Components should be spaced adequately to prevent shadowing of smaller devices on the board by larger, neighboring components. This is particularly important in IR ovens without the forcedconvection option. If the oven is characterized by poor uniformity across the tunnel width, the board should be oriented, if possible, to take advantage of the imbalance; i.e., the board edge with greatest thermal mass should be aligned to the oven edge with the highest recorded process temperatures. Board components should be designed far enough away from board edges to preclude any mechanical interference with the pin-chain conveyor. Since the oven rails can have an influence on the reflow or damage of bottom-side components, leave enough edge clearance on the board to accommodate the pin-chain pin length plus some additional margin in case the board is moved from one reflow oven to another. 47.3.7.6 Paste and Profile. The profile provided by the solder paste vendor is only a recommended starting point that needs to be optimized for the reflow conditions and process time requirements. Solder paste with finer solder particles are more troublesome to flux owing to their greater total surface area and corresponding greater surface oxide volume per unit volume of solder. It is important to limit exposure to high temperatures and particularly time above liquidus, as it is in this regime that the intermetallic layer growth is most vigorous.The thicker the intermetallic layer, the more brittle and less reliable the solder joint will be. 47.3.8 Impact of Pb-Free on Reflow Soldering Although the move to Pb-free soldering represents a major change for the industry, its impact on the reflow process and reflow equipment is minimal. In many cases, reflow ovens used for Pb-bearing solders can be used for Pb-free reflow. There has been concern that Pb-containing dust collected on the tunnel walls, fans, and such could contaminate Pb-free solder, but it is unlikely that the small number of Pb-bearing particles in a properly vented oven will provide enough contamination to exceed the RoHS limit for Pb of 0.1 percent by weight. As mentioned previously, to permit Pb-free reflow, reflow equipment may require some oven modifications. There is little impact to the reflow process and most of the changes are material related. Because many of the Pb-free solders are slower to wet out on leads and pads, extended wetting times may be required. Many Pb-free solder pastes are more prone to solder void formation. Extended soak (pre-heat) times and slightly higher peak temperatures are known to help with void reduction.
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