Circuits Assembly - November 2007 - (Page 41)

Reflow Soldering Aside from these key factors, planning an accurate thermal profile includes avoiding thermal shock by ensuring gradual temperature increases or decreases between different oven zones, carefully monitoring soak time and peak temperatures, and following solder paste manufacturer specifications. Solder paste manufacturers provide a specification chart associated with a particular paste. It provides detailed information about ideal temperature ranges in different oven zones. Included are preheat, soak, reflow and cool-down cycles during the final stages of reflow. If an assembler follows these charts carefully, it can nail the thermal profile. But if these specifications aren’t followed to the letter, the subsequent poorly developed thermal profile could result in voids and dewetFigure 3. Careful thermal profile monitoring involves following computer screen data. Here, a ting if too hot, and non-wetting and thermal profile shows 170°C in zones 3 and 4 and 195°C in zone 5. solder joint fractures if too cold. Proper thermal zone management is another major contributor to perfect thermal profile accuracy. Ovens must be well calibrated ponents. The result: extra rework, assembly costs and and routinely maintained. Specific segments must be product delays. The number and specific factors that lubricated on a regular basis, and manufacturer techniaffect a thermal profile’s accuracy include: cians must perform annual maintenance to calibrate the • Number of layers. ovens. An improperly operating heater in a zone can apply • PCB thickness. the wrong temperature in a specific zone, thereby creating • PCB dimensions. a defective and potentially unreliable product. • Number of planes. Finally, efficient reflow soldering demands a highly • Type of PCB material. effective exhaust facility. A reflow oven manufacturer’s • Component types. specifications detail the correct cubic feet per minute • Air pressure applied. (cfm) exhaust. It needs to be measured on a regular basis, Take, for example, the PCB thickness. A 0.062" board and if needed, calibrated. If an oven’s exhaust is lower or will have different thermal characteristics associated with higher than spec, the temperature within different zones the different zones compared to a 0.093" or 0.125" board. is jeopardized. The thicker board will take more time to heat the board’s If hot air stays in a zone longer than is required, the different segments. Likewise, layer count is important. board is subsequently heated more than what is specified The stackup structure containing the number of power for a given zone. If hot air is not present for a sufficient and ground planes determines the amount of heat the time, the board remains cold and proper reflow not perboard needs to absorb in any given cycle to create a performed. It may change flux activity at the solder paste fect thermal reflow. level because the air may be too hot or cold. Correct air Component types are also an important consideration pressure is also important to avoid sucking out or altering for generating a perfect thermal profile. If the board is location of such small devices as 0201s or 01005s. A good mostly populated with through-hole components and a rule-of-thumb is to routinely check exhaust levels and the few surface mount ones, it will require less heat than a cfm to ensure proper calibration. By implementing these board largely populated with SMT parts. Moreover, a fully major principles and practices, perfect reflow soldering is populated SMT board with CSPs, BGAs and 0201s will on the way to being ensured. have a vastly different thermal profile than a PCB popu■ lated with through-hole and SM components. Further, the reflow oven air pressure must be carefully monitored Zulki Khan is president and founder of NexLogic Technologies Inc. (nexlogic. to avoid blowing away smaller devices such as 0201s and com); zk@nexlogic.com. 01005s. circuitsassembly.com Circuits Assembly NOVEMBER 2007 41 http://nexlogic.com http://nexlogic.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - November 2007

Circuits Assembly - November 2007 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business Global Sourcing On the Forefront Screen Printing Better Manufacturing Pb-Free Manufacturing from a Tier III EMS Perspective ‘Living Documents, Subject to Change’ Creating Ideal Solder Joints An Alternative Drying Process for MSDs ‘Customer Satisfaction is More than a Score’ Optoelectronic Substrates: Will They Happen? Tech Tips Wave Soldering Process Doctor Test and Inspection Getting Lean Materials World Equipment Advances Product Spotlight Assembly Insider Technical Abstracts Ad Index

Circuits Assembly - November 2007

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