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114 SMT Magazine • September 2015 Galden ® , a proprietary heat transfer liquid from Solvay, is the most popular material typi- cally used in these processes, and is quite expen- sive, although most of the material condenses back into the bath at the end of a heating cycle, so it can last quite a while. Vapor phase is a very predictable method of heating, but because it is a very expensive system, it is more appropriate for extremely high-end precision applications such as MIL-spec electronics. Pros: precise temperature control; elimi- nates the need for a profile based on product mass or size; less overall programming required since a single recipe fits all Cons: extremely high cost, up to $100,000 versus a comparable convection oven at $15,000; material very expensive; typically batch process only with slow cycle times (10–20 min/cycle versus 4–5 minutes for convection) Infrared (Ir) IR systems were widely used throughout the 1990s because they were relatively inexpensive to buy and operate. IR heating can be used in either batch or continuous conveyorized units. While still available for specialized applications, IR heating is affected by the color of the target components, so different colored components absorb heat at different rates and can produce different heating profiles, not entirely desirable. Today, IR heating is often used for special applications where heat ramp rates need to go quickly with very little soak time. The direct heat- ing technique of IR makes modulation and con- trol very difficult. Factors that can be used to vary the heating profile include distance, speed and heat intensity. For these reasons, IR is not often used in low- to medium-volume production en- vironments which need to accommodate a wide range of board configurations and heat profiles. Pros: appropriate for specific applications requiring direct heating, but no longer widely used in circuit board assembly Cons: very unforgiving heat; boards may be easily damaged by overheating, delaminating, tombstoning, etc.; heat profiles difficult to set based on material type and color; trials are usual- ly necessary, making it impractical for short runs Convection Convection heating is the most widely used method for reflowing circuit boards today. In this system, air is circulated throughout one or more chambers (or zones), either vertically or horizontally, to surround the entire board as- sembly with even, uniform heat. Each zone in a multi-zone oven retains its own heat profile very reliably. Both the vertical and horizontal methods of heat distribution share the same end result—even heat around the boards. A plenum heat distribution system typical- ly directs air vertically from above and below toward the board surfaces. They often require preheating plenums to condition the air tem- perature before heating the board, but directing airflow from two different directions (top and bottom) allows the user to adjust heat settings independently and with good control to create a very precise profile. This type of system is usu- ally more complicated to build and thus more expensive than a horizontal convection type. Pros: even heat distribution; individual zone temperatures are extremely close to prod- uct board temperatures, making it easier to pro- file than IR; less trial and error testing needed Cons: certain methods of air circulation may not be as effective or easy to manage as horizontal convection In a horizontal convection chamber, heat is uniformly distributed throughout the entire chamber, resulting in even heat distribution around the board itself. The length of each zone and the conveyor speed determine the amount of time a board resides in a particular heating environment, ensuring very reliable and repeat- able processing quality. As opposed to air circu- lation moving vertically above and below the board, in horizontal reflows the air is circulated in one direction across the top of the board, and in the opposite direction beneath the board. This is a key benefit that prevents hot spots, and allows the parallel airstream 'angle of attack' to infiltrate the spaces beneath the components. Pros: Even heat distribution, as the top and bottom of the board receive the air from the outside to the center, which counters the com- Smt quiCK tipS seLeCtING A reFLOW OveN, pArt 2 continues

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