SMT007 Magazine

SMT-Nov2017

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32 SMT Magazine • November 2017 While there are many advantages to using a hot air rework system, there are some draw- backs the user needs to be aware of when trying to decide which reflow source to use. First, due to the nature of the hot air source and the ev- er-decreasing mass of SMT components, solder joints can be disturbed or parts can be skewed during reflow. This is especially true for 0201, µBGA and other micro packages. Secondly, to make sure the hot air blows effectively on the package and not onto the neighboring com- ponents, a customized nozzle is required. Too large a gap between the edge of the package and the nozzle and then the neighboring parts will tend to go in to reflow. A poorly-designed noz- zle leaves a temperature gradient in the hot air source thereby leading to inconsistent results. In addition, customized nozzles take several weeks to fabricate and are not cost-effective for small quantities. Finally, in many cases the peak temperature will be higher for a hot air profile versus that of a similar IR reflow profile. This may cause parts in the neighboring vicinity to those being reworked to reach their softening point (like plastic-bodied relays and connec- tors), thereby causing damage to them. What is IR? Infrared (IR) technology, the other wide- ly-used heating source for PCB rework, was first introduced into SMT repair equipment in mid-1980s. An infrared heater is a body trans- ferring energy to a body with a lower tempera- ture through electromagnetic radiation in the infrared spectrum with wavelengths from 780 nm to 1 mm. There are two basic styles of IR technology. The first is medium range IR (Fig- ure 2), which emits the energy and is "blocked" from some areas of the PCB by "shuttering." The second is a focused IR heat source (Figure 3) in which IR radiation is collimated and directed through a lens system. Advantage of IR The IR heating source presents some advan- tages to the PCB rework process. It is important to realize how passive and gentle the method is and, in fact, at full power the heating effect is so slight that you can hold your hand in the beam for some considerable time before any effect is felt. This makes the technology advantageous for applications where heat-sensitive compo- nents found in rework may not be damaged. TWO PREVALENT REWORK HEATING METHODS—WHICH ONE IS BEST? Figure 2: Medium IR heating technology for PCB rework.

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