Two Prevalent Rework Heating Methods--Which One is Best?
There are two prevalent heating technologies in use throughout most electronic assembly operations for advanced component rework. The first method employs the use of hot air gas to heat up the component through the package—this may or may not include the injection of nitrogen. A second much-used heating technology relies on infrared energy. This electromagnetic radiation, safe to the operators, is absorbed by the package, thereby sending the solder into reflow.
Hot Air Rework
A typical hot air rework system employs a heat source and an air source that forces the heated air through a nozzle (Figure 1) configured to the area of interest, which heats up the component to be reworked. There are numerous levels of such a hot air system, including a completely manual, a semi-automatic, and an automatic system, each with its own features.
Why Hot Air?
Hot air convection heating for PCB rework is advantageous for a variety of reasons. The absorption of heat by the component and circuit board is independent of a material’s color or texture. Additionally, inducing nitrogen to the site during component reflow has the advantage of making sure the metallurgical structure of the solder joint is the most reliable. In air-atmosphere rework, an oxide layer forms around the solder sphere as it reflows. Inducing nitrogen during this step displaces the oxygen and limits the oxide layer forming around the molten solder. Finally, a hot air convection system quickly delivers heat energy into thermally massive boards.
Figure 1: Nozzle delivers hot air in and around the package during rework cycle.
While there are many advantages to using a hot air rework system, there are some drawbacks 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 ever-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 components, 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 nozzle 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 connectors), thereby causing damage to them.
What is IR?
Infrared (IR) technology, the other widely-used heating source for PCB rework, was first introduced into SMT repair equipment in the mid-1980s. An infrared heater is a body transferring energy to a body with a lower temperature 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 (Figure 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 advantages 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 components found in rework may not be damaged.
Figure 2: Medium IR heating technology for PCB rework.
IR systems measure temperature empirically at the PCB or component, whereas hot air systems measure hot air temperature near the hot air exit source. This makes for a more-exacting profile based on achieved component temperature. In addition, IR heat sources require little tooling for specific BGA sizes. Another advantage is the ability to easily take in-process temperature data of the IC for each reflow operation. However, the primary advantage to the IR heating is the ability to not disturb very small components as no turbulent air flow exists along with the heating source.
Figure 3: Focused IR heating technology for PCB rework.
While the infrared heating source has some distinct benefits over the hot air systems, it too has some shortcomings as a reflow source for BGA rework. There are still some underpowered IR heaters on the market which means that the rework process cycle time is much longer than a properly-designed nozzle and hot air system. This means that for very large boards such as servers, backplanes and other high current-carrying PCBs, the IR heat source may not be sufficient. The user needs to be aware that IR systems using medium wavelength IR resulting in darker-colored components have a different absorption of heat energy than light-colored components. In some cases, this means that the user must employ an ESD-safe black tape to make sure the component heats up to the right heat energy levels. Process control can be difficult with infrared heating as the absorbency spectrum and therefore reproducibility from component to component and board to board results can be an issue.
Both hot air and IR heat sources have their place in PCB rework with specific choices of rework heat source dependent upon the application at hand.
This article was originally published in the November 2017 issue of SMT Magazine.