BGA Repair: Ensuring Process Control and Saving Money -- A medium wavelength infrared system can be used to quickly and economically repair a wide range of area array packages.
Advances in assembly equipment have allowed for an acceptable ppm failure rate during the production process. However, for many companies, the concept of quality repair remains an expensive nightmare. A more thorough understanding of area array packages and their production parameters can reduce fears of BGA repair, guarantee process control, and greatly save in rework costs.
Based on the historical experiences of most operators, three repair concerns remain paramount:
-removing the component off the board without damaging the substrate, lands and adjacent components during the process
-resoldering the components in a process controlled manner
- inspecting the quality of the procedure.
The density and performance requirements that drove the development of fine pitch, externally leaded devices such as quad flat packs (QFPs) cause enough repair headaches from a handling, hand soldering and inspection standpoint. While area array packages do not present the same handling problems, the soldering process cannot be achieved with typical repair tools, and inspection was, at one time, quite difficult. If the estimates are correct regarding the expected use of such components, area array package repair must become a viable, user-friendly and cost effective option for millions of repair operators worldwide.
The Reflow Process
Several key considerations exist during the reflow process. Uniform heat distribution and transfer across the entire surface of the area array package and its land pattern on the PCB are critical. The heating process and thermal profile must cause the package to reach reflow and then uniformly lower itself to the lands as the balls melt and form an intermetallic with the pads (Figure 3).
Figure 4 shows an x-ray image, optical picture and a cross section of a professionally installed PBGA. Note how the component has dropped, is parallel to the PCB, and how all balls are uniform in shape and are completely "wetted" or soldered to the lands.
In contrast, non-uniform heating would cause the package to unevenly drop or tilt toward the side or corner that has prematurely reached reflow. If the process is stopped at this point, the component will not lower itself uniformly, will not be coplanar and, therefore, will be insufficiently soldered.
In addition, a critical consideration for extremely small, lightweight CSP/flip chip components is the airflow rate in the convection reflow ovens. While a minimum air flow rate is required to transfer the heat to the component and PCB, this rate must not allow these light components to be either blown away or to move during the reflow process. When the extremely small eutectic balls are in a liquid state, any movement can cause the surface tension and support function of the balls to be disrupted, resulting in the component dropping completely onto the board during reflow.
BGA Repair Requirements
The repair requirements of area array packages are affected by the current shortcomings regarding reflow. While desoldering can be handled with most available hot air equipment, the resoldering process is most difficult to control. In rework, as in production, quality is the ultimate goal. Quality BGA reflow can be achieved for production in the enclosed environment of a reflow oven.
However, rework cannot be done in a completely enclosed environment because the heating conditions required for BGA reflow are difficult to achieve when blowing hot air through a nozzle. Success here depends on uniform heat distribution across the package and PCB land pattern without blowing or moving the component during reflow.
Convective heat transfer in a repair situation involves blowing heated air through a nozzle that has the shape of the component. Air flow dynamics, encompassing the effects of laminar flow, high and low pressure zones and circulation rate, is a complicated science. When combining these physical effects with those of heat absorption and distribution, the construction of a hot air nozzle for localized area heating and, therefore, proper BGA repair become difficult. Any pressure fluctuations or problems with the compressed air source or pump required by hot air systems will radically decrease the rework machine's performance.
Some hot air nozzles that contact the PCB to provide more even air circulation and heat distribution can experience co-planarity and spatial problems if adjacent components are too near. These nozzles will not contact the board, so the desired air circulation pattern in the nozzle will be disrupted and uneven heating of the BGA will result. Additionally, the heated air that exits the nozzle often heats adjacent chips and blows them away, or it burns adjacent plastic components.
Many semi-automatic convective re-pair systems often store several thermal profiles. However, this perceived benefit can be misleading unless the purpose of thermal profiling is clearly understood. In a production machine, an accurate thermal profile is the key to process control because it ensures that all joints heat uniformly and receive a sufficient peak temperature. The starting point for setting the production parameters is the actual board temperature. By analyzing the actual material temperatures, the process engineer can adjust the machine heating zone parameters to achieve the board's desired thermal profile.
A convective repair machine that can store various profiles of the heating element and/or air flow temperature can only approximately indicate the thermal situation on the board. A more accurate procedure is to monitor and document the actual board or component thermal profile by attaching K-type thermocouples to the PCB during reflow. Actual inspection of the solder joint during reflow is the ultimate form of process control (Figure 5).
Infrared Repair Systems
A viable repair alternative to these convective heat transfer problems is a medium wavelength (2-8 micro) infrared (IR) system. To demonstrate the effectiveness of infrared, three different hot air rework systems were compared with a medium wavelength infrared system. In the test, the hot air nozzles were lowered to a specially treated FR-4 substrate, and the machine performed its loaded profile. Although the profile might lead one to believe that the entire area is heated uniformly and reaching a hot enough temperature, the test results clearly showed hot and cold zones with the hot air systems (Figure 6). In contrast, the infrared system showed even heating, with no cold zones.
Infrared is not a new repair technology, but it became less popular because of the limiting physical effects of previously used short wavelength IR systems. Thermal radiation, while uniformly distributed, is unevenly absorbed and reflected by objects lighter or darker in color. Although such a heat source is perfectly acceptable for PCB preheating, short wavelength IR systems used for reflow often overheat dark component bodies and FR-4 substrate material before the reflecting leads reach proper reflow temperatures. In contrast, medium wavelength IR systems transfer heat perfectly uniform across a surface and also reveal an even absorption/reflection ratio between dark and light materials (Figure 7).
With an optimally designed medium wavelength IR repair system, K-type thermocouples can be easily placed on the board to monitor and document precise thermal profiles during the actual reflow process. This system can be used to repair microBGAs, CSPs and flip chips because of its inherent advantages.
First, an IR system has no air flow that could blow away or vibrate the component during reflow. Second, the IR system's heat can be uniformly focused, directed and transferred to an exact component size and location on the PCB.
In addition, adjacent component heating can be reduced or completely prevented by covering components with reflective foil. Even components situated as close as 0.5 mm from the component to be repaired can be safely blocked from the IR heating source. In contrast, due to the escaping air or required nozzle thickness, a hot air system cannot be used with densely placed components. Finally, a top and bottom medium wavelength IR repair system can also act as a "mini-reflow oven" for process controlled reballing of all types of area array packages.
A purchase decision for an area array package repair system must begin with a sound base of the technological requirements, as process control is paramount. The equipment must always be able to ensure quality by every operator while increasing ease of use. To provide a high return on its purchase, a repair system should be able to repair surface-mount devices, through-hole devices and plastic connectors without needing additional nozzles.
An open IR system instead of the enclosed environment inherent in hot air systems effectively eliminates "blindness" during the rework process. By optically capturing reflow in real-time during rework, the even collapse of the component's solder balls can be assured. Such process-controlled repair of area array packages is one of the hottest subjects in the industry today.
Area array packages tend to arouse fear on the part of repair operators from an ease of use standpoint, quality control inspectors from a process control standpoint, and purchasing agents from a capital investment and operational cost standpoint. However, such fears can be overcome with more understanding of the design of the various area array packages, their production reflow requirements, and the repair technology available. By going back to the basics, everyone wins.
For area array package repairs, medium wavelength IR systems provide ideal heat transfer and distribution, and are also flexible, user-friendly and cost-effective. Having an ideal repair solution, don't be afraid of BGAs; let them come in the trillions!
Cliff R. Bockard is the national sales manager with ERSA Soldering Tools and Inspection Systems, ERSA Inc., Old Lyme, CT; e-mail: firstname.lastname@example.org.
Copyright [copyright] 2001 Miller Freeman LLC