Proper PCB Preheating
One of the keys to insuring a proper reflow cycle is getting to and maintaining the proper preheat temperature. This is accomplished with a PCB preheater which heats the board to between 75 and 125º C prior to the application of heat to the part area requiring rework. This part of the rework process is a critical component as done properly it minimizes PCB warpage during component removal while at the same time limiting the thermal shock to the PCB. The maximum temperature is determined by the thermal expansion of the PCB. In order to maintain the integrity of the PCB laminate the maximum preheat temperature is set approximately 10ºC or below the Tg (glass transition temperature) of the PCB material. Higher thermal preheat temperatures minimize the potential thermal distortion and shock to the PCB during the reflow process.
ESD Safe Work Areas
BGAs should be handled in ESD safe work areas in order to prevent damage to sensitive components from electrostatic discharges. These areas must be designed and maintained to prevent ESD damage.
Proper Handling and Storage of ESD Sensitive Devices and PCBs
The following practices should be adhered to when working with BGA components and PCB assemblies:
PCBs should be handled at properly designated work areas only.
Designated ESD safe work areas must be checked periodically to ensure their continued safety from ESD. The areas should be monitored for the following:
- Proper grounding methods.
- Static dissipation of work surfaces.
- Static dissipation of floor surfaces.
- Operation of ion blowers and ion air guns.
- Designated work areas must be kept free of static generating materials such as Styrofoam, vinyl, plastic, fabrics or any other static generating materials.
- Work areas must be kept clean and neat in order to prevent contamination of the work area.
- Circuit board assemblies should be handled by the edges. Avoid touching the circuits or components.
- Components should be handled by the edges when possible. Avoid touching the component leads.
- When not being worked on, sensitive components and circuit boards must be enclosed in shielded bags or boxes. There are three types of ESD protective enclosure materials including:
Antistatic - Provides antistatic cushioning for electronic assemblies.
Static Shielding - Prevents static electricity from passing through the package.
Static Dissipative - An "over-package" that has enough conductivity to dissipate any static buildup.
- Whenever handling a circuit board assembly the operator must be properly grounded by one of the following:
- Wearing a wrist strap connected to earth ground.
- Wearing 2 heel grounders and have both feet on a static dissipative floor surface.
- Stacking of circuit boards and assemblies should be avoided to prevent physical damage. Special racks and trays are provided for handling.
MSD Control-Preparation of Area Array Devices
All moisture sensitive devices should be handled as per IPC/JEDEC J-STD-020A and IPC JEDEC J-STD-033 standards.
Areas near the area array device being reworked require their temperature to be controlled and limited to less than 150ºC or to the manufacturer's specifications paying special attention to "heat sensitive" devices such as connectors, plastic housing or similar lower melt temperature components. Heat shielding may be required in some cases to prevent temperature extremes from being developed.