The success of the Ball Grid Array (BGA) devices in several products related to telecommunications, consumer, office use, and more, has led to engineers experimenting and implementing stacking of packages for even higher densities. These stacked packages, termed Package on Package or PoP are simply one or more BGA packages stacked one on top of another. Broadly, there are two versions—one where the original component manufacturer stacks the packages, and the other where the printed board assembler stacks them. The main advantage of PoP is substantially enhanced functionality within the same footprint of a single BGA.
Methods for PoP Assembly
Assemblers usually follow one of two main methods for assembling PoP components—pre-joined and on-the-fly. Only the assembly process differs in these methods, not the results. Here, we consider one or more BGA packages soldered on to a Printed Circuit Board (PCB).
The pre-joined method of assembly is a two-pass process. In the first pass, the assembler assembles the devices that they will eventually solder on to the PCB, following standard SMT assembly processes. They place components onto other devices in a carrier. The entire carrier then travels through a reflow oven, which joins them. The assembler then repackages the pre-joined devices for pickup by the assembly system. The placement machine then stacks them on the PCB, which goes for the next reflow pass.
Sometimes, the assembler may add the devices one layer at a time, with the stacks going for reflow after addition of each layer. The assembler may also stack several pre-joined parts after initially assembling them.
As each step in the process is more like the traditional SMT assembly, the pre-joined method reduces the complexity for the placement machine. However, it adds other possible concerns such as handling.
The second method of assembly is on-the-fly, where the assembler places all the devices in the stack before sending them for reflow all at once. They place the first layer or the base BGA package on the PCB or carrier directly, and place additional layers successively. Once all the layers are in their proper sequence, the entire assembly goes for reflow soldering.
Steps for PoP Assembly
Assuming placement of PoP packages is only on one side of the PCB and not on both, the assembly sequence follows the steps as below:
Solder Paste Printing
Like any other part on an SMD board assembly, the termination dimensions, the pad size, and the device pitch of the BGA packages on the board-mounted layer defines the solder paste printing process. Most assemblers use a fine pitch stencil of thickness about 0.004-0.005 inch. Some also use a combination of a step stencil design using an electro-formed or laser cut foil. Fine pitch assembly, such as for high density fine pitch BGA packages requires type 4 particle solder paste.
There is no special requirement for printing solder paste for the first layer PoP package on the PCB than any other fine pitch package requires. Existing BGA packages also require the same—primarily, good control over registration, repeatability, and complete transfer of the paste. Confirmation of quality of paste deposition is also usually through visual inspection or by using a separate AOI system that measures solder volume.
Placement
Placing the first package on the PCB requires no special effort, as any major placement system should already be equipped to handle standard BGA builds or pre-built PoP devices. However, if the board assembler is configuring or building the PoP device, a review of the handling and controlling process of the upper level devices is necessary, as this may be critical.
For instance, along with issues regarding board vibrations and shock during transportation, accurate control of the Z-dimensions of the placement machine is critical. However, most high-end systems should be able to meet the above requirements.
However, not all machines may have a dipping module integrated with the placement system.
Dipping into Solder Paste
The second, and subsequent levels of PoP packages require dipping into flux or solder paste before they are placed on the previous layer. Assemblers prefer to dip PoP packages into solder paste rather than flux because the former ensures better solderability and reduces problems. Additionally, solder on the surface of the terminations is easier to inspect than it is when flux covers them.
However, this process of dipping requires a paste with special qualities. These are usually of the no clean type, halogen free, lead free, especially meant for PoP assembly. It contains optimized ultra-fine solder powder and flux, and leaves a clear, colorless residue with high electrical resistivity.
For earlier dip pastes, the transfer weight or the amount of solder sticking to the package balls after dipping would depend on process parameters such as dip thickness (depth of dipping), hold time, and rise velocity. Modern dip pastes have improved their composition so that hold time and rise velocities do not affect the transfer weight and only the dip thickness is of primary concern. This offers assemblers using PoP packages a broad process window for dwell time and rise velocity governing the assembly time. Typically, there are two methods of application of dip paste—a rotary type and a linear type.
The rotary type of applicator has a doctor blade fixed in place, but adjustable in height. It forms a part of the rotating dip tray that holds the solder paste as it spins under the blade. The blade brushing the surface of the solder paste offers a level surface and a known thickness of solder paste into which the dipping module dips the component.
In the linear type of applicator, rather than rotating, the dip tray moves from side to side under the fixed blade, which is also the reservoir for the solder paste. Although assemblers use the linear type more commonly, the rotary type offers more precision in depth control.
Along with the depth of paste in the applicator, it is also necessary to control the depth of insertion of the device into the paste. If the solder balls of the package dip more than half their height, the solder paste tends to wrap around them, and this increases the transfer weight with the excess paste leading to an increasing probability of generating solder shorts.
Considerations during Placement of PoP Devices
Inspecting the applicator paste surface after dipping shows the nature of deformation caused by the dipping process. As the size of the PoP device and the number of balls on it increases, so does the contact area on the paste surface, and therefore, the force holding the package to the surface of the paste is also higher. The size or area of the pickup tool and the vacuum force in the placement system may need changes to match this increase in the holding force to avoid incomplete, or poor pick up. In addition, any lateral movement of the PoP device when it is touching the paste surface increases the potential for excess paste on the balls, leading to solder shorts.
As the PCB travels via conveyor belt to the reflow oven, even minimal evidence of vibration can displace the stack of PoP devices it is carrying. The multi-layer structure of the stacked packages has more opportunities to misplacement from sundry vibrations. Since higher convection rates or chain stretches in the reflow oven may also cause random vibrations, regular process maintenance checks must be part of the routine process.
Inspecting PoP Devices
Before the PCB assembly enters the reflow oven, detecting missing or excess paste on the PoP balls is far simpler to handle than reworking after the reflow is completed. These defects, although rare, can happen because of changes in the paste viscosity, or incorrect insertion depth of the solder balls into the paste.
Optical or X-ray inspection of PoP devices can be time-consuming processes. Inspecting every device on every board in production will likely slow down the output drastically. Therefore, following statistical methods of inspection is a more prudent approach.
The quality of solder joints within the PoP structure is best examined with a 2-D X-ray inspection process starting at one corner of the device and moving around to cover the entire device. If undertaken at oblique angles, X-ray inspections can reveal variations in solder joint shapes after reflow. In general, X-ray inspection can reveal variations in solder ball diameter when measured on the same layer, open solder joints, and warpage and imperfections in the joint interfaces between the device and the board, or the device and the upper PoP layers.
Summary
The main problems the industry associates with PoP devices are open joints, warping of component substrates and issues related to the underlying PCB. Although the process of dip soldering overcomes the incidences of package warp and open joints largely, 2-D X-ray inspection can generate adequate data to interpret the inconsistencies.