Quality Considerations in BGA and Fine-Pitch Component Assembly
Surface Mount Technology (SMT) is moving towards smaller but higher input output (IO) components for new applications including portable and handheld products. Although most assemblers have stabilized their printed circuit board assembly processes for assembling BGAs and CSPs, those using fine-pitch components at pitches of 0.8 mm or under continue to face assembly challenges, mostly related to solder short prevention and detection.
Apart from PCB assembly capabilities, the yield of all surface mount devices (SMDs) depends on several common factors. According to a study by Rush PCB Inc., key factors influencing product quality of fine-pitch component assembly can be listed as follows:
- Electrode surface condition
- Size accuracy
- Board accuracy
- Pad design
- Pad surface condition
- Solder Paste
- Flux heat resistance
- Powder size
- Component tack force
- Stencil accuracy
- Insufficient print volume
- Print misalignment
- Mount load
- Mount accuracy
- Ambient conditions
- Preheat temperature and time control
- Insufficient reflow temperature and time
Component Size Issues
It is crucial that assemblers use the proper size of SMDs at the right places on the PCB. With SMDs as small as 0250125 (0.25×0.125×0.125 mm), pitches of 0.3 mm for QFPs, and 0.5 mm for BGAs being quite common in use, accuracy of size matters considerably.
Quality Issues with PCBs
Apart from the board accuracy and pad design, printed circuit board (PCB) planarity affects assembly of large packages. Typically, PCB manufacturers allow warpage levels up to 0.01 mm/mm, or 1%. For a 40 mm component, this equates to about 0.4 mm of warpage directly beneath the package. Therefore, with this level of bow, assemblers would find it difficult to solder devices larger than 20 mm, both peripheral leaded and BGA and this may require controlling board suppliers to tighter tolerances.
During printed circuit board assembly, pad surface conditions affect solderability. While it is not possible to test and guarantee solderability for each pad for all BGAs on a PCB, even a single pad with a problem of wetting can result in disaster. The issue of solderability may be consistent if the pads are uniformly non-wettable, resulting in multiple opens and high rework. A worse scenario is of nearly non-wettable pads, resulting in solder joints with small cross-section and a reduced thermal cycle fatigue life. That implies the PCB should have the lowest level of solderability defects possible.
However, the above PCB variables are not directly measurable, and therefore, the best solution lies in purchasing boards from suppliers with constant quality in both areas, as these built-in variables reduce the overall yield. Furthermore, assemblers need to focus on these issues more strongly when using BGA components, as there is no touch-up with BGA joints. Therefore, any yield detractors will drive up rework and consequently, the overall assembly costs. This implies buying the best boards that one can afford would be preferable in the best interests of the overall bottom line.
Quality Issues with Solder Paste
During PCB assembly using a mix of fine-pitch components and other SMDs, the selection of solder paste has to fit the entire component mix rather than being driven by the requirement of the BGA package alone. That means the paste must meet all the thermal and environmental requirements.
One of the issues relates to powder oxidation, which if high, can decrease the ability of the flux in the solder paste to remove oxide layers. This results in creation of solder balls and non-wetting of pads. Thickness of the oxide film depends on the powder size, and the surface area the powder presents to oxygen inside the reflow oven.
For instance, Type 6 paste has particles of size 5-15 µm, while Type 4 has them as 20-38 µm. Therefore, oxidation risk is more with the finer particle size, as surface area per unit volume of Type 6 paste is three times more than that of Type 4 paste. As flux must remove the oxidation on the component as well, with Type 6 solder paste, the ability to wet the pad and component reduces. This Implies one can get better soldering by optimizing the flux.
Therefore, when using a particular type of paste, proper soldering may not be achievable on all components. This includes some fine-pitch components such as BGAs. In such cases, it may be necessary to change the paste to allow a wider process window,
Quality Issues with Stencils
Proper deposition of solder paste with a stencil depends on its aspect ratio. For small apertures such as for fine-pitch components, a thick stencil is likely to produce a poor print definition and inconsistent transfer volume. Aspect ratio of the stencil is the ratio of the aperture width to the stencil thickness. Printing is better when the aspect ratio is between1.5 and 2.0.
Again, as with paste selection, the stencil has to meet the requirement of the entire component mix on the PCB, rather than for just the fine pitch component alone. The shape of the deposit from the stencil is preferably trapezoidal or hockey puck type with a flat top surface. Deposits of excessively cone shaped are usually indicative of paste release problems from the stencil, usually related to sidewall conditions of the stencil—it is critical to use stencils with smooth sidewalls. Improper viscosity or tack force of the solder paste may also cause inadequate volume of deposit.
As the balls of the BGA package are made up of eutectic solder, the volume of the paste does not present a highly critical factor to proper soldering or reliability. However, the paste must have adequate flux necessary to wet both the pad and the solder ball surface to allow soldering. Therefore, inadequate amount of paste deposit from the stencil reduces the amount of flux present, leading to an open after reflow.
For proper transfer of paste through a stencil, high durometer poly blades or metal blades provide higher and more consistent deposits. The quality of paste print is the single most important factor in producing high yields for PCB assembly with BGA. Optimizing the printing parameters is the key to ensuring high printing quality when the apertures are very small. These parameters are:
- Squeegee type—preferably metal or durometer poly blades
- Squeegee angle—preferably 60°
- Print speed—preferably 30-50 mm/s
- Print pressure—preferably 0.20-0.30 N/mm
- Separation speed—preferably 1.0-5.0 mm/s
- Print environment—preferably 22-28°C and 40-60% RH
Of the above, controlling the print speed and separation speed is of vital importance, as too fast or too slow of both causes greater variations in transfer volume. While too fast a print speed may leave the aperture inadequately filled, a high separation speed affects the adhesion of paste to the aperture walls, and influences the paste release and print definition significantly. On the other hand, a slow separation speed may cause poor print definition and lower transfer volume, leading to larger variations.
Excessive slumping of paste deposited or mis-registration of the stencil are other quality issues related to stencils, which may lead to solder connecting adjacent conducts, creating shorts after reflow. This necessitates performing some sort of paste inspection, mainly during early manufacturing runs.
Depending on the volume of the run and the overall philosophy of PCB assembly, the assembler may prefer to use visual inspection aided with a microscope or manual measurement. An automated system is preferable for high volume production. The idea is to be more comfortable with the capabilities of the process, and the assembler may alter the inspection requirements to fit the cost and throughput requirements of the overall product.
Mount accuracy is very important for fine-pitch components such as Quad Flat Packages (QFP). Placement machines equipped with optical recognition are able to meet this tight requirement. These are usually vision systems, facilitating the centering of the PCB as well as the components during pick-and-place. Engineers calculate any compensation required after a study of the placement accuracy.
Other fine-pitch packages, especially the BGA, have excellent self-centering properties. Therefore, it is possible to accommodate wide variations in placement during reflow. A thumb rule engineers typically follow for BGA package placement is the half on pad accuracy. However, half on pad accuracy is not simply half the pad dimension in X and Y directions. Rather, it is the radial accuracy and represents the square root of the sum of the squares of the misplacement accuracy on the X- and Y-axes.
Quality Issues with Reflow
For proper soldering of printed circuit board assembly, it is very important to set suitable preheat temperature/time profiles with a matching ramp-up speed. The preheat zone is important as oxidation is most likely to occur in this area. Although establishing the reflow profile by trial and error is necessary, it may vary for different PCBs and the components mounted on them, general recommendations are as follows:
- Ramp-to-Soak Zone—preferable rate 2-4°C/s
- Preheat or Soak Zone—preferable temperature 14-170C and time 70 seconds
- Ramp-to-Peak Zone—preferable rate 2-3°C/s
- Reflow—preferable peak temperature 230-245°C and time over liquidus over 30 seconds
Reflow of fine pitch components follows the same standard as normal SMT does. All IR, convection, and vapor phase reflow ovens are suitable for fine-pitch assembly. To prevent oxidation, engineers may use nitrogen atmosphere in IR and convection type reflow ovens, although this is not required in vapor phase types.
The choice of solder paste typically defines the profile and other reflow parameters. Recommended thermal profiles offered by paste manufacturers and other specifications for general SMT profiles are applicable to fine-pitch components as well.
Considering that it is impossible to manually inspect all the solder joints after reflow for fine-pitch components such as the BGA, thermal profiling for every new board is critical, as it is the only method to insure that all joints under a BGA package are completely soldered.
Engineers use thermal profilers with multiple thermocouple probes and associated software to generate the thermal profile of a new board. They do this by measuring temperatures and its variation at different points on the board as it travels through the oven and analyzing the collected data on a computer.
The process of generating a thermal profile is very important for prototype PCB assembly, as differences in surrounding components, cross-section of the board, density of parts, and the design of the fine-pitch component all result in varying temperatures under the fine-pitch component. For large BGA packages, the center balls will usually be at a lower temperature compared to those on the edges—requiring multiple thermocouple probes located within the solder joints themselves for accuracy of readings.
During prototype PCB assembly, another aspect necessary for the thermal profiler to cover is verifying the maximum temperature the fine-pitch component body reaches during reflow. For most large components such as BGA packages, manufacturers recommend the maximum body temperature as 220°C. Going beyond this temperature on the body during reflow can lead to popcorning and premature failure of the component.
High temperatures during reflow can cause the PCB to bow, leading to concerns for soldering large fine-pitched components such as QFPs and BGAs. Engineers usually tackle this problem by properly supporting the PCB assembly, and this is especially important for thin and flexible PCBs.
For achieving high yields of PCB assembly using fine-pitch components, Rush PCB Inc. finds it is important to select the proper solder paste, especially the solder powder size and the flux content. Equally important is the proper design of the stencil, especially its thickness and aperture design. Suitable parameters must be set for printing, while ensuring there is no contamination of the plating on the pads, and that the stencil alignment is proper. Finally, it is crucial to set a proper thermal profile specific to each board, with a suitable preheat temperature time combination and ramp-up speed.
Roy Akber, CEO RUSH PCB, Inc.