Ball Grid Array (BGA) and Chip Scale Packages (CSPs) are gaining popularity as the demand for smaller and faster circuitry increases in electronics manufacturing. The BGA and the more leading edge CSP are packages designed to be soldered to surface land patterns on an assembly substrate. Careful design consideration and process control is critical with these types of packages, making workforce training a key factor in the successful design and production of BGAs and CSPs. The EMPF offers a two-day course in BGA Manufacturing, Rework, and Repair. During this course, participants will learn about the issues that are associated with designing and manufacturing assemblies utilizing BGAs and CSPs. Some BGA process considerations and recommendations that are examined in detail during the course are highlighted in this article.

The characteristics of BGAs
There are many types of BGA and CSP packages. Most of these packages have some type of package substrate, providing mechanical support and an electrical interface. Characteristics of the more common types include high temp solderballs, eutectic solderballs, and Land-Grid Arrays (LGAs). LGAs do not have solderball attachments and provide only a land grid on the package substrate. BGAs with high temp solderballs rely on printed solder paste to attach the package to the assembly substrate. The solderballs are usually attached to the package with eutectic solder. When subjected to reflow, the balls are free to disengage from the package. This constitutes a difference between BGA and other SMT packages.

The solderball of a eutectic-attached BGA provides about 80% of the solder volume at the package-to-board interface. Solder paste provides flux, which promotes good wetting of the ball to the assembly substrate. The fact that the package-to-assembly interface melts to form the electrical interconnect is unique to this SMT package.

LGAs are quite similar to the old leadless chip carriers (LCC). The primary difference between the LGA and the traditional LCC is that the package-to-assembly interconnects are hidden under the package. The hidden interconnect presents the single biggest challenge to manufacturers of assemblies employing this family of components. It is not possible to touch or directly inspect the solder connections under the package.

BGA manufacturing issues related to materials
• Solder: The industry is transitioning from Lead (Pb) solder to Pb-Free solders. Some packages are available with Pb-Free balls. Some are available with Pb-Free balls only. Attempts at mixing Pb-Free BGAs with SnPb solder have yielded poor results. While it is standard practice for other SMT package types, do not attempt to mix Pb-free area array packages with solders containing Pb.
• Flux: Optimize your process so that no changes should be required. If you are using a no-clean process, make sure the flux under the package is processed adequately. The process goal is to expend all the flux and immediately enter reflow. Reaching reflow too quickly may produce voids. Burning off all of the flux and soaking just below reflow, prior to reflow, may result in non-wetting at the assembly interface.
• Paste: We have observed that some pastes have a tendency to produce a percentage of voids. A given paste may perform very well with traditional SMT package types and be simply unacceptable for use in assemblies that have area array packages.
• Underfill: Very small CSPs may require underfill adhesive to provide mechanical strength. The solder attachments alone may be inadequate to secure the package to the assembly. An underfill needs to be selected based on its compatibility with the other assembly process steps, the normal service environment, and assembly serviceability.

BGA manufacturing issues related to components
IPC J-STD-020 establishes test methods and criteria for moisture sensitivity in BGA packages (Figure 2-1). PBGA (Plastic Ball Grid Array) packages absorb moisture, making them susceptible to warp, swell, popcorn (a type of delamination) or crack when processed without appropriate care. IPC J-STD-033 provides guidance on how moisture-sensitive devices should be handled. Check on the JEDEC website (www.jedec.org) for status of the standard and to download the current version.
• Solderability: Solder joints exhibiting poor wetting can result in premature failure as a result of mechanical and thermal cyclic loading. Proper storage and handling of components will eliminate many of the solderability issues.
• Assembly: Stencil printing for area array packages is less difficult than for fine-pitch, leaded SMT components. However, the risk is greater. Placement requirements may not be as rigorous as those for peripheral leaded packages with similar IO count. Unlike a Quad Flat Package (QFP), the PBGA package tends to self-align on the land patterns during the reflow process. Placement errors less than about 50% of the ball diameter that do not generate bridges will move toward the center of the land pattern. CCGA packages will not “self-align” during reflow, nor can you expect an LGA to float to the target as a result of wetting forces and surface tension.
• Reflow: The process of reflow soldering does not change, but your ability to evaluate the process does. Preheat ramp and soak must be hot enough to drive the volatile compounds out of the flux, to reduce voids. The preheat cycle must be slow enough to allow all of the parts on the board to reach this state but fast enough to prevent oxides from forming on the substrate lands after the protective compounds have been driven from the flux. Developing a good time/temperature profile can be difficult and counterintuitive. Small CSPs can be harder to heat than large BGAs. Heavy PWBs, CBGAs, PBGAs, TBGAs, heat spreaders in packages, and high power components can prove to be a significant challenge even with the best reflow equipment.

Other BGA manufacturing considerations and issues
• Cleaning: In general, the cleaning process is the most overlooked and most difficult for manufacturers. The biggest challenge is with CSP and LGA components. The narrow gap formed between interconnects, the package, and the assembly can easily block cleaning media and trap residue.
• Inspection: X-ray, acoustic microscopy, and optical microscopy afford users limited ability to assess the results of their manufacturing processes. Generally, the cost of the equipment, training, and time prohibit 100% inspection in the manufacturing environment.
• Voids: The industry consensus on acceptability of voids is shifting. There is simply no evidence that voids in the center of the interface present reliability issues. Voids must not be confused with dewetting or non-wetting at the assembly or package substrate. Dewetting and non-wetting at the assembly or package land is a serious problem.
• Warp and collapse: It is possible to evaluate the output of the solder process visually and equipment suppliers have developed some useful tools to enhance the inspection process. As noted above, some types of BGAs and all CGAs do not melt during the reflow process. However, it is possible to evaluate process output by quantifying package warp and ball collapse. This may be accomplished through the use of pin gauges, laser inspection tools, and endoscopes. Endoscopes will also enable users to evaluate the characteristics of the outer rows of solder balls.
• Rework: Rework of BGAs frequently combines three methods of transferring heat energy into a workpiece: convection, conduction, and radiation. Successful rework requires special tools, knowledge, and experience.
• Receiving inspection: Many companies perform receiving inspection only as needed to isolate known vendor problems or to validate corrective actions. Experience has indicated that there are situations that require the users of BGA to inspect for voids in balls, variation in ball size, missing balls, and package bow.
• Wave solder: It is somewhat unusual for a manufacturer to develop a sequence that includes wave solder when an assembly employs BGAs. Do not allow molten solder to flow through vias directly adjacent to the interconnects under BGAs. The solder may heat the copper enough to weaken or break connections to the lands at the solder ball. Do not tent one side of the via with solder mask in order to leave the node open for test.
• De-paneling process problems: Breaking solder connections under the BGA during de-paneling is not unusual. It is easy to forget that the solder connections under a BGA, while they may be numerous, are individually fragile. Board panels should be carefully designed to prevent damage to individual assemblies at this step.

Board design considerations
Selecting a BGA package for use in a new board design may require an increase in the PWB layer count. It may also require the use of finer lines and spaces on the PWB. The BGA package can accommodate more IO (input/output) attachments than a package with peripheral lead packages at a given pitch and size. Be aware of the escape paths from the inner connections of the array. The escape paths can increase PWB design complexity and cost. Designers may be tempted to compromise manufacturing design rules to reduce layer counts. The process must be as robust as possible. A poorly designed PWB hinders the development of robust manufacturing processes.

Additional considerations include:
• Design the PWB with BGA packages near the edge of the board to mitigate the effects of warping of the board in process.
• Use industry-standard land patterns or design rules such as IPC-SM-782. Proper land pattern design is essential to assure proper solder joint formation.
• Copper-defined land patterns are more reliable than solder-mask-defined lands.

In summary, BGA manufacturing, rework, and repair can be challenging and is certainly ever-changing. The EMPF’s unique BGA Manufacturing, Rework, and Inspection course provides participants with two days of hands on training, a copy of the IPC standard, and access to trained instructors and manufacturing specialists that can assist you in developing enhanced BGA manufacturing skills to meet the unique challenges of your individual manufacturing environments. For more information on this course, visit our website at www.empf.org or contact us via e-mail at registrar@empf.org.