BGA Assembly

We have extensive expertise working with all different types of BGAs, ranging in size from small (2 mm x 3 mm) to big (45 mm), ceramic, and plastic BGAs. We can install BGAs with a minimum 0.2 mm pitch on your PCB.

Definition:

The full name of BGA is Ball Grid Array (solder ball array package), There are many arrays of solder pads on the substrate of BGA and corresponding numbers of solder pads on PCB, during the process, solderpaste will be printed on PCB solderpads and the pastes will be melt as balls between BGA and PCB. It is a packaging method for integrated circuits using an organic carrier board. In order to be able to determine and control the quality of such a process, it is necessary to understand and test the physical factors that affect its long-term operational reliability, such as the amount of solder, the positioning of the wires to the pads, and the wettability.

 

BGA Assembly

BGA assembly is the process of placing IC chips on a BGA board. The process of placing SMD packaged chips on a PCB board is called SMD assembly. However, unlike SMD assembly,It requires precise location and solderpaste controls, so as temperature during reflow process.

Characteristics of BGA packages

The plastic ball grid array (PBGA) has become one of the most popular packaging alternatives for high I/O devices in the industry.  It has many advantages in comparison with other packages.

Having no leads to bend, the PBGA has greatly reduced coplanarity problems and minimized handling issues. During reflow the solder balls are self-centering , thus reducing placement problems during surface mount. Normally, because of the larger ball pitch (typically 1.27 mm) of a BGA over a QFP or PQFP, the overall package and board assembly yields can be better.

1. Less package area.
2. Increased functionality, the number of pins increased;
3. PCB board can be self-centered when dissolved welding, easy to tin;
4. High reliability, good electrical performance, low overall cost.

BGA Assembly

What are the advantages of a BGA assembly?

PCBA processing has a large number of small holes in PCB boards in PCBA processing. Most customers BGA’s perforated designs are designed as the diameter of the finished pores 8 ~ 12mil. The distance from the surface of the BGA to the hole is 31.5mil, which is generally not less than 10.5mil.

1

Improves both electrical and thermal performance while making efficient use of space.

2

Reduces the overall thickness of the board.

3

Suitable for miniature packages with high pin count.

4

Offers improved solderability, resulting in a speedy assembly procedure.

5

Dissipates heat quickly due to low thermal resistance.

How to Guarantee BGA Assembly Quality?

PCB Pad and Via Design

BGA assembly quality is vital to the function realization of your design. Before BGA soldering, we need to know BGA packages. Generally, the solder-ball terminal on a BGA package is formed on a Solder-Mask-Defined (SMD) pad. Components with Non-Solder-Mask-Defined pads (NSMD) on the package are also available. Figures below shows the differences between both types.

The quality and reliability of interconnect solder joints to the PCB board are affected by:

Pad type (Solder-Mask Defined, SMD or Non-Solder-Mask Defined, NSMD)
Specific pad dimensions
Pad surface finish
Via layout and technology

The NSMD pad type on PCB is preferred since it allows for appropriate pad precision and good board reliability. Please choose a specific package when searching the BGA datasheet, which will then show an example of the stencil aperture layout for each package. Design details will depend on the PCB technology used, the capability of the PCB manufacturers and suppliers, and the planned routing. In most designs, PCB designers always use BGA solder balls with SMD pad design on the component side and with NSMD pad design on the PCB side. Investigations on board-level reliability have shown that in most cases this configuration results in the highest solder-joint reliability. The NSMD pad on the PCB allows the solder to grip the PCB pad sidewalls, improving the reliability of the solder joint.

If any kind of via hole is placed inside the pad area, its opening should be closed by plugging or plating in order to prevent solder flow into it. If closed microvias are placed inside the pads, their intersection with the pad should be flat as possible with Via-in-Pad manufacturing process.

Solder Paste Stencil Design

In BGA assembly, the solder paste is applied onto the PCB metal pads by stencil printing. The volume of the printed solder paste is determined by the stencil aperture and the stencil thickness. The stencil aperture for BGA packages should be circular. The stencil thickness can vary between 100μm (4mil) and 150μm (6mil) depending on aperture area ratio, the stencil material, and the aperture diameter.

To ensure a uniform and sufficient solder paste transfer to the PCB, laser-cut stencils (mostly made from stainless steel) are preferred. For individual design adaptations to use the optimum amount of solder, the stencil thickness, the PCB pad finish, quality and solder masking, the via layout and the solder paste type should be considered. In every case, application-specific experiments are recommended.

Component (including BGA) Placement

Although the self-alignment effect due to the surface tension of the liquid solder will support the formation of reliable solder joints, the BGA components have to be placed accurately depending on their geometry. Positioning the packages manually is not recommended, especially for packages with small terminals and pitch. An automated pick-and-place machine is recommended to obtain reliable solder joints.

Including BGA packages, component placement accuracies of +/-50μm and less are obtained with modern automatic component placement machines using CCD camera. With these systems, both the PCB and the components are optically measured and the components are placed on the PCB at their programmed positions. The fiducials on the PCB are located either on the edge of the PCB for the entire PCB, or at additional individual mounting positions (local fiducials). These fiducials are detected by a vision system immediately prior to the mounting process. Most such vision systems provide special lighting and algorithms for area array packages. For BGA components, it is recommended to use the ball recognition instead of the component outline recognition for centering. This approach eliminates potential tolerances between the solder ball and the package.

When placing BGA packages, the placement force should be sufficiently high, while avoiding excessive placement force that can squeeze the solder paste out of its intended location and cause solder joint shorts.

BGA Reflow Soldering

When the board with BGA packages placed comes to reflow soldering. The soldering profile should be in accordance with the recommendations of the solder paste manufacturer to achieve optimal solder-joint quality. The position and the surrounding of the component on the PCB, as well as the PCB thickness, can influence the solder joint temperature significantly.

BGA packages are generally suited for double-sided PCB mounting. That means that both sides of the PCB are fitted and reflowed one after another. As a consequence, the side that was initially assembled experiences two reflow cycles. During the second cycle, the components are hanging upside-down. Therefore, during the peak reflow zone of the reflow profile (where the solder is liquid), the components are only held by wetting forces of the molten solder. Gravity acting in the opposite direction will elongate the solder joints, unlike joints on the top side, where gravity forces the components nearer to the PCB surface. This shape will be frozen during cooling and therefore will result in a higher stand-off on the bottom side after the reflow process. Any mechanical impacts on components that are soldered upside down should be avoided.

BGA components have a natural tendency for warpage due to its layered construction. During package development projects the warpage is addressed by specific investigations in order to minimize a potential impact on the solder joint formation.

 

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