Fine I/O pitch and tiny solder balls require careful considerations for flux transfer and ball placement.
BY STANLEY TSUI, CHARLES J. VATH, III, AND C. W. CHENG
In the past ten years, acceptance of ball grid array (BGA) packages has increased tremendously; the overall BGA market is forecasted to grow at a rate three times faster than that of other commonly used packages.1 The demand for smaller, lighter and thinner consumer products, however, also has caused a significant increase in the use of chip-scale packages (CSPs).2 CSPs can be categorized as either BGA format or leaded (non-BGA) format. Chip-scale ball grid array (CSBGA) or mini BGA, has some advantages over non-BGA packages with respect to the potential for accommodating higher pin counts and better self-centering characteristics during attachment to printed circuit board assemblies.
Figure 1. CSBGAs in matrix format on a strip. |
Because CSBGAs have much smaller form factors than common BGAs, they require finer input/output (I/O) pitch and, therefore, a smaller interconnection medium (solder balls). Currently, having a 0.5-mm pitch with 0.3-mm solder balls has been proving very challenging. Equally challenging is the need to keep the cost of CSBGAs equal to that of traditional leaded packages, which is critical if CSBGAs are to become more competitive and migrate to larger production scale.
To reduce cost, packaging companies must switch from device-level manufacturing to strip/carrier-level manufacturing. Therefore, the handling format has been changing from multi-CSBGA devices on strip/carrier to a matrix of CSBGA devices on strip/carrier – simply called matrix CSBGA (Figure 1).
New Challenges
New requirements always bring about new challenges, and the packaging processes for CSBGAs provide great hurdles. Because ball placement is a critical process of BGA back-end packaging, it should address these new equirements. The demand for finer pitch and smaller solder balls necessitates a more accurate ball-placement process and higher process capability. The need to maintain cost efficiency means that matrix CSBGAs with higher throughput must be created, thereby obliging gang placement of solder balls with high yield and process reliability. The equipment should provide low cost of ownership, have a high yield, convert quickly and easily from one package to another, and provide high flexibility for integration. The interesting challenge is the ability to combine the characteristics of high accuracy and high throughput, as well as good process reliability, in one ball placement system.
High Accuracy and Throughput
Currently, the fine-pitch capability requirement of CSPs has migrated to 0.5-mm pitch with 0.3-mm solder balls. By very simple calculation, the ball placement accuracy requirement no longer is at the ±0.1-mm level, as the nominal gap between two adjacent balls is only 0.2 mm. It is not practical to have a tough accuracy requirement (e.g., less than ±0.05 mm), as solder balls will self-align to the center of pads because of surface-tension effect during reflow. According to experimental results, a fair figure is ±0.06 mm, which is accurate enough to avoid bridging balls and achievable in advanced ball-placement systems (Figures 2 and 3).
Figure 2. Balls with offset from pads of ±0.06 to 0.07 mm before reflow. |
The need to maintain concurrently high throughput and accuracy seems a paradox. In line with the emerging matrix CSBGA, gang placement of solder balls on an array of CSBGA devices, or on the entire strip, can provide high throughput. The following issues, however, still must be addressed when adopting this gang approach:
- Because the placement area is larger, good positional accuracy of ball placement (especially in the theta direction) is necessary.
- Alignment/locating holes no longer can be relied upon, as their accuracy/repeatability with regard to solder pads easily can be more than 0.1 mm.
- A higher yield and process reliability is expected.
Figure 3. Balls with offset from pads of less than ±0.01 mm after reflow. |
One U.S. patent-pending mechanism is designed to meet this challenging requirement of high accuracy and throughput (Figure 4). This design consists of two moving heads – a flux-pick head and a ball-pick head. Each head can move in Y, Z and 0 directions and carries a camera that can move along the X axis. Before performing the process of flux deposition and ball placement, the two fiducials on the BGA substrate can be recognized through the two pre-positioned cameras carried by the two heads. Therefore, subsequent positional compensation can be achieved with the position adjustments of both heads in the Y and 0 directions, and also with the adjustment of the work holder (substrate) in the X direction.
This configuration will have several advantages. With the dual-programmable camera design, an accurate placement no longer is dependent on the locating holes but relies on the fiducials that should be more accurate than locating holes; fiducial error should be in the range of 0.01 mm. An accurate placement of solder balls is guaranteed, and the flux-deposition accuracy that is required for fine-pitch CSBGA is improved significantly. Cycle time should not be compromised with accuracy, as throughput also is an important factor for the success of CSBGA packages. With this special dual-programmable camera design, additional pattern-recognition time will not incur a large increase in system-cycle time; therefore, gang placement for matrix CSBGAs becomes possible.
Yield and Process Reliability
Compared to the traditional plastic ball grid array (PBGA), the number of solder balls to be handled for matrix CSBGAs has been increasing from hundreds to thousands. Such an increase surely will create a new requirement for yield-per-ball defect from double-digit parts per million (PPM) to single-digit PPM. The major factors that allow this goal to be achieved are a high process reliability and robustness for both flux transfer and solder ball transfer.
Flux Transfer
The process of flux transfer for CSBGAs is required to deposit a controlled amount of solder flux reliably and accurately onto solder pads of the substrate, preferably by a gang-deposition technique. The three common types of flux transfer methods are pin transfer, screen-printing and direct-dipping. It is difficult to auto-align for screen-printing and, thus, inaccuracies arise when dealing with matrix CSBGAs. Direct-dipping will solve the inaccuracy problem because flux first is deposited on balls rather than solder pads. It has the potential risk of contaminating the solder-ball transfer head with flux, however, when solder-ball sizes become smaller for CSBGAs. The clearance between the flux surface and the template surface only can be about 0.1 mm across a length of the gang-placement dimension, which can be up to 250 mm (Figure 5). Hence, the template surface is contaminated easily by solder flux whenever there are minor imperfections in parallelism/flatness between the two surfaces.
Figure 4. A precise mechanism for ball placement (U.S. patent pending). |
Pin transfer allows auto-alignment, guaranteeing special design and accuracy, and a more robust flux-transfer process for parallelism/flatness because of the soft-pin design. With proper implementation, pin transfer is capable of placing a controlled amount of flux onto solder pads by a good match of pin geometry, flux characteristics, pin dipping-depth and contact time.
Solder-ball Transfer
Solder-ball transfer, which is the process of transferring a solder ball from a pool of balls to substrate solder pads, is the most critical process in the solder-ball placement system. As ball size and pitch become smaller, and as the number of balls to be handled increases, both the accuracy and the process reliability of solder-ball transfer are of paramount importance. There are many different methods for transferring solder balls. Generally, they can be classified into the following two types: gravity with stencil or vacuum and pick-and-place. The former makes use of a stencil with an array of through-holes. The pattern of the hole array is identical to that of the BGA substrate. Solder balls are supplied to fill these small holes; the solder balls will fall down through the array of holes onto solder pads when the substrate is mechanically aligned with the stencil. This method works well for applications of coarse-pitch and large balls, which do not require high accuracy or handle a large amount of balls. The lack of flexibility in applying a vision system before the solder balls fall onto the substrate or other in-process detection methods makes it virtually impossible to handle fine-pitch applications such as CSBGA.
Figure 5. Methods of flux transfer: (a) pin transfer, (b) screen print, and (c) direct dipping. |
The second method for solder-ball transfer makes use of a head with a template on which an array of small holes is arranged in the same pattern as the array of the substrate's solder pads. First, the head picks up solder balls by suction from a pool of solder balls and then transfers them onto the pads of the BGA substrate. This method involves more steps than the gravity-with-stencil method. It does, however, have the potential for achieving better placement accuracy by providing the flexibility to apply any vision technique before placing solder balls.
Figure 6. New vacuum and pick-and-place design (U.S. patent pending). |
Typically, solder balls are picked up randomly from a container where a large amount of solder balls are “blown up” when the head is approaching. This can be a process reliability issue when thousands of solder balls are to be picked up, as in the case of matrix CSBGAs. This problem can be solved by preparing an array of solder balls with a pattern matching that of the pads on the BGA substrate for the head to pick up. This “one ball for one hole” concept can reduce greatly the chance of missing or double balls and therefore improve process reliability (Figure 6).
Conclusion
As new and smaller versions of CSBGA packages emerge, new requirements for packaging them must be met. Ball placement – one of the most critical back-end processes for CSBGAs – is facing the great challenge of meeting the new accuracy and process capability requirements when handling smaller balls and finer pitches. In order to reduce cost and achieve high throughput, matrix-type CSBGAs are positioned to become the mainstream of ball-array CSPs. This new packaging configuration promises to lead to higher levels of process reliability, which the ball-placement system will address.
References:
- “Worldwide IC Packaging Market,” Electronic Trend Publications, 1999.
- TechSearch International Inc., CSP Markets and Applications, 1998.
STANLEY TSUI is general manager, post-encapsulation equipment, and C. W. CHENG is product manager, ball placement products, for ASM Assembly Automation Ltd., Hong Kong. For more information, contact CHARLES J. VATH, III, vice president of process and packaging technology, at ASM Technology Singapore Pte Ltd., 2 Yishun Avenue 7, Singapore 768924; 65-7526311; Fax: 65-7582287; [email protected].