Reworking CSPs-The Devil Is in the Details -- Consistent, cost-effective rework of CSPs requires proper training and equipment, as well as careful control of the repair process.

Economic slowdown or not, a few things remain constant in electronics manufacturing. Products are getting smaller, and the obvious market demand for portable devices will only serve to further reduce size. In turn, array packages continue to gain in popularity, particularly chip-scale packages (CSPs) and land grid arrays (LGAs).

These advanced packages offer low profiles and small footprints-making them ideal for today's most popular products such as cell phones, personal digital assistants (PDAs), pagers and digital cameras. One advantage of these devices over flip chips, for example, is that they are easily manufactured using current equipment.

Chip-scale packaging seems to offer the best combination of speed, density and manufacturability. In particular, what makes CSPs so enticing and economical is that the packages can be placed using standard surface-mount processes and equipment. Better than flip chips, CSPs require no underfill to compensate for an inherent mismatch of coefficient of thermal expansion (CTE).

Considering Rework 

Another industry constant: In spite of the best efforts to develop lights-out manufacturing, rework and repair remain a necessary and labor-intensive part of the assembly process. Ironically, the ability to cost-effectively repair and rework assemblies is growing in importance. And, as the number of solder joints is increasing on any given assembly, statistically so does the chance of a defect.

Conventional wisdom has it that rework is essentially a non-value added/non-profit making operation. Thankfully, that last-generation "wisdom" is beginning to change. Enlightened management understands that the ability to rework and repair printed circuit boards (PCBs) can be an important factor to overall profitability, particularly when prices are falling and profit margins are near the low-end of single digits. Maintaining profitability is often dependent upon the capacity to efficiently rework advanced assemblies and transform potential scrap into saleable product.

It's clear that the ability to rework CSPs gives the technology an advantage over other packages, particularly flip chips, which require underfill and cannot be reworked.

In many ways, reworking CSPs is the same process as reworking other surface-mount devices: identify the faulty component, desolder it, clean the area, then replace and resolder. Job done. But the devil is in the details-and, in fact, reworking CSPs is more intricate than it might first appear.

Following the Steps

The basic steps to the removal and replacement of CSPs include:

- establish thermal profile

- remove failed component

- clean and prepare site

- replace component

- reflow 

- inspect.

With traditional surface-mount components, those with visible leads, the first decision involves the choice of heat-either conduction (soldering iron) or convection. With array packages, that choice is already made. Conduction is not an option-if you can't make contact with the solder joints, convection or infrared (IR) are the only options available. In addition, convection delivers the increased process control necessary for reworking sensitive devices on high density PCBs.

Key to successful removal is the steady, uniform heating of the CSP at an ideal temperature-where the component will reflow but the substrate will not be damaged. Additionally, because the solder connections between the component and the PCB are within narrow tracks, replacing a CSP requires a high degree of precision and an optical placement capability on the rework station.

The requirements for a solder profile are generally well understood; they are normally dependent on the materials being used (solder paste or flux) and are typically as follows:

On any reflow soldering system that heats both sides of a PCB assembly, the required solder joint temperature is a function of how much heat is applied to both the top and bottom sides. Achieving the same solder joint temperature with a number of heat settings is possible, yet the incorrect combination can lead to low yields and, in some cases, catastrophic results-including warping due to excess heat on the topside. In fact, a minor deformation can lead to defects such as unsoldered joints and solder shorts.

For example, a normal ball grid array (BGA) device stands off the PCB by about 0.5 mm (0.020 in.) and lifting by even 0.1 mm (0.005 in.) across the device is enough to cause an open circuit. Even if the board/component survives the process with no apparent defect, the joint will be constantly under strain as the board returns to its normal shape, causing long term reliability problems.

Eliminating warpage generally involves improving the profile settings, particularly creating a better balance between bottom-side and topside pre-heat temperatures.

Convection Systems

The two basic types of convection systems are those using high airflow and those using low airflow. Generally, low flow systems are the better choice, as removing CSPs from tightly packed boards requires care that adjacent components are not reflowed and moved out of alignment-a possibility with high flow systems. As a rule of thumb, airflow levels below 15 l/min. will not disturb nearby components.

Component removal is generally accomplished using a straightforward vacuum pickup. Again, care must be taken during this seemingly simple process, as excessive vacuum pressure will cause the solder to collapse and adhere to the PCB, making cleanup slower and difficult.

Cleaning the Site

Once the component has been removed and pad integrity has been checked, pads must be prepared so that a new component can be reattached.

Many assemblers prefer the traditional method of removing residual solder with desoldering braid. Using braid allows the technician to achieve flat pads, which are preferred when solder paste is to be applied for reattach. When used correctly, and with the proper braid, the solder removal process works well.

If dome-shaped pads are preferred, for example, when only flux will be applied, a wave solder tip is best for site cleaning. Of course, this process can only be used when pad geometry is uniform; otherwise, solder joint heights will differ. When applicable, the method is a reliable and less damaging way to remove residual solder.

In either case, the process of removing solder is extremely critical. It takes operator skill to correctly use a soldering tool, braid and/or the wave solder tip. To help prevent thermal damage, and to speed the cleaning process by eliminating the need for constant setting and resetting of heating parameters, many rework facilities are moving away from "traditional" soldering irons in favor of advanced tools that precisely and automatically control tip temperature.

Material Deposition-A New Approach

When working with array packages, either flux (dip transfer method) or paste can be used for reattach. The device type and/or solderball composition will determine whether solder paste or flux is used. In either case, technicians must pay close attention to material deposition. How accurately the solder or flux is deposited will directly affect the yield. With a conventional surface-mount component, for example, a defect in the rework can be easily rectified by retouching. With array packages, however, a single failed solder joint would necessitate starting the entire rework process from scratch.

With paste, for example, stencils are still being used to apply solder paste to the PCB itself, albeit with great difficulty and greatly reduced yields. This traditional method of paste deposition creates problems, as stencil flatness and position are troublesome, and there are concerns about access due to the close proximity of the adjacent components.

However, a newer method of paste deposition using plates to print directly onto the CSP or LGA (or any array component) is gaining in popularity, in part because the process is easy to master and delivers better, more consistent results.

Printing directly onto the component is a relatively easy process. A specially designed plate is used to apply solder paste to the component with the same size and shape as originally done on an inline screen printer. The goal is to place approximately the same volume of paste as was originally deposited by an inline printer during the original manufacturing process.

The direct deposition process involves printing solder paste onto the component before it is picked up by the placement head. The optimum aperture geometry for applying the proper amount of paste was developed.

The process is straightforward, and results are superior to printing on the PCB. When working with LGAs, the component body outline is used to register the component to the stencil. Direct printing on the component is especially useful with LGAs with different pad dimensions on the same component, as technicians can print different amounts of solder at a uniform height across the bottom of the device. Note that on these devices flux and drag-solder pad cleaning methods cannot be used.

Plate printing is an easier process than stencils on PCBs-it's also faster and increases yields.

Replacing the Component

A skilled technician can "eye" component placement with a degree of success, but this technique is not recommended. Instead, a split-optics vision system is standard for those concerned with higher process yields. The component is aligned, placed with the vacuum pick-up tube and reflowed with hot air or gas. Here, the use of a pre-programmed, precisely determined temperature profile is extremely critical.

The thermal profile developed for removing the component is again employed so that the new device is not damaged during reattach. Adjusting the profile, however, may be necessary depending upon the type of flux or paste being used.

Inspection

With leaded components, it's a simple matter to look for good solder joints and the absence of shorts, using a low magnification inspection system. With CSPs and other array packages, inspecting the integrity of solderballs (size, positioning, bump geometry, etc.) requires either x-ray systems and/or optical inspection systems to see underneath the components.

X-ray inspection can highlight certain faults, such as voids, shorts, and undersoldered joints; however, equally serious problems like component or joint cracking and partially soldered joints are not easily found. Optical inspection, using an endoscope-type system, can be a useful addition to x-ray, allowing operators to inspect the solder joint from the component side.

One caveat: Underneath the typical BGA or CSP, a large number of solder joints resides, and 100 percent inspection may not be practical or cost effective. Both methods are, however, essential tools in good process development.

Conclusion

CSPs, LGAs and other array packages offer numerous advantages for today's high-speed assemblies. Reworking these devices may not require new processes, but it does demand adapting existing processes to the realities of hidden solder joints and temperature-sensitive components.

In some cases, standard methods, low tech soldering tools, stencils, and printing on the PCB may not be the best choices and may, in fact, slow rework and drive up costs, which, in turn, can adversely affect overall profitability.

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Paul Wood is the sales development manager, BGA/CSP Rework, Metcal Inc., Menlo Park, CA; e-mail: [e-mail].

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Copyright [copyright] 2001 Miller Freeman LLC