For a successful rework process packages chip-scale


The csp's small size presents new challenges in the rework process. although rework of csps is similar to rework for bgas, the tolerances for solder paste and flux deposition are greater.


Chip-scale packages, because of their diminutive size, present new challenges in the rework process compared to conventional package formats. The most common CSP today is Tessera's µBGA?package at 30 mil, 0.75 pitch, which is now in production by Intel and others with 40-56 solder balls.

Figure 1. Removal of a CSP in an encapsulated enviroment virtually eliminates adjacent compnent heating, since exhaust ports are located toward the top of the rework nozzle.


Figure 2. This four-stage rework profile, duplicates the original manufacturing profile.


Most PCMCIA cards and cellular phone boards employing CSPs are hard to hold. The edges are packed with connectors and sockets, and a special holder has to keep the PC board level and firm during rework.

These small PC boards are often 20 mil thick and are double-sided, making them quite susceptible to damage by heat. The boards often contain perimeter CSPs with ball counts as high as 196 on 20-mil pitch.

The reflow of adjacent parts is the most difficult to address because some manufacturers place discretes within 20 mils of the CSP.

Rework suppliers will feel more comfortable with 50 mil as a standard "keep out" zone. If customers insist on placing parts closer than 50 mil, then they should expect to pay for a custom tool to rework and remove the part.


Future CSP Challenges


Higher I/O (up to 625) will be common and 20 mil will be the normal pitch in the future. Finer pitches down to 12 mils are now being discussed by many device manufacturers, with 8-mil and 4-mil ball size. Some high power parts may also use solid copper balls instead of eutectic solder balls. This will require an increase in the stenciling of solder paste during CSP rework.

Benefits of CSP parts


CSP packages are light and robust compared to a TSOP or QFP. As a result, the components will self-center easily. BGA parts also self-center, but contain a much larger solder ball, which requires more heating due to its size.

The term "BGA" is becoming more commonly associated with larger area array packages, while "CSP" refers to any package whose area does not exceed 1.2x that of the die.

CSP Rework Process Overview


Rework of area array CSPs is quite similar to that practiced with BGA-packaged devices except for tighter tolerances, especially in the solder paste or flux-deposition process.

Although CSPs have superior self-centering characteristics, one must consider that greater alignment accuracy is required due to their smaller scale. The µBGA CSP features a ball size of .012", so placement should be within 50% =.006" or less.

If via construction is employed for this package, then the accuracy should be much tighter, otherwise solder paste might be pushed into the via during placement or reflow. Safe component removal begins the rework process. Degradation to the PC board, CSP and solder connections can be avoided by closely adhering to rise and dwell times prescribed by the solder paste and component manufacturers. To ensure time-and-temperature accuracy, microprocessor-controlled heating, including a preheat, thermal soak, reflow and cool-down zone should be employed. A controlled dwell time in each zone should be permitted.

The ideal thermal profile is typically a duplication of the original production reflow profile, although refinements can be made in the rework environment, when justified. For example, it may be that the original reflow profile was not optimized for the component being reworked, therefore, modifications are needed to compensate.

The removal process can be slightly different from the replacement profile, but generally the preheat and soak are the same. This is usually a function of mass due to the PC board and the number of board layers.

The reflow zone is usually between 20 and 60 seconds, but this depends on the type of solder paste and other factors. The reflow zone is the only zone that should be shortened for CSPs. To remove the part, we do not need to be in reflow for 60 seconds, as long as we can guarantee that reflow is complete so that removal can safely take place.


Figure 3. Residual solder removal with desoldering braid and low-temperature, highly thermally-stable conductive tool.


Reflow Profile


A four-stage rework profile duplicates the original manufacturing profile.

Other methods of controlling the quality of the rework process include:


1.   Thermal energy should be directed through the component body to the solder joints without heating adjacent components.


2.   Heating should ideally take place in an encapsulated, inert gas-purged environment (Figure 1), where temperature gradients do not exceed ?罜 across the heating zone. This condition will facilitate simultaneous reflow of all solder joints. It will also avoid inadvertent pulling of pads during component removal. Lack of oxidation on thermocouple or RTD will ensure accurate temperature readings (another advantage of inert processing, which can sometimes be neglected).


3.  During the reflow profile, the utilization of a convective bottom side preheater will maximize temperature uniformity, reducing top and bottom PC board temperature gradients across the area to be heated.


4.  Automated component lift-off can further enhance process control. Virtually all CSP rework systems use hot gas or hot air as their thermal transfer medium. Interchangeable nozzles designed with different geometries are used to accommodate different applications and to direct the airflow path.


When choosing the appropriate nozzle for CSP rework, keep in mind that it should be as large as possible so that the operator does not inadvertently "bump" the component, but not so large that it will contact adjacent components. Exhaust vents located near the top of the nozzle will prevent heating of adjacent components.


Temperature Measurement


During the initial stage of profile development, accurate temperature measurement can be obtained directly at the solder joint. Theoretically, thermocoupling of solder joints is critical, but the lack of space between the CSP and PC board makes thermocoupling difficult.

If the component is soldered with solderpaste in production with a standoff height of .010 mil or greater, a small thermocouple can be inserted between the part and the PC board with success.

Another possible method of thermocoupling is to drill a small diameter hole (slightly larger than the diameter of the thermocouple) through the bottom of the board to touch the ball/pad interface directly. This can only be done if a PC board can be spared. This method will allow the realtime solder-joint temperature to be fed back to the microprocessor controller. The operator will then be able to make "on-the-fly" changes to the time/temperature parameters based on this feedback while the profile is active.

Since the recommended rework system provides low temperature gradients (< 5°C) across the heating area, the temperature measured at that one particular solder joint/pad interface will be within a few degrees of the entire array of solder joints, ensuring profile repeatability and accuracy.

Although it is not practical to drill into the board, the profile parameters recorded during initial profile establishment (removal) can be utilized for subsequent attachment profiles. Air velocity should be kept at a minimum of about 15 l/m as a starting point. Excessive airflow settings can inadvertently cause the CSP to skew during reflow, especially in the cooling zone when solder is solidifying.

Vacuum lift-off should be automatically engaged at the transition point between the reflow and cool-down cycles without applying any stress to the solder joint. A low specification of vacuum (i.e., <15" Hg) will protect the pads from damage by allowing the vacuum seal to break if all the joints have not exceeded reflow temperatures, yet will provide enough vacuum to lift any CSP despite its total mass.


Land Preparation


Once a CSP has been removed, the site must be cleaned to prepare for package attachment. Care is critical; coarse or improper procedures can burn, lift-off or otherwise damage the delicate attachment area. The best results will be achieved with a low-temperature, blade-style conductive tool (matched specifically to the width of the pad array), in conjunction with desoldering braid (Figure 2).

The ideal conductive tool will provide maximum heat transfer at the lowest possible source temperature, thereby maintaining temperature stability. Rapid heat transfer will protect the integrity of the PC board. High temperatures are definitely not recommended.

The flux content of the desoldering braid must be compatible with the flux residues contained in the removed solder paste and with the paste or flux that will be used for placement. Active fluxes can be used effectively only if the area is properly cleaned and inspected before placement.

Due to the difficulties associated with cleaning in the tiny space between the CSP and PC board, the use of "no-cleans" throughout the entire process is recommended.

The process of employing a bottom-side convection preheater is also recommended during the residual solder removal process. This step will decrease top and bottom temperature gradients, further eliminating any accidental pulling of fragile pads.

Figure 4. When the stencil is aligned, printing can be done.


Figure 5. Example of artwork stencil apertures.


Eutectic Solder



The above process assumes that the solder bumps are composed of eutectic (Sn63/Pb37) solder, although some CSPs employ high temperature (Sn10/Pb90) solder spheres that are attached to the board with lower temperature (Sn63/Pb37) solder fillets. If the removed CSP contains these high-temperature spheres, not all of them will remain on the package after removal. Some high-temperature spheres will adhere to the package while others will remain on the board.

Melting the spheres is not recommended, since this practice will alter the metallic composition of the pad, resulting in an alteration of the reflow point during the subsequent attachment stage. To remove high-temperature spheres, heat the land area past eutectic temperature and "pluck" the high-temperature spheres from the PC board with tweezers.

This removal can be accomplished by "hovering" the rework nozzle above the land area (while maintaining the bottom-side heating). During the high-temperature solder removal process, the eutectic solder should be kept above reflow temperature.

After all the high-temperature spheres are removed completely from the PC board, residual eutectic solder can be removed with desoldering braid and a thermally-stable conductive tool at a low temperature.

Once all residual solder has been removed from the lands, they should be cleaned with an approved solvent, preferably one prescribed by the solder manufacturer. At this point, the operator can inspect for any neglected solder splashes or degradation to the printed circuit board.


Accurate Solder Paste or Flux Deposition


Most CSPs contain bump matrices in the area of 0.5mm to 0.8mm pitch, which create a significant challenge for depositing solder paste in the rework environment (Figure 4).


The ideal method for depositing solder paste in a precise, uniform fashion is through the use of an individual component miniature stencil, similar to the practice in the assembly environment.

Since alignment of the stencil apertures with the mating PC board lands can be difficult by eye (the pads are only 12 mil in size), this can best be done under 50x - 100x magnification using the prism to align.

Once precise alignment is accomplished (Figure 3), the stencil can be lowered onto the PC board, where the paste is applied with a small, metal squeegee blade. The width of the squeegee blade should be matched to the stencil, enabling the user to make one single pass to avoid any overprinting.

Since this is a manual printing process, technological advantages should be designed into the rework stencil. The optimum rework stencil should be trapezoidal laser-etched (tapered), so that the opening on the bottom of the stencil is 1-2 mil greater than the top opening. Square apertures are preferred, as opposed to circular apertures, with a radius on all four corners of each aperture (Figure 5).


Transfer Characteristics

Conforming to the above stencil specifications will significantly enhance solder paste transfer characteristics. Although excellent results have been recorded utilizing a standard -325 /+500 mesh size solder paste, a high-viscosity (i.e. >900 kcps) paste is strongly recommended. A no-clean flux should be specified, since cleaning beneath the CSP is virtually impossible.

Not every CSP manufacturer endorses the application of solder paste during the rework or assembly process. Certain manufacturers recommend only the application of a gel-based, high viscosity flux. Higher viscosity is recommended to provide adherence between the CSP and PC board during the rework/assembly process. For best results, use solder paste only if the stand-off height and strength are the same as the production PC board. The cost of the board and the reliability required also affects which process刾aste or flux刬s preferred.


Component Placement

CSP placement is notably similar to that of a BGA package. Both types contain I/Os in a bottom-sided array matrix, therefore, precise alignment between the solder bumps and mating land pattern cannot be achieved by eye. Alignment should be made through the utilization of a split-beam optical system where a dual image of the CSP solder bumps and mating land pattern can be viewed and overlaid on a high-resolution monitor (Figure 6).

Due to the small specifications associated with CSPs, alignment should be achieved within the window of 50x - 100x magnification. The optimum placement machine should give the operator the ability to make fine adjustments between the overlaid images in the X, Y and rotational axes. A CSP can properly align itself during reflow even if it is as much as 50% misaligned with the mating pads. Keep in mind, however, that the CSP's tiny specifications make 50% a fine tolerance.

Figure 6. An Optimum rework stencil


Figure 7. Dual image of alignment of CSP solder bumps and solder paste pattern


Figure 8. X-ray inspection of passed CSP; All solder joints are uniform in appearence with no evidence or voids or bridging.




Since the optimum rework profile parameters are developed during the initial removal process, the same reflow profile can be used for reattachment as well as for subsequent removal processes.

No additional thermocouple feedback or operator dependency is required, since all parameters are optimized during the removal profile and stored in the memory of the rework system. Therefore, profiles can be recalled with the touch of a button for both placement and removal.

One must not take shortcuts by shrinking the duration of the reflow profile simply because the CSP is a tiny component.

Unlike traditional leaded SMDs, the human eye cannot verify solder joint integrity, since the array of interconnections is hidden beneath the CSP. As with BGA inspection, X-ray equipment is the only optical method of inspection available today (Figure 7).

Although one can electronically test the final assembly for functionality, this method does not allow for verification of solder voids or optical measurement of solder volumes (i.e. inconsistent or excessive solder paste deposition).

The high reliability of well-controlled CSP rework processes actually makes the latter a reasonable alternative.

As with BGA packages, simple "touch-up" of discovered defects is not possible. Correction of any defect calls for the removal of the entire component, followed by the entire rework cycle. The impact, however, can be minimized by strict control of the rework process.


CSP Cleaning凥ow Feasible?


Removing contaminants or even inspecting for residue in the minute spacing between the CSP and PC board is generally impossible. Consistent use of no-clean fluxes on CSP assemblies is recommended for eliminating the need for cleaning. Remember, if a gel flux is used to rework the part, then the stand-off height of the component is virtually non-existent. When solder paste is used, a stand-off height is present so some cleaning can be done if needed.



Chip-scale packages share many positive attributes, as well as challenges, in the rework process with their larger predecessor, the ball grid array. By far the most challenging aspect of the CSP rework process is solder paste deposition. This deposition can be aided through the use of single component rework stencils by aligning their apertures with their mating land pattern under high magnification.

Engineers familiar with rework procedures for BGA packages will find the transition to CSPs quite simple if the correct rework equipment is specified.




1.   Tessera, V. Solberg, Editor, Application Note 5, January 1996, San Jose, Calif.


2.   V. Solberg, "Assembly Process Development for Chip-Scale BGA Devices," IPC BGA National Symposium, January 1996, San Diego, Calif.