Chip-scale packaging (CSP) parts may be attached using gel flux or liquid flux, instead of solder paste. Success may be achieved as long as the application is correct and full testing is carried out to verify the reliability of the process for a particular product.
When reworking CSPs with flux, remember these key points:
Chip-scale packaging (CSP) parts may be attached using gel flux or liquid flux, instead of solder paste. Success may be achieved as long as the application is correct and full testing is carried out to verify the reliability of the process for a particular product.
When reworking CSPs with flux, remember these key points:
* If the CSP has a flexible middle layer between the CSP die and the solder joint, the die can expand and contract due to thermal expansion, without putting stress on the solder ball joint.
* If the CSP has a solid construction and becomes hot, the coefficient of thermal expansion (TCE) difference between the part and the PCB may be under stress. Therefore, the solder joint strength has to be considered more seriously.
* The pad area of the PCB must be coplanar flat and adhesion to the PCB must be strong.
* The CSP must have eutectic solder balls, not gold bump or a 90/10 solder composition.
* The size of the joint will be smaller than a part put down with solder paste. When soldering to the PCB, a solder ball starting off with a 12 mil ball will reduce in diameter. This occurrence will also vary depending on pad size and whether solder mask-defined or nonsolder mask-defined.
* The standoff height is very low when using flux only - approximately 10 mil between the component and PCB. With solder paste, the standoff height is higher.
* When a part is soldered with solder paste, the ball size increases from 12 to 14 to 16 mil in diameter when using a 12 to 13 mil trapezoidal-etched stencil 5 mil thick.
* Thermal cycle tests should be carried out to check the strength of the joint and shear tests should be performed. If these tests meet the company requirements for reliability and quality, attachment with flux is acceptable.
* Gel flux should be spread evenly over the pad area with a small paintbrush. If not even, too much flux can hold the part off the PCB and cause openings and voids. Flux transfer can be an alternative to this brush method.
* If liquid flux is used, a small flux bottle for dispensing should be used with a paintbrush to spread the flux over the pad area. Although quick, this method is not always the most successful. All too often, flux is left in a tub with no lid and brushes are dipped in to apply flux; the flux solvent evaporates and puts inconsistent amounts of rosin down throughout the week's process.
* Flux transfer, better known as 'dip transfer', works well with no-clean gel flux.
Precise component placement
CSP placement is notably similar to that of a ball grid array (BGA). Both types of packages contain connections on the bottom-side array matrix; therefore, precise alignment between the solder bumps and mating land pattern cannot be achieved by simple human vision. Alignment should be made through the use of a split-beam optical system in which a dual image of CSP solder bumps and mating land pattern can be viewed on a high-resolution monitor (Figure 1). Due to the tight specifications associated with CSPs, alignment should be achieved within the window of 50X to 100X magnification.
The ideal placement machine should allow the operator 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 diameter of the mating pads or balls. Remember, though, that CSPs' tiny specifications make even that a fine tolerance.
Most of the problems with flux involve operators putting varying amounts of flux down; therefore, the results and residue left can vary. For example, if a PCB has activated flux all around a part that has been reworked, this flux has to be removed with either a spray or a brush. If the residue it not fully removed with spray or a brush, it will spread around the PCB, causing more potential failures later due to contamination and IRL (insulation resistance leakage). The use of a specially etched plate that holds a pocket flux is one method that can be used to transfer a dedicated amount of flux to the component/PCB.
The process
* A plate is etched the same size as the area of the balls and 4 mil deep for a CSP component.
* Gel flux is dispensed into the square and squeegeed flat with a manual squeegee.
* The component pick-up centre block is put onto the rework machine for component pick-up.
* The centre block is removed and the flux transfer plate put in its place.
* The component is then dipped into the plate until the balls contact the plate. This process puts a coating of flux on the bottom one-third of the solder ball on the part.
* This part is then placed onto the pads as usual with prism alignment of balls to pads viewed on a monitor.
* Reflow then occurs. The small amount of flux remaining usually cannot be seen around the part because the flux is only on the solder balls/pads.
CSP reattachment
If the optimum rework profile parameters were developed during the initial removal process, the same reflow profile can be used for reattachment and subsequent removal processes. No additional thermocouple feedback or operator dependency is required because all parameters were optimised during the removal profile and stored in the memory of the rework system. Do not take shortcuts by shrinking the duration of the reflow profile specifically because CSPs are small components. The reflow profile parameters employed in the rework process must conform to those specified by the flux manufacturer.
Inspection's further challenges
Unlike traditional leaded surface-mount devices, the human eye cannot verify solder joint integrity, as the array of interconnects is hidden beneath the CSP. As with BGA inspection, X-ray equipment is the only method of CSP inspection available today. Although the final assembly can be electronically tested for functionality, this method does not allow verification of solder voids or optical measurements of solder volumes.
As with BGA packages, simple 'touch-up' of discovered defects in CSPs are not possible. Correcting any defect calls for the removal of the entire component, followed by the entire rework cycle. However, the impact may be minimised by strict control of the rework process.
How feasible is CSP cleaning?
Removing contaminants or even inspecting for residue in the very small spacing between the CSP and PCB is often difficult to impossible. If acceptable to the customer's process, consistent use of no-clean fluxes on CSP assemblies is recommended to eliminate the need for cleaning. Remember, if only a gel flux is used to rework the part, then the standoff height of the component to PCB is virtually nonexistent. When solder paste is used, a standoff height is present so some cleaning may be done if required.
Conclusion
Chip-scale packages share many positive attributes and challenges in the rework process with their larger predecessor, the ball grid array. By far, solder paste deposition is the most challenging aspect of the CSP rework process. Deposition can be aided by using single component rework stencils and aligning the apertures with their mating land pattern under high magnification. Engineers familiar with rework procedures for BGAs will find the transition to CSPs simple if the correct rework equipment is specified.
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