Patent Application
20080083816
A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:
The solder paste deposition system 310 deposits solder paste on areas of the PCB defined by a solder paste stencil to produce a PCB 302 having solder paste deposits thereon. The solder paste stencil includes a plurality of apertures that are the same size and shape (and may also include other apertures of differing sizes and shapes). In one embodiment, a set of apertures of the same size and shape correspond to locations on the PCB (when the stencil is aligned over the PCB) at which bead probes are to be fabricated.
An automated inspection system 320 performs automated inspection of the solder paste deposits of the PCB 302. For example, the automated inspection system 320 may comprise an automated optical inspection (AOI) system or an automated X-ray inspection (AXI) system. During inspection of the solder paste deposits, the automated inspection system 320 obtains measurements 305 of various parameters pertaining to the solder paste deposits on the PCB 302. Examples of parameters that may be measured are listed in Appendix 1.
Based on the measurements 305, the automated inspection system 320 classifies the PCB 302 as either a good PCB 303 having solder paste deposits that meet predetermined passing criteria or a bad PCB 304 having solder paste deposits that do not meet the predetermined passing criteria. Good PCBs 303 are then sent to a component placement station 325 which places components on the PCB such that component leads align with pads on the PCB, and then to a reflow station 330 where solder is reflowed (i.e., melted) onto the surface of the PCB 303 to conductively adhere solder to the conductive areas of the PCB on which solder paste is deposited. Bad PCBs 304 may be repaired-and retested prior to the reflow step.
A statistical process control function 340 collects the solder paste deposit measurements 305. In particular, the automated inspection system 320 collects measurements including physical measurement data of repeated solder paste deposits across the PCB for use by statistical process control (SPC) 340 of the solder paste stenciling process.
In a production line, the above-described solder paste stenciling process is repeated over and over again for each PCB to be manufactured. During the production run, various problems may prevent the deposition of acceptable solder paste deposits on the PCB. For example, the stencil can get dirty with solder paste on its bottom surface (i.e., the surface contacting the PCB), which may result in solder paste residue in unwanted places on the PCBs. In another example, the solder paste may get depleted in the first apertures encountered by the squeegee, leaving apertures later encountered by the squeegee without any or sufficient solder paste. Likewise, solder paste can dry out prior to the squeegee step. In another example, the solder paste stencil may become misaligned over time. A multitude of other solder paste stenciling process problems may also occur, many of which are difficult to measure, and many of whose symptoms are difficult to correlate with a specific root cause of a problem.
Embodiments of the invention utilize PCBs that require repeated identical solder paste deposits across the PCB. As stated previously SPC techniques have been of limited use in the solder paste stenciling process because the shapes and sizes of solder paste deposits is unpredictable from board to board. In an illustrative embodiment, candidates for use in applying SPC techniques to the solder paste stenciling process may be locations on the PCB whose size and shape are repeated many times over across the board. For example, a recent development in the fabrication of PCBs is the use of “bead probes”. Bead probes may be solder bumps (or “beads”) attached along PCB traces that may be probed by in-circuit test (ICT) system probes to allow testing of the PCBA to verify that it was assembled correctly and that the components on the PCBA are correct and conductively connected. A more detailed description of the fabrication of bead probes is found in U.S. Patent Application Publication No. 20050061540, which is incorporated by reference herein for all that it teaches.
For the fabrication of bead probes on a PCBA, the bare printed circuit board (PCB) is designed and manufactured, and locations of bead probes on traces or other metalized areas of the outer surface of the PCB are determined. During PCB design, apertures of a repeated specific size and shape are defined in the CAD artwork that are used to fabricate the solder paste stencil. The solder paste stencil is aligned over the manufactured PCB directly over the bare metal exposed by the openings in the solder mask. During the normal surface mount assembly process step of solder paste stenciling, solder paste is deposited through the apertures to lie directly on the bare metal. After normal placement of surface mount components on the PCB, the solder on the PCB is reflowed, making solder joints that physically and electrically attach the components to the PCBA. Concurrently, the solder paste deposits over the bead probe sites also melt during reflow and the surface tension of the molten solder pulls the solder into a bead that covers the bare metal. As the PCBA cools, each bead of solder retains that shape, and is called a “bead probe”.
With the advent of bead probes, many PCBAs now have multiple solder paste deposits of repeated size and shape in predictable locations across the entire PCBA after the solder paste stenciling process. Furthermore, in a given manufacturing line which may use bead probes in the post-fabrication testing process (e.g., in in-circuit testing) due to the tester itself being equipped with specialized probes for probing bead probes, all PCBAs entering into the manufacturing line may in fact be required to be fabricated with bead probes. Because of the repeatable nature of these solder paste deposits, subtle differences in their physical characteristics such as, but not limited to, size, thickness, volume, and position that are caused by variations in the stenciling process can be measured with an automated inspection system, independent of the actual PCBA design. This data can then be used very effectively for statistical process control of the solder paste stenciling process not only from board to board of the same PCBA design, but also from PCBA design to PCBA design. This makes SPC practical for the first time.
In one embodiment, physical bead probe measurement data is collected and organized into a useful form for use by an SPC system. The SPC system performs statistical calculations on the data. In one embodiment, the SPC system correlates results of the SPC analysis of the solder paste deposits to root causes of problems with the solder paste stenciling process, such as but not limited to the parameters and features listed in Appendix 1. For example, the automated inspection system may collect physical data pertaining to solder paste deposits for fabrication of bead probes including length, width, height, volume, position, etc., that may be assimilated by the SPC system.
When a solder paste deposit on a PCB is determined to be bad, a technician may investigate the physical solder paste deposition system to determine the root cause(s) of the rejected solder paste deposit. This information may then be fed into the SPC system to allow it to correlate similar measurement data associated with other bead probe solder paste deposits with the root cause. Thus, root causes of solder paste deposition problems, which are not easily identified or controlled with prior art methods, may be identified, controlled, and/or eliminated to improve the solder paste stenciling process, which leads to reduction of the number of solder paste deposit defects, and therefore reduction in failure rate of manufactured PCBAs. For example, the addition of measurements of paste area, height, placement and volume can be used to more directly link visible defects with specific actions. For instance, typical AOI measurements such as paste to pad offset may be correlated by the SPC system to a stencil alignment problem whose corrective action would be screen printer adjustment. Likewise, low area measurement may be correlated by the SPC system with dried solder paste on the stencil apertures. Corrective action such as stencil cleaning and/or stencil paste replacement may be linked to this correlation.
The statistical process control system may correlate the collected data with at least one of a plurality solder paste stenciling process problems to identify at least one of the plurality of solder paste stenciling process problems occurring in the solder paste stenciling process (step 507). The identified solder paste stenciling process problem(s) may be indicated to a user (step 508). The user may correct the identified solder paste stenciling process problem(s) (step 509). Representative data collected may include, for example only and not limitation, solder paste deposit volume, uniformity, length, width, thickness, offset of solder paste deposit with respect to bare metal on the printed circuit board, presence of solder paste on non-predefined solder paste deposit areas of the printed circuit board.
Although this preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.
