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Page 62


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Rapid Electronic Assembly Continued from page 58


department to department beyond that point. There are no shortcuts that do not compromise the integrity of the PCB. For this reason, a 24-hour turn


on a four-layer PCB is a marketing ploy. Yes, the actual processing time in the shop does just fit inside a 24- hour timeframe. However, getting into that process is not practical in an on-demand situation. Few PCB shops run three shifts, and those that do, do not offer all processes on all shifts. A common practice is to use engineering questions (EQs) to delay the start time and make it practical for the shop. Shortcuts with the PCB can be


disastrous for a rapid electronic assembly. The entire process depends upon the fabricator precisely duplicat- ing the design in traces/spaces, impedance, Tg, surface finish, through-hole copper thickness, and cleanliness. Ignoring any of these crit- ical parameters could prevent the assembly from functioning. Component procurement has


improved due to the Internet links to major distributors and the software innovations by these distributors. DigiKey and Mouser have taken steps to simplify the process of BOM scrubbing, part shape identification and component procurement. In many cases, a BOM can be


uploaded, costed and sourced for next-day delivery. Allocation and


long component lead times for stan- dard parts are almost non-existent due to slow economic growth over the past few years. The CAM systems used to pro-


gram the major pieces of assembly process equipment have not advanced. The software literature would lead one to conclude that CAM programs are also a mouse click away. Reality is much different. The


systems are generally cumbersome and fairly crude in their capability. There seems to be little cooperation between the equipment manufactur- ers and the software providers. This may be due in part to the unique soft- ware offered by each equipment manufacturer. While their offerings are lack-


ing, they appear to protect them by poor cooperation with general-pur- pose CAM vendors. Many CAM pro- grams are less than impressive. Assemblers that have found ways to speed the CAM process are generally doing so with custom scripts and pat- tern formatters that front-end the software from the equipment manu- facturers. In the assembly process, there


are multiple machine programs that must be created from the customer’s CAD files. These include: stencil design, stencil printer, SPI, place- ment, oven profile, AOI, selective sol- der, conformal coat, and flying probe ICT. With all of the programming decisions and independent software


THE NEW


BENCHMARK IN WIRE STRIPPING


systems. RPT has had to rely on other process enhancements or new technologies for speed. Combining CAM systems into one environment is the next major assembly frontier.


Reducing Processing Time There are four innovations that


merit special attention for their abil- ity to reduce uncertainty and pro- cessing time.


PCB Sourcing. When a new design does not perform, the engineer has four possibilities to consider. Did the PCB match the design net list and are there any opens or shorts? Did the assembler make an error with solder shorts or open connections? Are the components defective? Did they make a design error? An issue with the PCB is most


likely. One solution is to obtain the PCB through a source that offers an independent laboratory validation of its critical parameters. While an in- house source has a one-day time advantage due to shipping, is also suffers from allowing the “fox in the hen house.” An independent source with a lab has no vested financial interest in continuing to process a PCB that doesn’t meet specifications. Remaking a PCB delays the pro-


gram. However, in the rapid environ- ment, there is only one set of parts. Exhausting those parts on a defective PCB not only delays the program, but consumes valuable debug time on the part of the customer, greatly increas- ing overall cost.


Stencil Printing. Few assemblers can make their own stencils. This is particularly true for stencils that have advanced features to make the openings “slicker.” For most assem- blers, this process consumes a mini- mum of 24 hours. However new paste dispensing systems, such as the MY600 from Mycronic, eliminate this wait time. These systems program from the stencil file, but use a direct injection system to dispense solder paste onto the pads. The one-day improvement is


The


Mira 230 has a new robust and


sturdy design that supports an extraor- dinary processing range from AWG 32 to AWG 8. It sets a new benchmark by its unique sequencing capabilities that can be stored in a library for quick reproduction. The Mira 230 can strip and cut inner conductors with a variety of parameters without a program change. It comes equipped with a simple graphical user interface that ensures a quick learning process.


komaxwire.com


huge, but the other advantage is the ability to quickly change the aper- ture size, increasing or decreasing paste volume. Any change of aper- ture geometry with a fixed stencil would require another 24 hours to obtain a new stencil, plus the time lost in determining what change should be made. Also, not all aper- ture geometries can be achieved with a metal stencil. Direct paste dispens- ing has become one of the greatest time improvements for electronic assembly.


Oven Profile. There is a saying that goes, “Occasionally, even a blind squirrel finds an acorn.” The same can be said for oven profiling. Occasionally, you get lucky. But more often, the thermocouples are unkind and hours are spent trying to get the thermal geometries to cooper- ate, such that heating high-mass parts does not destroy small chips. Sometimes, the mass mismatch


is so great that no solution is accept- able. An old idea has become more mainstream for solving both prob- lems. Vapor phase soldering. Profiling time goes from hours to minutes and much larger thermal mismatches can be accommodated. Old timers will remember this process from the 70s and early 80s. However, the working fluids


were not environmentally friendly and most systems were abandoned.


Solder paste dispensers reduce process time and can adjust dispense geometries easily.


profile in all areas, the temperature of each oven zone is tweaked and the conveyor speed adjusted. This process repeats until the desired results are obtained. The time spent configuring the oven profile is a bottleneck for rapid assembly.


Selective Soldering. Recent advan - ces in the graphical interfaces of selective solder systems have radi- cally reduced the programming time, moving these from desirable to essential. While the quantity of through-hole components has been diminished by SMT technology, vir- tually all assemblies have some through-hole parts. Most notable are the connectors. Wave soldering is efficient for


large numbers of solder joints. But, SMT assemblies have widely dis- persed through-hole components that are usually surrounded by bottom- side SMT parts. Wave soldering requires that solder pallets be pro- duced to mask areas where the solder wave is not desired. Designing and producing these pallets is not usually an option for a rapid electronic assem- bly. The options then become hand solder or selective solder. Hand soldering is becoming more


and more problematic as the number of skilled hand solderers decreases. Consistency and time have always been a concern. Selective solder sys- tems generally solve both issues. The time is a predictable two


seconds per isolated joint, and consis- tency and repeatability are easily achieved once programmed. Graph - ical interfaces have dropped pro- gramming time to less than 30 min- utes. Some systems also provide off- line programming from the Gerber file and even allow for remote machine operation to test a program.


Rapid Prototyping RapidProto can fabricate a dou-


ble-sided PCB in 24 hours and have it available day two or three with overnight shipping. Excellent internet sourcing programs allow components to be specified and procured in 24


Continued on next page


June, 2017


The current systems lose very small amounts of the working fluid and do not have the massive boiling sumps of their ancient predecessors. The beauty of these machines is that they do not require repeated attempts to get the mass of the PCB to follow the desired thermal profile. In principle, the profile is entered


and the machine matches the thermal demand required to produce the pro- file. In a fusion system, there is no feedback of the temperature of the assembly once the profile is estab- lished. All up-front work is done with a sacrificial assembly and measured with a series of thermocouples travel- ing through the heat process. If the assembly does not follow the desired


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