FastPrint

OPTIMIZING SOLDER PASTE RHEOLOGY TO SATISFY THE FASTPRINT PROCESSArden D. JonesMulticore Solders, Inc.ABSTRACTA need for a faster printing solder paste has evolved because of theever-increasing demands for faster assembly throughput. Fastprint printing materials are available to meet this demand. Ideally this type of solderpaste will have excellent printer open time, tack time, slump resistance, and extended “between print” abandon time. The ideal material will be of a flux chemistry that will provide high activity to deal with poor componentsolderability yet produces safe residues, i.e. IPC Type LR3CN and theBellcore specification. Overall this material will improve high volumeproduction yields, dramatically decrease cycle time, and enhance the postprint inspection time opportunity. Materials capable of fast printing, i.e.>100mm per second, have a unique rheology, which insures that the highshear rate viscosity will remain low while the thixotropic index is highenough to insure excellent print definition. In order to achieve the maximum result from these unique materials, it is imperative that optimization of theprinter and oven be reviewed. This paper will review the fast print process, the reflow process, and the associated benefit to overall manufacturingyield and throughput.Keyword: Fastprint-a family of solder creams designed for high speed printing processes.IntroductionThe need for continuous improvement is driven by the bottom line. Equipment and process improvements have been made that effect all aspects of surface mount assembly. Present replacement machines have throughput capabilities far exceeding what was available a few years ago or even a few months ago. Every aspect of surface mount assembly is under the eye of scrutiny for signs of continuous improvement. In many situations printing throughput has not felt the pressure to print faster because the placement machine is always the bottleneck. With the advent of faster and faster placement machines printing is now inheriting some of the past pressure for improved throughput previously relegated to the placement machine. The fact is that most printing machines have the capability to easily increase throughput. However this can not be accomplished unless a high-speed (Fastprint) solder paste is available to accommodate the increase in throughput speed. In order to meet current and future demands of high speed solder paste printing a new material technology must be developed. This new family of materials must continue to have the characteristics expected of materials that currently print and reflow in air with consistently high yields. As a minimum the new faster printable materials must have the following attributes:• Suitable for fine pitch, high speeds (100-250mm/sec) stencil printing• Extended printer open and tack time• Extended “between print” abandon time• Activity high enough to deal with poor component solderability• Produce safe residues• Have excellent slump resistanceMaterial ValidationFastprint materials have been qualified to Bellcore Specification TR-NWT-000078 Issue 3 (December 1991) and IPC-SF-818 (January 1988).PrintingFaster printing materials can be printed utilizing equipment and methods commonplace to the surface mount assembly industry. The term Fastprint does not exclude the material from applications involving dispensing or hand printing. In order to achieve the maximum effectiveness in productivity improvement with this material the only requirement is an increase in the squeegee speed. It is common practice to characterize the rheology of solder paste by making a viscosity measurement at single specified shear condition. Increasing metal content increased the measured value and at higher metal contents, decreasing the mean solder powder particle size can have the same effect. A plot of viscosity Vs shear rate can be a more informative indicator of the rheological properties of the paste. The Thixotropic Index enhances the assessment of real application performance and is an important tool in ensuring that the paste will do what we expect them to do. The Thixotropic Index for seven different types of paste is compared. The materials chosen represent paste formulated as Rosin Activated (RA), Rosin Mildly Activated (RMA), Low Residue No Clean, RMA No Clean and Fastprint No Clean. All of these materials are commonplace in the SMA marketplace with the exception of the Fastprint families. Because the high shear rate viscosity is relatively low and the Thixotropic Index high the Fastprint material will maintain good roll and drop off behavior at high speeds (Fig. 6). Materials that do not have this characteristic when printed at higher speed will typically result in skips and incomplete aperture fills.1The Thixotropic Index (TI) is defined as:TI=log (viscosity at 18s-1/viscosity at 18s-1)Figure 1Unlike other paste high squeegee pressures are not required. Lower squeegee pressures make this paste particularly useful in minimizing PCB flexing while second side printing. Increased stencil life, squeegee blade life, better aperture fill, less skips, less chance of aperture overpressure smearing, and less solder paste waste are all advantages realized by lower squeegee pressure printing. Fastprint paste is innovative in chemistry and different than traditional solder pastes in the following ways.•Different solvents with high boiling points and slower evaporation rates •Viscosity ranges of 630,000 to 770,000 Centipoise•Different gelling agents having higher shear resistanceBecause of the differences in chemistry Tack Time is improved (Fig. 2).Tack Comparative DataType Initial Peak Open TimeRMA 1.2g/mm 1.2g/mm16hrsLow Res 1.4g/mm 1.6g/mm16hrsFASTPRINT 1.4g/mm 2.0g/mm48hrsFigure 2Stencil printing equipment is available that can be easily set-up and adjusted by software inputs. This type of equipment precisely controls speed and pressure so that the advantages of faster printing can be maximized. A typical printing set-up for panels with fine pitch apertures yielded zero print defects (Fig. 3).Typical Printer Set-upSqueegee8 inch metalStencil 5 mil laser cutSqueegee speed102mm/secSqueegee pressure 4.5KgUnderside wipeEvery 6 panelsfrequencyUnderside solvent IPA or SC01Figure 3ReflowFastprint paste is unique in rheology but does not require special profile conditions. Any of the available methods of heating to cause reflow may be used including IR, convection, hot belt and vapor phase. Because of the material’s characteristics a very generous window for tack time is allowed. In the event that the placement or reflow oven is down for extended periods, tack times for Fastprint materials can be increased easily to 8 hours. It is not practicable to recommend a profile for every conditions of reflow. While this paper is directed towards an application utilizing inline printing, component placement and reflow the following profile conditions can apply (Fig. 4)(Fig. 5).1. Ramp to 140/150°C at 2°C/sec.2. Hold at 140/160°C for 60 second3. Ramp to peak reflow at 2°C/sec4. Optimum peak temperature 210-215°C5. Dwell above liquidus 45 seconds6. Cool rate 3°C/secFigure 4Typical Reflow Oven Set-upBelt Speed: 100cm/min.Zone1Zone 2Zone 3Zone 4Zone 5Zone 6Zone 7Zone 8Top 120140160150150150200255Bottom120140160150150150200255Figure 5Throughput and YieldIncreasing print speed improves throughput (17%) and affords greateropportunity for Post-print inspection while maintaining a zero printing defect yield (Fig. 6). It has been shown many times by Pareto analysis that printing defects have the greatest impact on final assembly yield. Because of the paste generous tack time boards can be printed and queue for Post-print inspection. State of the art automated Post-print inspection machines are available to take advantage of this opportunity. If given enough time span between printing and placement inspection machines can do a 100% inspection of the print deposition thus maximizing the chance for high assembly yields.Fastprint Paste Profile 050100150200250123456789101112T e m p . i n C e n t r i g r a d e 20 40 60 80 100 120 140 160 180 200 220 240Figure 6ConclusionIt has been demonstrated that faster printing applications can have the following effects on the manufacturing operation:1. Increase in Tack Time and Abandon Time translates to decreased downtime.2. Controlling squeegee speed and pressure easily attains the advantages ofFastprinting paste.3. Depending on the application, printer throughput can be increased by 17% ormore.4. The opportunity for Post-print inspection and assembly yield improvement isafforded.5. Lower printing pressures extend squeegee blade and stencil life.Innovative chemistry allowing high-speed printing has been successfully developed and tested. This chemistry is ready and should be the next step of surface mount assemblers. With the continued pressures for productivity improvements, i.e. throughput with high yield, faster printing materials will assist in maintaining a cost-effective competitive edge.。