1 - Al Kossow's Bitsavers

Solder Paste ApplicationSolder paste can be applied using several methods: screeningthrough a stencil or stainless steel mesh, pneumaticdispensing or by hand application with a syringe. TexasInstruments recommends screening through a stainless steelscreen. The screen mesh must be chosen in accordance withthe mesh of the solder paste to provide an adequate emulsionof the solder paste and to prevent screen clogging. Ingeneral an 80-100 mesh screen should be used with 200 meshor finer solder paste particle. There are a number of factorsthat need to be considered when selecting a solder paste, afew key factors are as follows:1. Particle size2. Particle shape3. Percentage of metal content4. Temperature rangeComponent PlacementThe components can be placed via several different modesinto the still moist solder paste. In a production environment,the components are most efficiently placed with an automaticpick-and-place machine to achieve both speed and accuracy.Presently, pick-and-place machines can place between 600and 600,000 components per hour and are priced accordingly.In a research and development environment, hand placementcan be adequate due to the forgiving nature of surfacemounting. When a component is placed off center it will tendto self-align during reflow due to the surface tension of themolten solder. Naturally there are limits to the amount ofmisalignment that can be corrected. Two important aspectsof self-alignment are provision of adequate solder pad area,and proper placement of the solder pads with respect to thecomponent.Oven DryingAs solder pastes have evolved over the past several years,the drying process following component placement is notalways necessary. In the past, drying was necessary to driveout the solvents in the solder paste. If the solvents were notdriven out, the formation of solder balls was frequent dueto the out gassing of the solvents prior to reflow. Todaymanufacturers of solder pastes report that drying is no longernecessary when using many of the new solder paste formulations.As a wide variety of solder pastes exists, it is necessaryto consult the manufacturer before determining if drying isnecessary in your process.Solder ReflowWhile several methods of solder reflow are available,vapor phase soldering has been the most successful and isbecoming the industry standard.Two types of vapor phase systems are the batch and thein-line. The batch system is a two-vapor system that uses afluoroinert liquid such as FC-70 for the primary vapor anda clorofluorocarbon such as trichlorotrifluoroethane (R113)as the secondary liquid (see Figure 6). The secondary liquidhas a lower boiling point (47.6°C) than the primary liquid(215°C) thus acting as a blanket to prevent loss of the expensiveprimary liquid. The in-line system (see Figure 7)is a single-vapor system using only a primary vapor (suchas FC-70). The batch system is the forerunner of the in-lineand is more suited to development and small productionwhere the in-line is tailored to a mass production atmosphererequiring good throughput and minimal operating expense.Although the two systems are targeted to different marketstheir basic operation is the same. Both are capable of singleand double sided surface mount.Batch System OperationThe PC board complete with components is plated on anelevator and lowered into the secondary vapor. The elevatorascent-descent rate and dwell in the two vapor zones can bepreset via the vapor phase machine front panel. The descentrate and hold time in the secondary zone should be set soas not to unnecessarily disrupt the secondary vapor blanketor cause defluxing of the solder paste. Lowering the boardinto the 215°C primary zone causes the solder to reflow.A dwell time of 10-30 seconds in the primary zone is generallysufficient for most PC boards. The dwell time in theprimary zone is a function of the PC board mass. Once thesolder is reflowed, the PC board is raised back into the secondaryzone where the molten solder is allowed to solidify.In the batch system it is necessary to pay particular attentionto the ascent-descent rate of the elevator as the disruptionof the two-vapor zones will cause unnecessary loss ofthe expensive primary liquid.In-Line System OperationThe operation of the in-line system is similar to that ofthe batch system except that there are no secondary vaporor dwell times with which to contend. The PC board is placedon a conveyor belt that transports it through the system ata constant speed. Passing through the vapor zone the solderbecomes molten and solidifies as it moves toward the systemsexit. Where the ascent-descent rate and dwell time are criticalto the batch system, the conveyor speed is critical to the inlinesystem. The speed at which the conveyor should be setis also a function of the PC board mass.r::o'';:CUE.......o.EC/)r::o'';:CU,~C.c.