49 www.johansontechnology.com www.johansontechnology.com a pplication n otes for c eramic c hip c apacitors g eneral ceramic chip capacitors exhibit excellent reliability characteristics providing that proper circuit design techniques and controlled assembly processes are utilized. due to the ceramic capacitor?s crystalline micro-structure these components are susceptible when exposed to excessive thermal or mechanical shock during circuit processing. it should be noted that micro-cracks in ceramic can be difficult to detect with normal post assembly visual and electrical testing and can pose a significant threat to reliable field operation. for this reason it is recommended that the assembly qualification process employ suitable testing to expose the presence of micro-cracking conditions. c hip c apacitor a ttachment lasertrim ? capacitors - offered with gold flashed nickel-barrier terminations only. due to the unique internal construction of the lasertrim ? it is recommended that a conservative reflow temperature profile be used ( fig. 1). wave soldering is discouraged. high frequency capacitors & inductors - offered with standard tin plated nickel-barrier terminations compatible with solder flow and reflow processes. microwave single layer capacitors - offered with titanium-tungsten/gold and titanium-tungsten/ nickel/gold thin-film termination as well as legacy platinum/palladium/gold terminations. please refer to the attachment compatibility table (page 31) specific to these devices. s oldering i ron ceramic capacitor attachment with a soldering iron is discouraged due to the inherent limitations on precisely controlling soldering temperature, heat transfer rate, and time. in the event that a soldering iron must be employed the following precautions are recommended. preheat circuit and capacitors to 150c never contact the ceramic with the iron tip 30 watt iron output (max) 280c tip temperature (max) 3.0 mm tip diameter (max) limit soldering time to 5 sec. s older p re -h eat c ycle proper preheating is essential to prevent thermal shock cracking of the capacitor. the circuit assembly should be preheated as shown in the recommended profiles at a rate of 1.0 to 2.0c per second to within 65 to 100c of the maximum soldering temperature. s mt s oldering t emperatures solders typically utilized in smt have melting points between 179c and 188c. activation of rosin fluxes occurs at about 200c. based on these facts a minimum peak reflow temperature of 205c to 210c should be established. a maximum peak reflow temperature of 225c should be adequate in most circumstances. many reflow process profiles have peaks ranging from 240c to 260c and while ceramic capacitors can withstand soldering temperatures in this range for short durations they should be minimized or avoided whenever possible. use of pcb mounted multiple thermocouple m.o.l.e. profiling is advised for accurate characterization of circuit heat absorption and maximum temperature conditions. r eflow s older the general term ?reflow? refers to several methods used in heating the circuit so that solder paste reflows, or ?wetting? of the ceramic capacitor and pcb contacts occurs. these methods include infra-red, convection and radiant heating. the size of the solder fillet may be controlled by varying the amount of solder paste that is screened onto the circuit. recommended temperature limits for solder reflow are shown in figure 1 for lasertrim? and in figure 2 for standard capacitors. v apor p hase a typical vapor phase soldering process consists of several temperature zones created by saturated vapor from a boiling liquid. as the circuit passes through the zone the vapor condenses on the solder paste, pad, and termination resulting in heat transfer and reflow of the solder paste. vapor phase reflow produces consistent circuit heating with reflow occurring at a relatively lower temperature that is determined by the known boiling point of the liquid used, typically 215c. recommended temperature limits for vapor phase reflow are shown in figure 3.
50 www.johansontechnology.com www.johansontechnology.com a pplication n otes for c eramic c hip c apacitors s older w ave wave soldering is perhaps the most rigorous of surface mount soldering processes due to the steep rise in temperature seen by the circuit as it is immersed in the molten solder wave, typically at 240c. recommended temperature limits for wave soldering are shown in fig. 4. c ool d own c ycle after the solder reflows properly the assembly should be allowed to cool gradually at room ambient conditions. attempts to speed this cooling process or immediate exposure of the circuit to cold cleaning solutions may result in thermal shock cracking of the ceramic capacitor. 0 100 25 50 75 125 150 175 200 225 250 275 60 30 90 120 150 180 210 time (sec.) temperature ( c) 240 soldering time 25 sec. max. @230 c soak: 160 5 c 60 sec. min. gradual cooling at room temperature pre-heat: 1.0 to 2.0 c/sec. figure 1: solder reflow profile for lasertrims? 0 8 0 20 40 60 100 120 140 160 1 8 0 200 220 8 0 40 120 160 200 220 240 2 8 0 260 time (sec.) temperat u re ( c) grad u al cooling at room temperat u re pre-heating zone: 1.0 to 2.0 c/sec. max soldering time 20 sec. max. @ 260 c figure 2: solder reflow profile for mlccs 080 20 40 60 100 120 140 160 180 200 220 80 40 120 160 200 215 240 280 time (sec.) temperature ( c) gradual cooling at room temperature pre-heating zone: 1.0 to 2.0 c/sec. max soldering time 40 sec. max. @ 215 c figure 3: vapor phase profile for mlccs 080 20 40 60 100 120 140 160 180 200 220 80 40 120 160 200 220 240 280 260 time (sec.) temperature ( c) gradual cooling at room temperature pre-heat @ 1.0 to 2.0 c/sec. max soldering time 5 sec. max. @260 c figure 4: wave solder profile for mlccs please refer to our web site for solder profile information for other component types.
51 www.johansontechnology.com www.johansontechnology.com b oard l ayout & p ad d esign solder pad design, solder application, and component placement are important elements of the soldering process. excessive transfer of thermal or mechanical stresses to the mlc can result from oversized solder fillets. nominal pad designs for solder reflow process are listed in table 1. these guidelines represent a starting point in printed circuit board (pcb) design. further information is the institute for interconnecting and packaging electronic circuits (www.ipc.org) has developed and published ipc-sm-782a ?surface mount design and land pattern standard?. s older f illets to avoid detrimental effects of thermal and mechanical stress it is essential that the solder fillet be limited to 2/3rds of the overall height of the mlc termination as illustrated in the figure below. the solder fillet can be controlled by solder paste deposition and pad design in reflow and vapor phase processes and by pad design and use of hot air knives in the wave process. t omb s toning / c hip m ovement tomb-stoning or draw bridging is illustrated in the figure below. tomb-stoning or other undesirable chip movements may result if unequal surface tension forces exist as the molten solder wets the mlc terminations and mounting pads. this tendency can be minimized by insuring that all factors at both solder joints are equal, namely; pad size, solder mass, termination size, component position and heating. tomb-stoning is easily avoided through proper design, material selection and proofing of the process. s l l w end terminations ceramic body mounting pads printed circuit board (pcb) solder fillet solder fillet table 1 reflow pad dimensions chip size (l) (s) (w) length separation width min max min max min max 0201 in 0.008 0.014 0.008 0.012 0.008 0.016 0603 mm 0.20 0.35 0.20 0.30 0.20 0.40 0402 in 0.014 0.018 0.012 0.020 0.016 0.024 1005 mm 0.35 0.45 0.30 0.50 0.40 0.60 0603 in 0.024 0.028 0.024 0.031 0.024 0.031 1608 mm 0.60 0.70 0.60 0.80 0.60 0.80 0805 in 0.024 0.028 0.039 0.047 0.031 0.043 2012 mm 0.60 0.70 1.00 1.20 0.80 1.10 1210 in 0.039 0.047 0.079 0.094 0.071 0.091 3225 mm 1.00 1.20 2.00 2.40 1.80 2.30 a pplication n otes for c eramic c hip c apacitors
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