Elektronika 2012-04 I.pdf - Instytut Systemów Elektronicznych ...

Tabl. 2. Percentage content of leaded alloy in solder jointTab. 2. Procentowy udział ołowiowego stopu lutowniczego pasty w połączeniulutowanymType of structureStencilaperture[mm]Leaded solder alloy content[%]100 µm 125 µmCSP84T. 5-DC123 0.28 39.5 49.3CSP132T. 5-DC145 0.28 39.5 49.3BGA676T1.0-DC269 0.56 18.8 23.5ADSP-BF561 SBBZ600 0.48 16.0 20.0Soldering profilesGenerally, each manufacturer of solder paste offers the most optimalmelting profile of solder paste to create optimal conditionsfor the formation of solder joints with the highest possible quality.The real profile of solder paste melting depends on many factors,such as:– construction of PCB (shape and thermal capacity);– number and differentiation of assembled electronic components(type and package dimensions);– placement of components on the PCB.Moreover, the melting profile of solder paste in the same electronicpacket may be different, and usually it is, depending on thelocation on the PCB, which is related to the factors listed above.Therefore, the elaboration of solder paste melting profile, fora particular electronic packet, should be proceeded by the “initial”soldering with placed all electronic components, and additionallythermocouples located at the strategic points where it canbe expect the greatest differences in temperature-time course.Figure 1 shows location of thermocouples (T1÷T6) in functionalpacket signed 10xx_H_dsp6, which contains many different typesof assembled electronic components. That packet is a control unitdesigned in Tele and Radio Research Institute.leaded electronic components, and too high soldering temperaturecan damage the leaded electronic components. The first fromadditional profiles was signed as P2, and it was elaborated toachieve the temperature of spherical leads (measured by thermocoupleT6, Fig. 1) slightly higher than 217°C, which is equal tothe melting point of lead-free alloy (SAC305) used on electroniccomponent leads. It was assumed that in this case the maximummeasured temperature should not exceed 220°C. The second additionalprofile was elaborated to achieve temperature measuredby thermocouple T6 on the level about 225°C, and it was signedas P3. Figure 2 shows the elaborated profile P2, and Table 3summarizes the important parameters of elaborated profiles. Themost important parameter is soldering temperature measured byT6 thermocouple (presented in Table 3 in gray coloured row) locatedin spherical lead of BGA package, in central area of it.Temperature [°C]250.0200.0150.0100.050.00 120 240 360Time [s]Fig. 2. Soldering profile P2 of functional electronic packet assembledin mixed technologyRys. 2. Profil lutowania P2 funkcjonalnego pakietu elektronicznego montowanegow technologii mieszanejTabl. 3. Parameters of elaborated profiles P1, P2 and P3Tab. 3. Parametry opracowanych profili lutowania P1, P2 i P3T1T5T4T6ProfileParameterUnitP1 P2 P3Preheating temp. °C 118-122 170-172 185-186Soldering – temp. T1 | time °C | s 228 | 32 234 | 38 241 | 41Soldering – temp. T2 | time °C | s 234 | 37 232 | 38 238 | 44Soldering – temp. T3 | time °C | s 236 | 40 228 | 41 239 | 49Fig. 1. Location of thermocouples in functional electronic packetRys. 1. Rozmieszczenie termopar w funkcjonalnym pakiecie elektronicznym22T2TOP sideT3BOTTOM sideT1– near the edge of PCB; T2, T3 – near the corner of PCB;T4 – near the DIP component; T5 – on the BGA; T6 – in spherical lead of BGAAs the base point, the settings of reflow oven (VIP70 from BTUcompany) for typical leaded melting profile of chosen solder paste(HM-1 RMA V16L) were selected, and that profile was signed asP1. From the fact that in this specific case there are both leadedsolder paste and lead-free solder alloy from spherical leads ofcomponent package, the typical leaded melting profile may beinsufficient to create the uniform and reliable solder joints. Fromthis reason it was decided to elaborate two additional profiles,which will be something intermediate between typical profiles ofmelting leaded and lead-free solder paste. It was excluded toapply the standard lead-free profile because it is very probablethat in the same electronic packet there will be both lead-free andSoldering – temp. T4 | time °C | s 226 | 26 228 | 22 233 | 32Soldering – temp. T5 | time °C | s 227 | 29 225 | 22 234 | 48Soldering – temp. T6 | time °C | s 218 | 07 220 | 17 226 | 32Time to T maxs 210 240 211Cooling rate (measured at T6) °C/s 1.67 1.70 1.65In conclusion, it should be noticed that the investigations weremade using HM-1 RMA V16L (Sn62Pb36Ag2) solder paste andlead-free electronic components (presented in Table 1) assembledon PCBs with Ni/Au and immersion Sn protective coatingsof solder pads. Tests were made with all combinations of two thicknessof stencils (100 and 125 µm) and three soldering profiles(P1, P2 and P3). As a result 12 tests with three repetitions of eachwere made.ResultsAs the result of realized investigations the quality verification ofsolder joints was used. The solder joints of multi-lead structuresare very difficult to control because they are not visible directlyElektronika 4/2012