1、BSI Standards PublicationEnvironmental testingPart 3-12: Supporting documentation and guidance Method to evaluate a possible lead-free solder reflow temperature profilePD IEC/TR 60068-3-12:2014National forewordThis Published Document is the UK implementation of IEC/TR 60068-3-12:2014. It supersedes
2、PD IEC/TR 60068-3-12:2007which is withdrawn.The UK participation in its preparation was entrusted to TechnicalCommittee EPL/501, Electronic assembly technology any IEC National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental and no
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11、rights. The main task of IEC technical committees is to prepare International Standards. However, a technical committee may propose the publication of a technical report when it has collected data of a different kind from that which is normally published as an International Standard, for example “st
12、ate of the art“. IEC TR 60068-3-12, which is a technical report, has been prepared by IEC technical committee 91: Electronics assembly technology. This second edition cancels and replaces the first edition published in 2007 and constitutes a technical revision. PD IEC/TR 60068-3-12:2014 4 IEC TR 600
13、68-3-12:2014 IEC 2014 This edition includes the following significant technical changes with respect to the previous edition: the content has been adapted to the state-of-the-art of reflow-oven technology and termination finishes; minor language adjustments were performed. The text of this technical
14、 report is based on the following documents: Enquiry draft Report on voting 91/1158/DTR 91/1177/RVC Full information on the voting for the approval of this technical report can be found in the report on voting indicated in the above table. This publication has been drafted in accordance with the ISO
15、/IEC Directives, Part 2. A list of all the parts in the IEC 60068 series, under the general title Environmental testing, can be found on the IEC website. The committee has decided that the contents of this publication will remain unchanged until the stability date indicated on the IEC website under
16、“http:/webstore.iec.ch“ in the data related to the specific publication. At this date, the publication will be reconfirmed, withdrawn, replaced by a revised edition, or amended. PD IEC/TR 60068-3-12:2014IEC TR 60068-3-12:2014 IEC 2014 5 ENVIRONMENTAL TESTING Part 3-12: Supporting documentation and g
17、uidance Method to evaluate a possible lead-free solder reflow temperature profile 1 Scope This part of IEC 60068, which is a technical report, presents two approaches for establishing a possible temperature profile for a lead-free reflow soldering process using SnAgCu solder paste. This process cove
18、rs a great variety of electronic products, including a large range of package sizes (e.g. molded active electronic components, passive components and electromechanical components). Study A addresses requirements needed in the production of high-reliability electronic control units (ECU), as for exam
19、ple in automotive electronics. These requirements include measurement and production tolerances. Study B represents consumer electronics products and includes reflow oven capability, board design and package sizes. 2 Basics The process temperature for SnPb solder paste has a wide margin due to the l
20、iquid temperature of the solder alloy. During reflow soldering, temperature differences between components exist but are not critical. The process temperature of SnAgCu solder paste is about 20 K to 30 K higher than SnPb solder paste. Furthermore, the temperature difference between components (T) be
21、comes wider and sometimes the heat resistance temperature of components can become critical. To avoid soldering failures which could be very harmful in safety-related applications and also generate higher failure costs, the capability of the soldering process is very important. A compromise between
22、the temperature requirements of highly reliable solder joints and the limited solder-heat resistance of the electronic components has to be sought. In addition, the different aspects of mass production have to be considered. To achieve a reliable solder joint, the conventional reflow soldering proce
23、ss with eutectic SnPb solder paste is usually performed at a minimum peak temperature of about 203 C at the coldest solder joint (i.e. at least 20 K above the liquid temperature of SnPb Tliquid= 183 C). The selected lead-free solder is SnAgCu with a melting point at around Tliquid = 217 C 1 1. It is
24、 a generally preferred material for lead-free reflow and wave soldering in mass production 2. Using SnAgCu solder paste, it is not possible to solder the coldest solder joints at least 20 K above the liquid temperature (Tliquid= 217 C), which would result in minimum temperatures of 237 C. When the c
25、oldest solder joint is 237 C, the temperature spread between small and large components, small semiconductor, and passive components, as well as the printed circuit board (PCB), will be too large for the components to survive the heat impact. Despite the aim to achieve a relatively low temperature a
26、t the coldest solder joint, the reliability of the solder joint has to be assured. _ 1Numbers in square brackets refer to the Bibliography. PD IEC/TR 60068-3-12:2014 6 IEC TR 60068-3-12:2014 IEC 2014 To reach this target in Study A, the temperature at the coldest solder joint is taken to be Tmin= 23
27、0 C, for a minimum time of 20 s, which is just 13 K above the melting temperature. Considering the peak shape (see Figure 1) this condition corresponds to 1 s at 233 C. From a physical point of view, the risk of insufficient solder wetting during mass production is significantly higher if the solder
28、 joint temperature is lower than the above mentioned temperature of 230 C. In addition, lead-free termination finishes (like tin layers with a post-bake process or very thin NiPdAu finishes) are known to exhibit a poorer wetting behavior than conventional SnPb pin finishes. Figure 1 Curve shape for
29、a peak temperature of at least 20 s at 230 C and 1 s at 233 C The experiments had been performed under mass production conditions (850 mm/min) using state-of-the-art reflow equipment, i.e ovens featuring multiple heating zones, full convection and N2atmosphere. 3 Boards under investigation 3.1 Test
30、board approach For the experiment in Study A, a special test PCB was designed. Polyimide resin with a glass transition temperature of Tg= 260 C was used as base material for the PCB. Such a test board can represent the entire automotive ECU spectrum. The largest temperature difference (T) between th
31、e coldest solder joint and the hottest point existing on this printed circuit assembly (PCA) spectrum is reflected on this test board (T can be even larger for even more complex PCAs). The coldest solder joint was represented by a defined thermal mass, to represent large integrated circuits (ICs), c
32、oils or aluminium electrolytic capacitors. Its temperature behavior was correlated with the temperatures of the coldest solder joints on serial boards. 3.2 Production board approach For Study B, PCB and reflow oven were taken from actual series production. T,Ct, s Coldest solder joint 20 s at 230 C
33、IEC PD IEC/TR 60068-3-12:2014IEC TR 60068-3-12:2014 IEC 2014 7 4 Temperature tolerances 4.1 Temperature tolerances in Study A For tolerances during temperature profiling, different systematic failures shall be considered. First of all, there is an error associated with the temperature measurement it
34、self. The measurement was performed in the centre on top of the packages with a well defined and repeatable preparation technique. Nevertheless, the failure due to preparation had to be fixed within 1,0 K. In addition, the thermocouple (NiCrNi), together with the evaluation unit has an accuracy of 1
35、,5 K for pre-selected thermocouples. According to IEC 60584-2 6 the NiCrNi thermocouples, class K, tolerance class 1 are specified with a tolerance of 1,5 K for just the thermocouple itself without the measurement unit. Based on suppliers indication and own measurements, the furnace tolerance based
36、on furnace load is 0,5 K and the furnace tolerance for long term stability is 2,5 K. Thermocouple with measurement unit: 1,5 K Preparation of thermocouple: 1,0 K Furnace load variation: 0,5 K Long term stability of furnace: 2,5 K Because these variations are independent, the Gaussian error propagati
37、on can be applied, which results in a total tolerance of 3,0 K, due to measurement errors and variations in mass production. The tolerance of 3,0 K results in the requirement to profile the coldest solder joint at 236 C, instead of 233 C (i.e. 233 C + 3,0 K). This tolerance is known as the “lower to
38、lerance”. In addition to the measurement errors and variations due to mass production, the influences of the test board have to be considered. The measured temperatures of the electronic components depend also on the position on the test board because of the longitudinal and transversal temperature
39、spread in the furnace and along the test board (see Figure 2). These temperature differences are the result of the heat flow conditions in the furnace and around the test board. The actual temperature of a device can be up to 3,5 K higher than the measured values at the position where the device is
40、mounted on the test board. The temperature dependence on the device position was measured independently before measuring the device temperatures on the assembled test board. Figure 2 Position of the assembled devices and temperature dependence on the device position The thermal mass on the test boar
41、d, which represents the coldest solder joint on the serial boards, was designed to include the relevant position-dependent tolerances. The upper temperature tolerances consist of the position-dependent temperature tolerances of 2 K to 3,5 K and the above mentioned +3 K. This leads to a total upper t
42、olerance of 5 K to 6,5 K. IEC Temperature Transversal temperature profile Direction of transportation Device position on test board Longitudinal temperature profile PD IEC/TR 60068-3-12:2014 8 IEC TR 60068-3-12:2014 IEC 2014 Regarding the whole temperature window of the lead-free soldering process,
43、a total position-dependent temperature tolerance of 8 K to 9,5 K has to be added to the measured T spread of the devices (see Figure 3). NOTE Electronic devices are divided into three temperature classes. Figure 3 Lower and upper temperature tolerances of the reflow solder profile 4.2 Temperature to
44、lerance and board size influence in Study B In the consumer board study, the measured temperature includes lower temperature tolerance and upper temperature tolerance. Therefore at the coldest solder joint temperature of 230 C, the “worst-case“ temperature becomes 227 C (i.e. 230 C 3 K) which is sti
45、ll 10 K higher than the melting point of the SnAgCu solder alloy (see Figure 4). Figure 4 Temperature tolerance and board size influence IEC Total tolerance 3 K 227 C min. 13 K Melting point of the SnAgCu: 217 C T = 10 K T = 15 K T = 20 K SmallsizePCBDigitalcameraCamcorderMid size PCBPCSettop boxLar
46、ge sizePCBNon consumerproductsT between coldest solder joint and hottest component within the board 230 C Minimum condition for high reliable soldering Temperature: 230 C Soldering time: 10 s to 30 s IEC SmalldevicesLargedevicesVerylargedevicesT devices 3 K 236 C 233 C 230 C 20 s Upper tolerances (m
47、easurement, equipment and position on test board 5,0 K to 6,5 K) Measured T between coldest solder joint and hottest device within each class Lower tolerances (measurement and equipment) 20 s at 230 C corresponds to 1 s at 233 C Minimum temperature requirement; 20 s at 230 C PD IEC/TR 60068-3-12:201
48、4IEC TR 60068-3-12:2014 IEC 2014 9 5 Peak form and width 5.1 Peak form and width in Study A The requirements were to maintain a temperature of at least 230 C for 20 s at the coldest solder joint, and to limit the peak package temperature of the smallest devices (e.g. SOT23, small LQFP, TopLEDs and p
49、assive components) to Tpeak 260 C. In order to meet these requirements, a soak-type preheating, as well as a hat type soldering peak were necessary in the investigation. The soak-type preheating allowed the temperatures of the individual packages to be close to each other upon entering the peak zone (see Figure 7). The hat type form of the soldering peak was used to minimize the temperature differences between t
