Meet the challenge of lead-free welding process an

2022-08-12
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To meet the challenges of lead-free welding process and electronic technology

the electronic industry is changing from lead-containing welding system to lead-free welding system. This change has increased the melting and flowing processing temperature of the new lead-free solder, resulting in a variety of traditional polymer materials used in electronic accessories can not meet the requirements. Therefore, high temperature resistant polymeric materials such as Victrex Peek (poly (aryl ether ketone) polymeric materials are increasingly important in the manufacturing process of lead-free solder. This paper compares the properties of Victrex peek polymer with other thermoplastic materials from the perspective of lead-free welding process, analyzes its dielectric properties, and lists some successful application examples. It can be seen that Victrex peek is an ideal high-performance polymer material that challenges the demanding requirements of lead-free welding process and electronic technology

challenges of lead-free welding process and electronic technology

with the promulgation and implementation of RoHS (2002/95/ec) and other directives, the application of heavy metals including lead, mercury, cadmium and hexavalent chromium in electronic products has been limited. It is precisely because of the promotion of this regulation that the electronic industry is currently shifting from lead-containing solder systems to lead-free solder systems. It is estimated that by 2008, the proportion of lead-free soldering in the global electronic production industry will exceed 50%. This change improves the melting and flowing processing temperature of the new lead-free solder, which makes a variety of traditional polymer materials used in electronic accessories unable to meet the requirements. The lead-free welding process temperature is much higher than the traditional tin lead welding temperature (the lead-free welding system is 260 ℃ ~300 ℃, while the tin lead welding system is only 230 ℃). Moreover, in the lead-free process, the high temperature exposure time is also longer than the traditional process

there are many accessories assembled by surface mount technology on the circuit board of electronic equipment, such as signal relays, capacitors, mobile backup batteries, connectors, telecommunications coaxial connectors, etc. Many of these accessories are made of polymeric materials as part of the shell or accessories. Such applications involve a variety of polymeric materials, and their heat resistance can fully meet the requirements of the traditional tin lead solder process. However, because the application of lead-free solder increases the processing temperature, higher requirements are put forward for the heat resistance (melting temperature and thermal deformation temperature) of the thermoplastic used. For example, because the lead-free welding process needs to adopt higher temperature on the whole welding surface, the insulation materials with lower heat resistance such as PP and pet used in the dielectric layer of film chip capacitors are replaced by materials with higher heat resistance such as PPS

in addition to heat resistance, polymeric materials used in electronic accessories need to meet many other requirements. This includes: high mechanical properties (such as tensile strength, creep, toughness and impact strength) at room temperature and high temperature are critical factors; Dielectric properties are particularly important in the application of high frequency (above 1 GHz) wireless transmission; The dimensional stability of polymeric materials must be as close as possible to the metal or other polymeric materials with which they are matched to ensure the reliability of the system. Only when the properties of polymeric materials meet all the stringent requirements mentioned above, can they meet the needs of today's high-performance electronic products

meet the requirements of lead-free welding process

Figure 1 compares the melting temperature and thermal deformation temperature (HDT) of various polymeric materials often used in electronic applications. As mentioned above, the melting temperature and thermal deformation temperature (HDT) should generally be higher than the processing temperature range of lead-free welding process (260 ℃ ~ 300 ℃). However, materials with poor high temperature resistance such as PBT and PA66, and even PPS, cannot meet the requirements of lead-free welding process for high temperature resistance. Because at this temperature, such materials will melt. Although the melting temperature of LCP is very high, which fully meets the temperature requirements of lead-free welding process, its thermal deformation temperature (HDT) is not satisfactory. Once any related load occurs, it will endanger the application itself. In contrast, the melting temperature and thermal deformation temperature (HDT) of Victrex peek polymer materials are beyond the temperature range of lead-free welding process

Figure 1 melting temperature and thermal deformation temperature of various polymeric materials in electronic applications

for accessories requiring lead-free welding process, in addition to the temperature characteristics of polymeric materials, the tensile strength (the main indicator representing the mechanical properties of polymeric materials), gas release performance at high temperature and water content of polymeric materials used must also be considered. Table 1 compares various glass fiber filled thermoplastics based on these three properties. It can be seen from the table that Victrex peek polymeric materials have the highest tensile strength and the lowest gas release level. Since the moisture contained in the material may be converted into steam at high processing temperature, high moisture content may have a negative impact on reliability. Therefore, low water absorbent materials can be sent directly to local municipal waste collection points or community retail stores, which is the best choice. The water absorption level of Victrex peek polymer is much lower than that of PBT and PA66, but higher than that of PPS and LCP. Table 1 performance comparison of various glass fiber filled polymer materials commonly used in electronic applications

for further comparison, we compare the mechanical properties of several materials at high temperature. Figure 2 shows the change of bending modulus of 30% glass fiber filled Victrex Peek (90gl30), 60% glass fiber filled Victrex Peek (90gl60) and 30% glass fiber filled PPS

Figure 2 Comparison of bending modulus of glass fiber filled Victrex peek and glass fiber filled PPS measured by DMTA method

at low temperature, the bending modulus of 90gl30 is equivalent to that of 30% glass fiber filled PPS, about 10GPa. However, the glass transition temperature (TG) of PPS is much lower than that of Victrex peek polymer (87 ℃ and 143 ℃ respectively). Therefore, the properties related to Tg of PPS will decline when the temperature is significantly low; When the temperature is higher than 100 ℃ and lower than TG, 90gl30 has obvious advantages over PPS; When the melting temperature of PPS is higher than TG and lower than about 250 ℃, their moduli are basically similar. When this temperature is reached, PPS can no longer be considered; Beyond this temperature, 90gl30 performs well until 300 ℃ or even above. Victrex peek is obviously better than PPS at this point because components must maintain a certain degree of rigidity when reflow soldering at relatively high temperatures

for higher mechanical performance requirements, 90gl60 can be met. Because 90gl60 has a very high modulus in the whole temperature range, which is far more than 90gl30 and PPS + 30% GF, its modulus at about 200 ℃ is similar to PPS at room temperature, and the decline mode of bending modulus is very similar to 90gl30. Thanks to its high glass fiber filler content, the bending modulus of 90gl60 is always maintained at a higher level than that of 90gl30

in addition, we also carried out experiments and studies on the possibility of applying Victrex peek polymer materials in the accessories of lead-free welding process equipment. Place Victrex peek 450g test strip in the tin furnace of industrial lead-free solder processing machine for one month. The solder is composed of tin (96.5%), silver (3%) and copper (0.5%), and the temperature is 260 ℃. The tensile modulus, tensile strength and elongation were measured before and after immersion. The experimental results are shown in Table 2

it can be seen from the test results that the tensile strength increases, but the elongation decreases. The annealing of samples at high temperature is the main reason for this result. However, this change in performance is very limited, so this material can be used to manufacture relevant accessories in welding and reflow equipment (such as welding claws and wet treatment gears). However, for this specific use, it still needs to be further tested

application examples

1. Battery gasket

in today's medium and high-end mobile, backup batteries are widely used to maintain the minimum power required for mobile timing and memory functions when the main battery is removed from the mobile. Figure 3 is a cross-sectional view of the backup battery. The part indicated by the arrow is a gasket made of Victrex peek polymer material. The standby power can realize the full digital 3 closed-loop control pool of force, deformation and displacement, which is directly installed on the mobile circuit board. In the past, manual welding and manual plugging were often used for installation. In order to improve production efficiency, automatic welding process is now widely used. Since other materials cannot meet the requirements, Victrex peek polymeric materials are specified for this application. The reason why this material is specified is that it meets the requirements of heat resistance of lead-free welding process and chemical resistance of electrolyte in two electrode chambers, as well as the requirements of stress resistance to maintain mechanical integrity during assembly. In addition, because the gasket is very thin (only 0.2 mm in some extreme cases), the machinability of the material is very important. This is undoubtedly a challenge for many polymer materials, but for Victrex peek polymer materials, its good thin-wall molding and filling performance can ensure the perfect molding of gaskets

Figure 3 cross sectional view of mobile backup battery. The light gray components indicated by the arrow are Victrex peek gasket

2, molded connection device

molded interconnection component (MID) is a new technology used to manufacture electronic devices such as sensors, antennas, MEMS, LED packaging, etc. This technology has the advantages of reducing costs, increasing design flexibility and parts integration. Many mids are installed on circuit boards and systems through lead-free soldering process, so high temperature resistant materials are required. Other requirements also include good electrical performance, chemical resistance and processability

the molding technology used by mid is usually secondary injection molding, that is, two different polymer materials are injected in two consecutive injections. For the secondary injection molding process, whether there is good bonding strength between two different polymer materials is particularly important. Therefore, we studied and compared the bonding strength of various polymeric materials, including Victrex peek. The results show that Victrex peek polymer has relatively good bonding strength with ABS, and the bonding strength with polyetherimide (PEI) is lower than middle, which depends on the injection sequence of the material. From our preliminary investigation of mid applications, the performance of Victrex peek is sufficient to meet the requirements of these applications. However, the validity of this view needs to be further confirmed. Table 2 Comparison of mechanical properties of Victrex peek 450g before and after immersion in lead-free solder

3. The loudspeaker diaphragm

has gradually developed into a multimedia device with movement, which puts forward higher requirements for the performance of its loudspeaker. In order to improve the playback quality of audio, the input power of speakers is constantly improving. Since 97% of the electric energy supplied to the loudspeaker is released in the form of heat, the working temperature of the high-power loudspeaker can be as high as 260 ~ 270 ℃ in the state of full power, which leads to another high temperature resistance requirement besides the lead-free welding process. Since the manufacturing process involves chemical substances (such as organic solvents), the material also needs to have chemical resistance. Victrex peek can meet the requirements in terms of high temperature resistance and chemical resistance. In addition, vibrating films are generally manufactured by thermoplastic molding of polymer films. Experiments show that the processing cycle of amorphous Victrex peek film (APTIV film) is much shorter than that of some other high-temperature polymer materials (such as polyimide)

4. Connector

victrex peek polymeric materials have been successfully applied to various electronic devices

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