Patent Application
20100299919
1. Field of the Invention
This invention relates to a method for making a positive temperature coefficient (PTC) device, more particularly to a method for making a PTC device that includes crosslinking a crosslinkable preform after soldering a pair of conductive leads to a pair of electrodes on the crosslinkable preform.
2. Description of the Related Art
A PTC composite material consisting of polymer and electrical conductive filler exhibits a PTC property such that the resistance of the PTC composite material is increased exponentially when the temperature thereof is raised to its melting point. Hence, the PTC composite material is commonly used as a fuse, such as a thermistor, for protecting a circuit from being damaged.
Referring to
Since the reflow oven 15 is required to be operated at a temperature sufficient to melt the lead-free solder paste 14 for the soldering operation, which is relatively high, undesired breaking of hydrogen bonds of the molecular structure of the crosslinked blend 11 of the PTC composition is likely to occur, which, in turn, results in a deviation from the specification in the resistance requirement for the products of the PTC device 1 and a reduction of the production yield.
In addition, the way of heating during the soldering of the leads 13 to the electrodes 12 in the reflow oven 15, i.e., by heating the upper one of the leads 13 through a heated gas blown from above and the lower one of the leads 13 through a metallic support 151 of the reflow oven 15 that is in contact therewith, can cause a non-uniform temperature distribution throughout the PTC device. As a consequence, when the PTC device is cooled down, the cooling rate throughout the crosslinked blend 11 of the PTC composition will be uneven, which results in an increase in the resistance of the crosslinked blend 11 of the PTC composition, which, in turn, results in an increase in power consumption during the use of the PTC device 1.
Therefore, the object of the present invention is to provide a method for making a positive temperature coefficient device that can eliminate the aforesaid drawbacks associated with the prior art.
According to this invention, there is provided a method for making a positive temperature coefficient device. The method comprises: (a) forming a crosslinkable preform of a positive temperature coefficient polymer composition containing a polymer system and a conductive filler; (b) attaching a pair of electrodes to the crosslinkable preform; (c) soldering a pair of conductive leads to the electrodes using a lead-free solder paste having a melting point greater than 210° C.; and (d) crosslinking the crosslinkable preform after step (c).
Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawings, in which:
Before the present invention is described in greater detail with reference to the accompanying preferred embodiments, it should be noted herein that like elements are denoted by the same reference numerals throughout the disclosure.
Preferably, the soldering operation in step (c) is conducted at a working temperature greater than the melting point of the lead-free solder paste 5 and not greater than 260° C. More preferably, the working temperature of the soldering operation in step (c) ranges from 240° C. to 260° C.
Preferably, the polymer system contains a crystalline polyolefin selected from the group consisting of non-grafted high density polyethylene (HDPE), non-grafted low density polyethylene (LDPE), non-grafted ultra-low density polyethylene (ULDPE), non-grafted middle density polyethylene (MDPE), non-grafted polypropylene (PP), and combinations thereof, and a copolymer of an olefin monomer and an anhydride monomer. For example, ethylene/maleic anhydride (PE/MA) copolymer and ethylene/butyl acrylate/maleic anhydride (PE/BA/MA) trimer can be used as the copolymer in this invention.
Preferably, the conductive filler is selected from the group consisting of carbon black, metal powder, such as Ni powder, and a combination thereof.
Preferably, the crosslinkable preform 2 is formed by compounding and extruding the positive temperature coefficient polymer composition. The electrodes 3 in step (b) are attached respectively to two opposite surfaces 21 of the crosslinkable preform 2 through laminating techniques so as to form a laminate 20.
Preferably, the first preferred embodiment further includes thermally treating the crosslinked preform 2 after step (d) (see
Preferably, the crosslinking operation in step (d) for the above preferred embodiments is conducted by irradiating the crosslinkable preform 2 to a dosage of at least 10 kGy using Cobalt-60 gamma-ray irradiation generated by an irradiating apparatus 7.
It is noted that the crosslinkable preform 2 can be partially crosslinked before the soldering operation to an extent that causes insignificant deviation from the specification in the resistance requirement of the products of the PTC device.
The merits of the method for making the PTC device of this invention will become apparent with reference to the following Examples and Comparative Examples.
Table 1 shows different PTC polymer compositions of six formulations (F1˜F6) for preparing PTC materials of the following Examples and Comparative Examples.
aHDPE with a melting point (Tm) of 140° C., purchased from Formosa Plastic Corporation, Taiwan.
bPE/MA copolymer with a melting point of 132° C., purchased from Dupont.
cPE/BA/MA trimer with a melting point of 108° C., purchased from Arkema Incorporation.
dPE/MA copolymer with a melting point of 105° C., purchased from Dupont.
ea carbon powder purchased from Columbian Chemicals Company.
fa product purchased from Inco Special Products.
Six PTC materials, having different PTC polymer compositions (F1-F6) listed in Table 1, for Example 1 were prepared based on the method of the first preferred embodiment as illustrated in
Six PTC materials, having different PTC polymer compositions (F1-F6) listed in Table 1, for Example 2 were prepared based on the method of the second preferred embodiment as illustrated in
Six PTC materials, having different PTC polymer compositions (F1-F6) listed in Table 1, for Example 3 were prepared based on the method of the third preferred embodiment as illustrated in
Six PTC materials, having different PTC polymer compositions (F1-F6) listed in Table 1, for Example 4 were prepared based on the method of the fourth preferred embodiment as illustrated in
Six PTC materials, having different PTC polymer compositions (F1-F6) listed in Table 1, for each of Examples 5-8 were prepared based on the method of the fourth preferred embodiment as illustrated in
Six PTC materials, having different PTC polymer compositions (F1-F6) listed in Table 1, for Comparative Example 1 were prepared. The procedures and operating conditions for preparing each PTC material were similar to those of Example 1 (E1), except that the crosslinking operation by irradiation was implemented before the soldering operation.
Six PTC materials, having different PTC polymer compositions (F1-F6) listed in Table 1, for Comparative Example 2 were prepared. The procedures and operating conditions for preparing each PTC material were similar to those of Example 3 (E3), except that the crosslinking operation by irradiation was implemented before the soldering operation.
Table 2 shows the measured resistances and the resistance change in percentage (R %) of each PTC material for Comparative Examples (CE1-CE2) and Examples (E1-E4). The measured resistance of each PTC material in Table 2 is an average value of measured resistances of ten specimens obtained from the PTC material. The resistance change in percentage (R %) is defined as (R1/R0)×100%, wherein R0 and R1 represent the initial resistances of the laminate (before soldering) and the PTC device (after soldering) of each PTC material, respectively.
From the results shown in Table 2, the resistance changes of Examples (E1-E4) in percentage are much lower than Comparative Examples CE1 and CE2 under the same polymer composition or formulation. Moreover, since formation of the PTC devices of Examples 3 and 4 (E3-E4) involves the use of the hot pressing machine 6 during soldering operation, a uniform heating of the crosslinkable preform 2 can be achieved through the heating and pressing of two metallic plates 61 of the hot pressing machine 6 (see
Table 3 shows the PTC effect test results for the PTC devices for Comparative Examples (CE1˜CE2) and Examples (E1˜E4). The measured resistance of each PTC material is an average value of measured resistances of ten specimens obtained from the PTC material. In the test, each PTC material was placed in a hot air oven, and was heated from 20 to 200° C. under a heating rate of 2° C./min. The measured resistances at 140° C. and 170° C. (see Table 3) were recorded using a data acquisition instrument (Agilent 34970A) with a scanning rate of 1 time/sec. A positive value of the resistance difference R170-R140 shown in Table 3 is an indication that the PTC device has the PTC effect at the temperature range, while a negative value of the resistance difference R170-R140 is an indication that the PTC device does not have or lost the PTC effect at the temperature range. In addition, the magnitude of the resistance difference R170-R140 must be sufficient to provide the PTC effect.
From the results shown in Table 3, Examples (E1-E4) exhibit good PTC effect at the temperature range. Although the formulations F4-F6 of Comparative Example 1 and the formulations F4-F5 of Comparative Example 2 have positive values of the resistance difference R170-R140, the magnitudes thereof are insufficient for providing the PTC effect at the temperature range.
Table 4 shows the cycle test results under DC voltage for Comparative Examples (CE1˜CE2) and Examples (E1˜E4). The measured resistance change in percentage (R %) of each PTC material in Table 4 is an average value of ten specimens obtained from the PTC material. The cycle test was conducted according to the endurance test of UL1434 (having test conditions: 20 VDC, 100 A, 100 cycles, each cycle including a power-on operation for 1 minute and a power-off operation for 1 minute).
The resistance change in percentage (R %) shown in Table 4 is defined as (R100/R1)×100%, wherein R1 and R100 represent resistances measured at initial and the 100th cycle for the PTC material of the PTC device, respectively.
From the results shown in Table 4, all of the PTC materials of Examples E1-E4 passed the cycle test under DC voltage, while not all of the samples of Comparative Examples CE1 and CE2 passed the cycle test.
Table 5 shows the cycle test results under AC voltage for Comparative Examples (CE1˜CE2) and Examples (E1˜E4). The measured resistance change in percentage of each PTC material in Table 5 is an average value of ten specimens obtained from the PTC material. The cycle test shown in Table 5 was conducted according to the endurance test of UL1434 (having test conditions: 30 Vrms, 10 A, 50 cycles, each cycle including a power-on operation for 1 minute and a power-off operation for 1 minute).
The resistance change in percentage (R %) shown in Table 5 is defined as (R50/R1)×100%, wherein R1 and R50 represent resistances measured at initial and the 50th cycle for the PTC material of the PTC device, respectively.
From the results shown in Table 5, all of the PTC materials of Examples E1-E4 passed the cycle test under AC voltage, while none of the PTC materials of Comparative Examples CE1 and CE2 passed the cycle test.
Table 6 shows the thermal runaway test results for Comparative Examples (CE1˜CE2) and Examples (E1˜E4). The failure voltage of each Example or Comparative Example in Table 6 is an average voltage of ten specimens. The thermal runaway test was conducted according to the thermal runaway test of UL1434 (having test conditions: the applied voltage being increased stepwise from an initial voltage of 10 VDC to a final voltage of 90 VDC under a fixed current of 5 A sufficient to cause the test specimen to trip at the initial applied voltage, in which the applied voltage is raised at increments of 10 VDC per step, the time interval between two steps is two minutes, and the time interval at each step is two minutes).
From the results shown in Table 6, most of the PTC materials of Examples E1-E4 passed the thermal runaway test, while none of the PTC materials of Comparative Examples CE1 and CE2 passed the test.
Similar to Table 2, Table 7 shows the measured resistances and the resistance change in percentage (R %) of each PTC material for Examples E5-E8.
It is found from the results of Examples 2 and 4 (E2 and E4) shown in Table 2 and the results of Examples 5-8 (E5-E8) shown in Table 7 (note that no pressure was applied to the assembly during soldering for preparation of the PTC device of E2, while the pressure P applied to the assemblies of E4-E8 was 50 psi, 10 psi, 30 psi, 70 psi, and 100 psi, respectively) that the PTC device can achieve a lower resistance when a suitable pressure, ranging from 10-50 psi, is applied to the assembly during the soldering operation, and that the resistance of the PTC device is significantly increased when the pressure applied to the assembly is higher than 50 psi.
In conclusion, by crosslinking the crosslinkable preform 2 after soldering the conductive leads 3 to the electrodes 4 on the crosslinkable preform 2 in the method of this invention for making the PTC device, the PTC device is able to have a lower and stable resistance, a lower power consumption during the use thereof, and a high production yield.
While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.
