APPLICATION NOTE

Plastic Material Properties

ANE018 BY GERALDINE MORISSON

01. INTRODUCTION

It has been observed that the natural color of halogen-free polyamide (PA) plastic used in Würth Elektronik connectors turns brown during the SMT process (Reflow soldering). The aim of this technical note is to understand this phenomenon and demonstrate whether and how it may impact the plastic material properties of our connectors. This was achieved by conducting spectroscopy IR and differential scanning calorimetry tests.

02. PHENOMENON/EXPLANATION

Halogen-free polyamides are a group of polymers that are characterized by a repeating amide bond (Figure 1). Polyphthalamide (PPA) ranks among the high performance polymers. The repeating unit in the polymer chain is composed of a combination of terephthalic (aromatic) and methylendiamine: PAnT [1].

The color change is a consequence of the thermos-oxidation when the product is exposed to high temperature in an oxygen environment (initiation phase). This is characteristic of halogen-free PA materials of natural color. The degradation process depends on the nature and the structure of the macromolecular chain, as well as on the chemical nature of the branched groups. As regards to polyamides, oxidation occurs by hydroperoxides (ROOH) within a short-chain mechanism.

The thermo-oxidation of a hydrocarbon polymer results from the extraction of an H atom because of temperature. The resulting radical (R•) reacts very quickly with the oxygen (O₂) available to form a peroxy radical (ROO•). During the reaction process, the ROO• strips the hydrogen atom from a methylene (RH) and forms a hydroperoxide (ROOH). The hydroperoxide (ROOH) comes along with a radical (R•) and thus propagates the chain reaction process (Figure 2). This is greatly accelerated at higher temperatures.

Figure 2 : Cyclic propagation of ROOH hydroperoxides.

Oxygen absorption starts after a certain time (induction time). It is followed by an accelerated rise in concentration of the peroxy groups. When the peak is reached (autocatalytic stage), this is followed by the depletion of peroxy groups (termination stage). The temperature and diffusion coefficient of oxygen determines the rate of the oxygen absorption while the induction time depends on the polymer nature, structure and density. The oxygen absorption is also proportional to the polymer open surface.

The absorbed oxygen mass in the thermal oxidation process is inversely proportional to the crystallinity degree of the polymer. The oxidation process starts within its amorphous phase or interfibrilar areas. A branched polymer offers less resistance to oxidation than a linear polymer.

03. TESTS RESULTS

In order to observe this phenomenon, we tested the behavior of two polyamide products under reflow soldering at 260 °C. The plastic surface of the housing turns brown (Figure 3 and Figure 4).

Figure 3 : ZIF connector 6861xx148922 before and after reflow

Figure 4 : LECO Connector 695402400222 before and after reflow

3.1 Contact resistance analysis

Contact resistance measurements carried out after reflow soldering indicated that they comply with the values stated in the technical data sheets (Table 1), i.e. less than 50 mΩ per contact depending on the products.

Table 1: Contact resistance of connectors stated in the data sheet and measured after reflow.

3.2 Amide group analysis

In order to identify the surface oxidation phenomenon, our affected products were tested by spectroscopy before and after the reflow 260 °C process of a ZIF connector 6861xx148922 (PA6T) (Figure 5) and LECO connector 695402400222 (PA9T) (Figure 6). Spectroscopy is a fundamental exploratory tool in the field of physical chemistry that enables the identification of the elements and compounds of materials.

Figure 5: Spectroscopic analysis of amide functional groups (ZIF – PA6T)

Figure 6: Spectroscopic analysis of amide functional groups (LECO – PA9T)

The position of the peaks shows no difference before and after reflow. As a conclusion, the color browning neither altered the plastic material composition or structure, and therefore its properties. However, looking at each material independently, there is too much interference between the groups to interpret the absorbance rate before and after reflow. To interpret the graph, the importance is the position of the peaks (absorption wavelength) and not their heights (absorption value).

3.3 Phase change analysis of polyamides

Differential Scanning Calorimetry (DSC) is a plastic material characterization method. DSC is a thermal analysis technique used to measure the differences in heat exchanges between the analyzed sample and a reference product during a physical transformation, such as the glass transition temperature (the material changes from a glassy, solid, rigid state a rubbery state) represented by the green curve of the thermogram, and the melting temperature (the material moves from solid to liquid) represented by the red curve of the thermogram.

3.4 Melting Point

When conducting the DSC tests, the melting temperature for the samples were also recorded. Three measurements were taken for each part: an initial sample without any previous heating process, a sample after a 260 °C reflow profile and a sample after a 260 °C DSC profile under a N2 atmosphere, which simulates the reflow profile without oxygen.

We could observe the melting point under the second phase (red curve of the thermogram), which corresponds to the rubbery state and reaches the melting point (passage from solid state to liquid state).

When the temperature drops, the sample becomes more rubbery (blue curve) then solid again (khaki curve).

6861xx148922: for PA6T, the theoretical value is 295 °C.

Table 2: Measured Peak – ZIF 6861xx148922

Figure 7 : ZIF initial

Figure 8: ZIF after reflow 260°C

Figure 9: ZIF after DSC 260°C

695402400222: for PA9T, the theoretical value is 305°C.

Table 3: Thermograms show two melting temperature peaks, which is characteristic of PA9T

Figure 10 : LECO initial

Figure 11 : LECO after reflow 260°C

Figure 12 : LECO after DSC 260°C

On each of these curves, the melting temperature remains the same before and after soldering processes. This means that heating processes do not cause any change in material or any damage of the plastic material.

3.5 Surface Oxidation

In order to prevent the surface oxidation of a halogen-free polyamide material, there must be no oxygen present when exposed to high temperature. Reflow oven conditions without the presence of oxygen were simulated using the DSC test equipment. As opposed to the reflow atmosphere, DSC analyses are performed with an inert gas (nitrogen) atmosphere in order to prevent any material reaction with the atmosphere.

Figure 13 : On the left, a portion of a ZIF 6861xx148922 heated in the DSC to 260°C with Nitrogen (N₂) vs. Reflow at 260°C with O₂

Figure 14 : On the left, a portion of a LECO 695402400222 heated in the DSC to 260°C with Nitrogen (N₂) vs. Reflow at 260°C with O₂

We can observe that a nitrogen atmosphere prevents the surface oxidation of the plastic material during high temperature processes.

04. CONCLUSION

Halogen-free PA of natural color turns brown when exposed to high temperature in an oxygen atmosphere in order to solder connectors on printed circuits when using reflow soldering, for example. However, the tests conducted confirm that the solder joints, as well as the plastic properties, remain compliant. Only the top layer of the polymer has absorbed oxygen which is only an aesthetic change but it does not impact the product specifications, performance or behavior. Indeed, spectroscopic and DSC tests do not show any change in the physical or molecular structure of the plastic after the soldering process.

In addition, this surface oxidation phenomenon has been taken into account during the product design and qualification. Our electrical and mechanical tests were conducted after soldering of the connectors on printed circuits, that is, after the surface oxidation phenomenon, and under operating conditions of the customer applications. The successful test results demonstrate that the products remain compliant even with the color browning. If the aesthetic of the product is important to the design, surface oxidation can be prevented by performing the reflow soldering process under a nitrogen atmosphere.

This note is not relevant for white colored and halogenated PA plastic products.

05. BOM

A.1 References

[1] Sorin ILIE; Radu SENETSCU/TE-VSC/Cern/Polymeric Materials Review on Oxidation, Stabilization and Evaluation using CL and DSC Methods

IMPORTANT NOTICE

The Application Note is based on our knowledge and experience of typical requirements concerning these areas. It serves as general guidance and should not be construed as a commitment for the suitability for customer applications by Würth Elektronik eiSos GmbH & Co. KG. The information in the Application Note is subject to change without notice. This document and parts thereof must not be reproduced or copied without written permission, and contents thereof must not be imparted to a third party nor be used for any unauthorized purpose. Würth Elektronik eiSos GmbH & Co. KG and its subsidiaries and affiliates (WE) are not liable for application assistance of any kind. Customers may use WE’s assistance and product recommendations for their applications and design. The responsibility for the applicability and use of WE Products in a particular customer design is always solely within the authority of the customer. Due to this fact it is up to the customer to evaluate and investigate, where appropriate, and decide whether the device with the specific product characteristics described in the product specification is valid and suitable for the respective customer application or not. The technical specifications are stated in the current data sheet of the products. Therefore the customers shall use the data sheets and are cautioned to verify that data sheets are current. The current data sheets can be downloaded at www.we-online.com. Customers shall strictly observe any product-specific notes, cautions and warnings. WE reserves the right to make corrections, modifications , enhancements, improvements, and other changes to its products and services. WE DOES NOT WARRANT OR REPRESENT THAT ANY LICENSE, EITHER EXPRESS OR IMPLIED, IS GRANTED UNDER ANY PATENT RIGHT, COPYRIGHT, MASK WORK RIGHT, OR OTHER INTELLECTUAL PROPERTY RIGHT RELATING TO ANY COMBINATION, MACHINE, OR PROCESS IN WHICH WE PRODUCTS OR SERVICES ARE USED. INFORMATION PUBLISHED BY WE REGARDING THIRD-PARTY PRODUCTS OR SERVICES DOES NOT CONSTITUTE A LICENSE FROM WE TO USE SUCH PRODUCTS OR SERVICES OR A WARRANTY OR ENDORSEMENT THEREOF. WE products are not authorized for use in safety-critical applications , or where a failure of the product is reasonably expected to cause severe personal injury or death. Moreover, WE products are neither designed nor intended for use in areas such as military, aerospace, aviation, nuclear control, submarine, transportation (automotive control, train control, ship control), transportation signal, disaster prevention, medical, public information network etc. Customers shall inform WE about the intent of such usage before design-in stage. In certain customer applications requiring a very high level of safety and in which the malfunction or failure of an electronic component could endanger human life or health, customers must ensure that they have all necessary expertise in the safety and regulatory ramifications of their applications. Customers acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of WE products in such safety-critical applications, notwithstanding any applications-related information or support that may be provided by WE. CUSTOMERS SHALL INDEMNIFY WE AGAINST ANY DAMAGES ARISING OUT OF THE USE OF WE PRODUCTS IN SUCH SAFETYCRITICAL APPLICATIONS

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ANE018 | Plastic Material Properties

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