End-use environment

Construction of test samples that can be subjected to actual exposure to the end-use environments which are tested to destruction to determine the possible modes of failure and the conditions that cause the failure. 

Plastics. Part of the trend to substitute plastic and composite substrates for metals can be attributed to a desire to avoid the process of metallic corrosion and subsequent failure. Relatively little attention has been called to the possible failure modes of plastics under environments considered corrosive to metals. More extensive work should be conducted on the durability and life expectancy of plastic and composite materials under end-use environments. A further consideration is the potential for polymer degradation by the products of metal corrosion in hybrid structures comprising metal and polymer components. Since it is expected that coatings will continue to be used to protect plastic and composite substrates, ancillary programs need to be conducted on the mechanisms by which coatings can protect such substrates. 

The measurement of durability in a given system under conditions of illumination (spectral distribution and intensities), mimicking closely the intended end-use environment, and as determined by changes in appearance and/or physical properties is the ultimate measure of successful formulation in systems requiring photostable behavior. Although many laboratory and field methods exist for the direct measure of photoactivity, these often do not sufficiently reflect the performance that will be experienced in the actual end use. 

This book puts a wealth of information at your fingertips. It represents an exhaustive compilation of data on how chemicals and other exposure environments affect the properties and characteristics of thermoplastic materials. How a plastic will perform in its end use environment is a critical consideration and the information presented here gives useful guidelines. However, this or any other information resource should not serve as a substitute for actual testing in determining the applicability of a given part or material in a given end use environment. 

In materials science, the materials properties depend not only on composition, but also on morphology, microstructure, and other parameters related to the material-preparation conditions and on the end-use environment. As a result of this complexity,... 

Detailed discussion of large-scale fire tests is outside the scope of this book, and those requiring any such fire tests to be carried out are advised to get in touch with specialist organizations. A useful appraisal of large-scale tests and especially on the need carefully to design the tests to meet the end use environment, and also to establish the limitations of such tests, is given in BS 6336 [6]. 

Resistance to moisture, chemicals, and oils Service conditions and assembly configuration often play a part in total resistance to end use environment, but polychloroprene exhibits excellent performance for most conditions. Constant immersion in any of these materials is not recommended. 

Temperature or humidity fluctuations can be accelerated only to the point of maintaining uniform penetration that is likely in the end use environment. If creep or vibration is expected in service, time-temperature superposition may often be applied to accelerate laboratory testing. This technique mathematically predicts the material s response in service, based on laboratory characterization of the material over a... 

Since the effect of temperature varies with type of polymer aud its formulations, temperatures different than those encountered in end-use environments can distort the stability rankings of materials in addition to causing unrealistic aging behavior. For example, it was shown [141] that change in air temperature from 30 to 60°C in an artilicial weathering test changed the rank order of the stabilities of polyamide, polypropylene, and polyester yam based on reduction... 

Modified PPE resins will soften or dissolve in many halogenated and aromatic hydrocarbons. Laboratory data are available on the chemical resistance of plastics. However, such data should only be used as a screening tool. If a material is found to be incompatible in a short-term test, it wiU usually be found to be incompatible in a similar end-use environment. The converse, however, is not always true. Favorable results in a short-term test are no guarantee of actual performance in long-term, end-use conditions. The amount of molded-in stress found in any particular part wiU have a pronounced effect upon the relative chemical compatibility of a polymer. The acceptable chemical compatibility of a polymer in an application can be determined only by exposure or immersion of prototypes and suitably stressed samples in this type of environment under actual operating conditions [21]. 

Design calculations used with polycarbonate resin are no different from those for any other thermoplastic material. Physical properties of all thermoplastics are dependent on the expected temperature and stress levels as defined by the application s end-use environment. Standard engineering calculations can be used to predict part performance for polycarbonates. It is important to take into account the notch sensitivity inherent to... 

The ductile-brittle transition temperature of each part molded of polycarbonate resin may indicate the overall robustness of that part and may give indications as to the longevity of the part in the end-use environment. The lower the ductile-brittle transition temperature, the more robustly the component may perform and the longer the part may last, assuming proper design and processing. 

Thermal history, also known as heat history, describes the memory effect in polymers. In simple terms, polymers are affected by hot or cold temperatures and remember the last effect. This is considered a variable that should be controlled in the laboratory. Thermal history has a strong influence on the material s glass transition, melting, and crystallization temperatures. Temperature changes, both hot and cold temperatures, induce thermal stresses in the material. The extent and implication of thermal stresses to a polymer are usually unknown and uncontrolled. This variable may be of interest for those who want to know more about the sample as it was received in the laboratory or how it reacts to an uncontrolled end-use environment. Usually this is of interest, but because it is uncontrolled it is very difficult to rely on for material comparisons. 

In this chapter we are concerned with how the designer discharges this responsibility. The effect of the use environment will be discussed as well as the effect that it can have on the performance of plastic parts. The design of suitable tests to determine if the part will perform in the end-use environment will be covered, as well as the cost of such tests. The test data will be reviewed to determine the... 

The next section covers the effect of the end-use environment on product performance so as to permit the design of a product for a reasonable lifetime with realistic economics. Included is the design engineer s involvement in testing and evaluating, as well as a definition of the specific function of the part, to prevent its abuse. The involvement of the product designer is such that his responsibility extends to having the product serve the end-user effectively, and the text indicates the extent and means whereby he can do this. 

Perhaps most important of all, polymer chemists now understand in great detail how to use functional ingredients such as cross-linking agents and other specialised monomers to manipulate the polymer architecture in order to satisfy virtually all the needs of a very wide range of end-use environments. 

A further component that can be varied, to take into account the requirement or specific needs of the end-use environment, is that of the continuous phase oil. For products to be used as thickeners in general industrial operations, such as Improved Oil Recovery or Print Paste formulations for textiles, this phase is usually based on a mineral oil hydrocarbon. In products developed for more specialised or demanding end uses, such as cosmetic and toiletry formulations, special grades are available where the continuous phase is a medicinal-grade high-purity white oil. Hydrophobic esters and even silicone-based fluids have also been used as the continuous oil phase. 

However, it is worthwhile considering some of the underlying principles on which the science and measurement of rheology is based, in order to understand how the different polymer types discussed in this chapter behave in end-use environments. 

The rotational instruments already discussed do not simulate this sort of condition. Nonetheless, a number of end-use environments exist where extensional flow is the predominant effect. Examples of this are listed below ... 

Fill Materials. Since the fill material is an additional fabrication material that becomes a part of the design construction, procurement documentation is required to specify a fill material type and thereby implement the via fill process. The selection and documentation of the fill material require the same consideration as the base laminate preference. This is especially critical when targeting a lead-free-compatible process. Currently, an industry-based material specification for via fill material does not exist.Therefore, specific fill-material brands may be named on the drawing, or some other form of user/supplier agreement must be established.The fabricator has preferences for the type of material used for via fill. Just as suppliers often have preferences for a specific solder mask brand, they also often prefer to use of a specific via fill material around which they have developed their principal processes. Supplier preferences can be driven by specific via fill material characteristics, such as accessibility, equipment compatibility, process supportability, plateability, and/or shelf/pot life. This may complicate source selection, or it might influence the use of a dedicated service center for the hole-fill process. The fabricator may not always know the reliability of its preferred material for a given via structure or end-use environment. 

Functionality through operational life should be the ultimate criteria for acceptance. To account for the fact that a PCB will see different end-use environments and conditions, acceptance criteria are typically broken into three classes of service. Each company must estabUsh the level of acceptance as it is dependent on the functional criteria to which the PCB will be subjected. The IPC has established recommended guidehnes for acceptance, based on end use, categorized as Classes 1,2, and 3.The following subsections describe the classes. 

CLASS 2 Dedicated Service Electronic Prodncts Class 2 includes products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical. Typically the end-use environment would not cause failures. CLASS 3 High-Perforniance Electronic Prodncts These include products where continued high performance or performance-on-demand is critical, equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required. Examples include life support or other critical systems. 

Defect This condition may be insnfficient to ensure the form, fit, or function of the assembly in its end-use environment. Defect conditions need to be dispositioned by the manufacturer based on design, service, and customer requirements. Disposition may be to rework, repair, scrap, or use as is. Repairing or using the assembly as is may require customer concurrence. A defect for Class 1 automatically implies a defect for Class 2 and 3. A defect for Class 2 implies a defect for Class 3. 

Computer room environments represent the other extreme, with temperatures and hnmid-ity controlled to very tight tolerances. While servers, switches, hubs, routers, and other devices are housed in a controlled environment, their extremely large packages, coupled with power and mini cycles (discussed in the next chapter), create a set of loading conditions that are different from those found in automotive applications, bnt that are equally challenging. Worst-case loading conditions for a variety of end-use environments can be found in Table 58.1. Note the broad variation in temperature extremes, frequency of thermal cycles, and expected service life. 

End-use conditions can be estimated by attaching thermocouples, humidity, and shock/vibration sensors in the final product because it is used in field operating conditions. This data needs to be analyzed over time and different expected field conditions to get useful estimates of the end-use environment as illustrated in Table 58.1. 

TABLE 59.6 Examples of Typical Shock Conditions Used for Different End-Use Environments (the Specific Conditions Tested Depend on the End Users)... 

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