Screening and Reliability Analysis
Release time:
2014-06-04
Several issues that the entrusted screening party must pay attention to: 1. Not all devices can introduce the screening items specified in the national military standard GJB597A. For example, epoxy encapsulated devices, plastic sealed devices, or other non-cavity non-gas-tight packaged components are not subject to PIND and leak testing according to GJB597A regulations. Parallel sealed welding devices or some other components with thinner shell walls cannot undergo constant acceleration tests because testing under specified acceleration conditions can easily cause deformation of the device shell, increasing the possibility of short circuits at internal bonding point cover plates or gas leaks from the shell. Programmable devices that can only be written once cannot undergo programming/erasing verification.
Several issues that the entrusted screening party must pay attention to:
1. Not all devices can introduce the screening items specified in the national military standard GJB597A. For example, epoxy encapsulated devices, plastic sealed devices, or other non-cavity non-gas-tight packaged components are not subject to PIND and leak testing according to GJB597A.
Parallel sealed welding devices or other components with relatively thin shell walls cannot undergo constant acceleration tests because testing under specified acceleration conditions can easily cause deformation of the device shell, increasing the possibility of short circuits at internal bond point cover plates or gas leaks from the shell.
One-time programmable devices cannot undergo programming/erasure verification tests.
Fuse structures such as fuses and circuit breakers cannot undergo current tolerance withstand voltage tests.
2. Cavity devices that pass three-temperature testing do not guarantee reliable operation across the full temperature range.
Three-temperature testing refers to measuring various electrical parameters of a device at three temperature points: -55°C, +25°C, and +125°C.
In practice, it is occasionally encountered that some devices meet specification values for three-temperature electrical parameters but exhibit abnormal operation near 0°C during actual use. Further measurement and analysis of moisture content may reveal that this is due to gas leakage or excessive moisture content in the inherent atmosphere inside the device. The distance between adjacent bonding points on integrated circuit chips is usually ≤100um. At -55°C test points, moisture condenses into ice or frost and does not conduct electricity. However, near 0°C, which is close to the 'dew point' (the 'dew point' varies for different devices), moisture turns into water droplets, which can reduce electrical insulation between adjacent bonding points or even cause short circuits, leading to abnormal device operation. Once temperatures exceed the 'dew point', water droplets revert to vapor and normal device operation resumes.
This situation arises from non-standard packaging processes by component manufacturers and high humidity in the atmosphere; however,
conventional testing may not detect it. Therefore, it is recommended that customers use components in important situations; it is best to randomly sample a few (as specified based on batch quantity) from each batch after secondary screening for moisture content testing at a qualified unit. Components with internal moisture content ≥15000ppm should be used with caution. Components exceeding 20000ppm must not be used.
3. Components that pass leak detection and PIND tests are not necessarily qualified after assembly.
In practice, there are occasional complaints from clients who believe that components deemed qualified by your unit through leak detection and PIND tests have issues during use; when sent for third-party testing after failure, leaks and excess materials are found inside.
This situation should be analyzed objectively and factually. There may indeed be misjudgments during secondary screening (e.g., due to lack of responsibility from operators or equipment exceeding measurement validity without timely calibration), but for any relatively standardized testing unit, the probability of misjudgment is very low.
Therefore, it is recommended that system users pay more attention to the following points:
(a) Soldering temperature and time:
Most manufacturers specify maximum soldering temperatures and longest soldering times for external pins of their devices in their manuals under 'Absolute Maximum Ratings'. Some manufacturers also specifically state requirements for soldering methods, clarifying suitability and risk assessments for manual soldering/wave soldering/reflow soldering.
Soldering beyond maximum temperature and longest time can easily cause cracking or damage to device shells (ceramic packaging) or low-temperature glass insulators (metal shells), leading to gas leaks or excess materials being produced. This is especially true for SO/LCCC ceramic micro-packaged devices.
In practice, when conducting failure analysis on a device if corrosion of internal leads or aluminum oxidation in bonding areas is found, one should first consider gas leakage as a cause allowing oxygen and moisture into the device's interior.
(If mass spectrometry analysis shows a significant proportion of tin and lead inside the device indicates that there has been a 'large leak' in its shell allowing solder material direct entry into its cavity!)
(c) Power dissipation capability of component shells: The power dissipation capabilities (Power Dissipation) for different packaging forms are specified under 'Absolute Maximum Ratings' in product manuals.)
(4) For components with high static power consumption or power components results given by computer-aided testers may differ from actual values.)
(Of course different components have varying warm-up durations so not all results provided by computer-aided testers are incorrect.)
(In practice a pragmatic approach involves creating some manual test stations performing secondary measurements on key parameters for high static power consumption/power components; if measured parameters differ significantly from those provided by computer-aided testers then manual test station results should take precedence.)
在＋125℃下工作一定要降额使用，尤其对一些线性功率驱动器／放大器，线性调整电源、效率≤60%的DC／DC电源等。脉冲功率过高或长期使用在内部热功耗超过器件外壳能够承载的功率耗散能力，或者没有关注不同封装形式功率耗散能力的差异，均容易造成器件漏气。因为器件外壳的热膨胀或收缩是有极限的。超过极限会对器件的性能或长期可靠性造成不利影响。
⒋对于静态功耗较大的器件或功率器件，计算机辅助测试仪所给出的测量结果有可能与实际值不符。
这两类器件（如OPA544、541、2541、LM143、78H05 LM137K、LM117K、一些DC／DC电源等）都存在一个加电后预热（Warm UP）问题。在这个预热过程没有结束前，这些器件的各种测量参数是不真实的。而计算机辅助测试仪对各类器件的参数测量恰好是在ms级时间间隔内完成的，所以不等效。
当然，不同器件的预热过程的长短各有差异。因此也不是所有计算机辅助测试仪给出的结果都不正确。
值得关注的另一个问题是，通常计算机辅助测试仪对被测器件施加安培级电流是做不到的。那么它测出的结果不能反映某个器件在较大工作电流条件下的真实参数。
实践中，一个较为务实的做法就是自制一部分手动测试台，对静态功耗较大的器件或功率器件的部分关键参数进行二次测量。这个过程测出的部分参数若与计算机辅助测试仪给出的数据不吻合时，则要以手动测试台的结果为准。
The appearance and solderability of the devices after secondary screening may deteriorate.
During the secondary screening process, when testing the devices, it is necessary to place the tested device into an adapter, which can easily cause marks of pressure or insertion/removal on the external pins of the tested device.
After high-temperature aging of the devices, especially for external pins made of nickel-chromium alloy or palladium-silver structure, oxidation may occur, affecting solderability.
The leak detection process requires placing the first and second batches of devices in a fluorinated oil pressure tank at a specified pressure for a designated time. It is difficult to avoid one device's external pins touching or scratching another device's surface, leading to deterioration in appearance. If considering placing devices parallel in the fluorinated oil pressure tank, it would result in low efficiency, extended screening cycles, and increased screening costs; thus, it is hard to completely avoid this issue. The client should consider these factors comprehensively during secondary screening.
Determination of secondary screening conditions:
According to the consensus formed in the mid-1990s regarding reliability standards, electronic components' reliability is divided into inherent reliability and operational reliability. Inherent reliability is determined by the design and process level of the device. For designed and finalized devices, inherent reliability is determined by manufacturing processes. The manufacturing process is influenced by various factors such as materials, process environment, and conditions; therefore, it is impossible to manufacture 100% qualified devices. Additionally, some defects in devices can only be eliminated by applying certain environmental stresses and electrical stresses. Therefore, military product manufacturers conduct extensive screening tests on their military products before leaving the factory. By applying certain levels of environmental stress and electrical stress to eliminate early failure devices and assess whether component quality meets corresponding military standards. These screening activities conducted by manufacturers are commonly referred to as primary screening.
For military electronic components, primary screening is crucial; most costs for military components come from primary screening. Any type of component packaging form that has not undergone primary screening cannot be classified as a military component but only as a military-grade component. Its inherent reliability cannot be guaranteed.
Therefore, primary screening is a production procedure that any manufacturer must carry out. This also explains why qualified product preferred lists (QPL) defined in MIL-M-38510 transition to qualified manufacturer lists (QML) defined in MIL-M-38535.
It should be noted that primary screening does not affect the lifespan or operational reliability of military components.
Secondary screening should fall under operational reliability.
Secondary screening refers to activities commissioned by users before assembling devices due to concerns about procurement time for that batch of components, procurement channel issues, storage environment issues, transportation issues etc., which verify results from primary screenings conducted by third parties to ensure or improve the reliability of installed components. Typically, secondary screenings should not exceed either environmental stress conditions or electrical stress conditions set during primary screenings; otherwise it could adversely affect operational reliability.
In light of these circumstances, when providing secondary screening conditions, clients cannot simply replicate original manufacturers' primary screening conditions nor exceed those conditions. Stricter screenings or additional condition screenings should also consider the quality grade of screened components. For example, NASA has defined three risk levels for critical applications involving components and indicated that additional screenings can improve risk levels. Components with a quality grade at level III can be upgraded through stricter screenings to level II usage; level II can be upgraded to II+ usage but cannot be upgraded to level I usage. This point is very important.
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