-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
In-field Assembly Failures — Expensive and Avoidable?
December 31, 1969 |Estimated reading time: 9 minutes
The interaction of cleaning with protective coating, together with weak-point analyses, all contribute to the fulfillment of long-term reliability requirements.
By Helmut Schweigart and Andreas Muehlbauer
More and more assemblies are being subjected to climatic stresses of increasing severity. The degree of cleanliness and the coating quality of assemblies are the decisive criteria for climatic reliability, which are established by ionic-equivalent measurements and various humidity and dew point tests. Their evaluations can be time-consuming and cost-intensive, yet as environment-simulation tests, they do not guarantee complete protection against in-field failure.
Industry Examples
The number of assemblies used in the automobile industry and in communications and sensor technology continuously increases. The functional reliability of these modules determines the overall reliability of the products that are incorporated therein. The circuits are exposed to climatic influences of increasing severity, particularly with regard to moisture and harmful gases. Also, miniaturization and the use of high-ohmic components render these assemblies more sensitive to environmentally-induced disruptions.
This situation contrasts with current warranty demands of 15 to 20 years at product-cycle times of five to seven years, particularly in the automotive industry. In addition to avoiding costly, image-damaging product recalls, there is the urgent need to focus on the prevention of expensive operating standstills, particularly in communications technology. Large-scale serial manufacture and the ever-growing trend toward company mergers throughout the world make it imperative to reveal assembly weaknesses as early as possible to avoid logistical problems. The effect has been to extend climatic reliability and environmental testing beyond the traditional realms of military and aviation applications to consumer-orientated markets. And as a guarantee in the age of ISO 9000 and QS 9000, keen visual inspection by experienced personnel is no longer sufficient: What is demanded is traceability to confirm guarantee claims.
Figure 1. The distribution of contaminants is made visible by charge contrast.
null
The Influences of Climatic Reliability
The climatic reliability of modules is determined in great part by the cleanliness of the circuits' surfaces and their screening against the influences of moisture. Contamination favors moisture absorption, and with it, electrochemical migration and corrosion-induced leakage currents. Because of their conductivity, most production-induced contamination reduces the assembly's surface resistance:
- Flux residues change the capacitance of through-connection contact areas, which primarily affects the signal integrity of high-density integrated and high-frequency circuits.
- Dust on non-conformally coated miniaturized circuits, in conjunction with moisture, can result in the formation of so-called dust dendrites, i.e., tree-like short-circuit bries.
- Contaminants can cause long-term delaminations, i.e., the peeling off of protective coatings. Both hygroscopic effects and hydrolytic processes impair the adhesion of coatings and the protection they typically provide.
Figure 2. Delamination of protective lacquer at a soldered point as a result of hygrosocopic contaminants and moisture.
null
Confirming Climatic Reliability
Climatic reliability testing of modules has become particularly important to overcome long-term process risks. Such procedures usually consist of a sequence of fitness-for-use tests, such as those for temperature changes, salt-spray mist, dusting, harmful gases, Tesa peel-off and simulated operation tests. Most incur cost-intensive testing of assemblies, while the testing times often are in the order of three to six months — durations that clash with the often-stipulated short development times for electronically controlled systems.
Climatic reliability testing commences with an assessment of the components to be processed. For instance, the ionic contamination of bare boards and parts is assessed by ANSI/J-Std 001A (formerly MIL-P-28809). Fluxes are released according to J-STD-004, IPC 2.6.13 and 14 or DIN Draft 32513, Part 5.6. These specifications are necessary but are not an imperative precondition for manufacture without cleaning processes.
Repeated attempts have been made to use ionic-equivalent tests to assess the climatic reliability of finished assemblies. However, while such tests are conditionally possible with cleaned assemblies, they furnish no information on contaminant distribution. This can be established only by preceding the ionic equivalent with nondestructive scanning electron microscopic (SEM) examinations plus a simple reaction test based on activator residues. Charge contrast makes the distribution of the contaminants visible (Figure 1). With non-cleaned assemblies, the ionic equivalent can be used only for statistical process control (SPC).
Figure 3. Critical reduction of the specific resistance through hygroscopic contaminants after 14 hours drying was carried out for two hours.
null
Climate Testing Methods
The resistance of finished assemblies to humid ambient conditions without dew point is established by the MIL STD 810 E test, which is conducted at 95%RH at 24°C for up to 120 days. Because it is essential that dew point be avoided in this test, these testing conditions correspond with the maximum operating load, thus explaining the long testing times. Further, because there is no acceleration resulting from a stringent test, the test furnishes information only on a limited scale.
Dew point conditions are stipulated by DIN ISO 6270 (continuous condensation) or by in-house standards. In-house standards, in particular, are based on experiences gained with in-field failures that are reproducible via selected test conditions during a short testing period.
In-situ climate testing of in-field modules, particularly those housed in cases or behind special covers, is gaining increasing importance. Temperature change and shock tests (e.g., MIL-STD-883 C, IEC 68-2-14, GMI 12558, IEC 68-2-30), with test products of adequate size, produce a brief dew point, depending on the test specimen's type and thickness and from the ambient humidity. However, this test primarily establishes a given assembly's mechanical stability.
Protective Coating Adhesion. Climate reliability is important in connection to the adhesion of protective coatings to assemblies. This is established by the Tesa peel-off or cross-cut adhesion test in conformity with DIN 58383. Such tests can be performed only on unpopulated surfaces. However, adhesion tends to fail at contaminated areas around solder joints (Figure 2). Hence, passing the Tesa test is a necessary, but insufficient, condition for adequate protective coating adhesion.
Corrosion Tests. Safety from harmful gases is established with mixed-gas tests, according to IEC 68-2-60, which are regarded as accelerated environment-simulation tests. A correlation to service life, however, is problematic because the time-accelerating factor is established by comparing the mass loss rate (or the corrosion-layer formation of test and service conditions) with the use of material probes, e.g., copper strips. However, they can provide only an insufficient simulation of the corrosion behavior of materials of assemblies, particularly hybrid materials. Also, field experience measurements with such material probes generally are lacking.
Corrosion spray tests (e.g., the salt-spray mist test as defined in DIN 53167) cannot predict the expected service life. This is because they are corrosion-stress processes, which generally are not related to service conditions. Because none of the spray tests provide adequate certainty on their own, they typically are combined. This explains the long testing times and the extensive testing equipment involved. Despite these elaborate procedures, most manufacturing errors remain undetected and can result in costly product recalls.
Figure 4. Adhesion strength of frequent lacquer systems (AQ = water-based system).
null
Protective Coating Demands
To avoid climate-induced failures, as those indicated in the tests discussed, assemblies generally are given a protective lacquer coating:
- Water-absorption tests on delamination-free, fully hardened coatings indicate that a technically relevant influence on the insulation resistance currently is possible only for hygroscopic type contamination (Figure 3). Next to the binder and the hardening parameters of time and temperature, the layer thickness of incompletely hardened coating systems also determines the water-absorption rate because thicker layers take longer to achieve the same hardening level.
- Tests for water-vapor permeability indicate a proportionality of permeation resistance in relation to the root of the coating thickness. Consequently, the improvement of the protective effect by increasing coating thickness does not appear to be suitable. This is because the lacquer's permeation resistance only rises by 30 percent due to increased coating thickness, while hardening time is adversely lengthened because of the local lacquer accumulation.
- Mechanical testing of incompletely hardened coatings results in an incomplete cohesion or material failure on EP-FR 4 (Figure 3). Most commercial polyurethane (PU) and acrylate (AY) coatings fail on tin, primarily due to adhesion. To ensure adequate climate protection through a covering of the free metal surface, lacquer adhesion strength must be higher than that of the coating (Figure 4). Insufficient hardening on substrates on which lacquers have good adhesion strength will result in a weakening of cohesion strength. This means that the already weak adhesion of most PU systems on tin/lead solders is decreased even further. In response, treating the solder surface with an appropriate cleaning agent can improve the poor PU lacquer adhesion on tin/lead (Figure 5).
In-service Experiences after Five No-clean Years. Typical corrosion-induced damage mechanisms result in exponentially distributed failure rates. Most of such failures occur during the first six months to one year after the device is first operated. The logarithmic normal distribution rates give an indication of the electro-diffusion-controlled mechanisms, which typically are generated, moisture-independent, by conductive contaminants. In these instances, the damage usually arises at an increasing failure rate after one year. The reliability of such an interpretation depends on the statistical quality and the disciplined, planned acquisition of data. However, for cost reasons, the failure data of several types in different application areas under differing climates often are collected jointly.
Unfortunately, statistical reports based on phenomenological and causal combinations cannot be analysed meaningfully. Accordingly, data on the distribution and concrete causes of climate-induced damage are unclassified. However, it is well known that tin-organic compounds impede hardening in the adhesion zone, particularly with silicon coatings. In addition to contaminants, peeling caused by long-term moisture endangers coat durability.
Damage Analysis Potential. Technical product malfunction cannot be avoided with absolute certainty. However, precise cause analysis of the damage often is a key means of failure prevention via corresponding optimization of design and manufacture together with detailed quality assessments through effective weak point testing. Appropriate cost-effective measures for damage prevention can be taken only when all causes and influencing factors are known.
Figure 5. How an improvement of lacquer adhesion can be achieved through primer cleaning.
null
Prevention Via Weak-point Analysis
Often, acquiring all operating conditions in practice is just as difficult as zero-fault manufacturing. Weak-point analysis based on worst-case considerations regarding operating and mounting conditions is necessary as a safeguard against in-service failure. Weak-point analysis also reveals a product's development potential with regard to consistent design while minimizing company liability. Foreseeable risks are tested automatically in keeping with the latest standards of science and technology and with the highest possible strictness.
Economic optimization requires the discussion and examination of the faults that arise on the basis of in-service and damage experience as well as in-service climate measurements.Appropriate testing level adaptation also permits a gradual adaptation of the weak-point tests. However, the degree of severity in weak-point analysis always will be higher than the service life tests based on tailoring. A flowing transition can be created through a step-by-step reduction of testing intensity, which will make it possible to establish the critical climate-limit values required to specify the permissible service conditions.
The water-stress test is a typical example of climate testing to establish, within a few hours, weak points, e.g., to the sensitivity of assemblies to humid climates without missing failures and at a favorable cost (Figure 3). The test's intensity ensures that failures are detected and that the failure mechanism of electrochemical migration is maximally accelerated, but without changing the mechanism. Moreover, appropriate evaluation will disclose irregularities that are not related directly to climatic reliability. However, a critical assessment is essential, particularly regarding corrosion products, to avoid false alarms. Weak-point analysis also furnishes a statement on the protective coating's purity and adequacy so that the question of climate reliability becomes a general yes/no decision.
Helmut Schweigart, Ph.D., and Andreas Muehlbauer, Ph.D., may be contacted at Zestron Corp., 21641 Beaumeade Circle, Suite 315, Ashburn, VA 20147; (888) 999-9116; Fax: (703) 821-9248; E-mail: h.schweigart@zestron.com; a.muehlbauer@zestron.com.