REG NASA-LLIS-0460-1996 Lessons Learned - Glass Cased Components Fractured as a Result of Shrinkage in Coating Bonding Materials Applied in Excessive Amounts (1963).pdf

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1、Lessons Learned Entry: 0460Lesson Info:a71 Lesson Number: 0460a71 Lesson Date: 1996-10-3a71 Submitting Organization: JPLa71 Submitted by: J.W. BottSubject: Glass Cased Components Fractured as a Result of Shrinkage in Coating/Bonding Materials Applied in Excessive Amounts (1963) Abstract: Shrinkage d

2、uring the curing of hard set, epoxy based, conformal coating caused cracked glass cases in many axial lead diodes in Ranger and Mariner flight computers. The lesson recommended investigating coating and bonding material properties, avoiding excessive amounts of coating, and performing post-cure insp

3、ection.Description of Driving Event: Microscopic inspection under 10X magnification of a completed Ranger Central Computer and Sequencer (CC&S) Subsystem revealed a relatively large number of cracked glass cases in axial lead diodes. The fractures frequently extended lengthwise along the glass case

4、from lead entry to lead exit at the opposite end. The fractures usually occur at the edge of heavy accumulations of hard set epoxy based conformal coating and/or spot bonding materials used to secure the components to substrates. The problem was found to re-occur with several similar types of coatin

5、g/bonding materials then and still available. This problem necessitated the development of a new process to soften the coating/bonding material, and stripping the material away from hundreds of glass diodes. A large number of fractured diodes were replaced. The problem was also evident in other subs

6、ystems which had been built prior to discovery of this problem.Subsequent investigations into other applications where these materials were used revealed the existence of cracked glass diode cases in a Mariner Attitude Control (AC) Subsystem under construction at another contractor.The source of the

7、 cracked glass problem was traced to an .8% Shrinkage factor during the cure cycle in the coating/bonding material, which also had very good adhesive properties. Contraction in a Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-heavy application of th

8、e hard set coating or spot bonding material which enclosed 50% or more of the diode case diameter literally pulled the glass case apart. Those which do not fracture during the cure cycle may be assumed to be severely stressed, and possibly subject to failure under subsequent environmental exposure,

9、including acceptance testing and flight exposure.Lesson(s) Learned: 1. Unnecessary or excessive application of materials used for coating or spot bonding may have unexpected side effects.2. Materials used for coating and spot bonding applications may have unexpected side effects. Thermosetting epoxi

10、es and others which cure to a hard, inelastic consistency may have destructive side effects, i.e. shrinkage, outgassing, unusual viscosity characteristics, tacky surfaces, etc.3. Some coating/bonding materials go through a low viscosity watery state during the cure cycle, which can lead to slumping

11、of the material, allowing it to run into unintended locations where its presence was not anticipated.Recommendation(s): 1. Investigate the properties of any coating or bonding materials being considered for use with respect to vulnerabilities in the intended application, particularly with respect to

12、 new materials for which there is no prior use history.2. Avoid the use of excessive amounts of coating or bonding materials, and specify the thickness and configuration to be used for applications.3. Perform a post-cure inspection of the coating/bonding material to verify thickness and configuratio

13、n.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): N/AAdditional Key Phrase(s): Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Parts Materials & ProcessesAdditional Info: Approval Info: a71 Approval Date: 1996-10-11a71 Approval Name: Carol Dumaina71 Approval Organization: 125-204a71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-