Are there guidelines or specifications for using silicone to secure large parts against vibration?
Are there guidelines or specifications for using silicone to secure large parts against vibration?
(OP)
On a lot of electronic assemblies, I see silicone, or even hot glue sometimes, used to secure large components against motion or vibration. I know that solder is weak structurally.
Some assemblies have silicone on darn near everything that has more than several millimeters of height. I've seen computer power supply teardown photos that look like that: Whole sections are buried in silicone. Others only put it on very large components.
-- Is there an IPC or EIA spec, or some kind of standard that says when it is necessary or advisable to use something to secure a component in addition to its solder joints?
Right now, the silicone processing step is seen as nothing more than a waste of labor. Unless I can provide some good evidence and proof of it from some external source, that step will likely be eliminated. I'm all in favor of reducing labor and making an efficient process, but not if it means creating failed parts.
Most of my experience on the mechanical side of things with regards to fatigue loading was with steel (good old endurance limit) and aluminum (tends not to show a flattening of that particular curve). I know that tin/lead solder is fairly weak in general, and I wouldn't want to entrust it with very much stress.
Some assemblies have silicone on darn near everything that has more than several millimeters of height. I've seen computer power supply teardown photos that look like that: Whole sections are buried in silicone. Others only put it on very large components.
-- Is there an IPC or EIA spec, or some kind of standard that says when it is necessary or advisable to use something to secure a component in addition to its solder joints?
Right now, the silicone processing step is seen as nothing more than a waste of labor. Unless I can provide some good evidence and proof of it from some external source, that step will likely be eliminated. I'm all in favor of reducing labor and making an efficient process, but not if it means creating failed parts.
Most of my experience on the mechanical side of things with regards to fatigue loading was with steel (good old endurance limit) and aluminum (tends not to show a flattening of that particular curve). I know that tin/lead solder is fairly weak in general, and I wouldn't want to entrust it with very much stress.
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
TTFN
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RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
I'm guessing that the silicone was added to your subject assembly at some time in the distant past, in response to vibration testing or analysis of failed assemblies, and whoever did it may not have recorded the complete reasons for the change, or you did not find the historical record, or did not look.
Anyone who suggests removing silicone adhesive from an existing assembly is an idiot, or worse, an MBA.
Mike Halloran
Pembroke Pines, FL, USA
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
Even so, some manner of IPC/ASME/EIA spec would go a long way. Even if I show a failure on a shaker table, because it's a small in-house test and not an exact replica of the vibration seen in the field (which will vary greatly depending on installation), it may not be seen as valid evidence.
The silicone is a recent addition to our processes. That means that it's new, and new things can meet resistance. Old designs used heavy transformers that were bolted to the PCB. The new design is lighter, but has some parts like bulky capacitors. I think that using silicone is quicker than attaching nut+bolt+insulating washer, and is simple and cheap way of preventing failures due to a known mechanism.
- I got some free CRT TVs over the years that had been trashed because they didn't work. Reason: Cracked solder joints on anything from a 1x0.5x0.25" capacitor, up to the bulky flyback transformer. Reheat joint = working TV.
- Solder is not strong to begin with, and I can only imagine what tin/lead would do under fatigue loading, or copper foil, especially if it's got a capacitor flopping around on the other end.
- Vibration results in behaviors that you wouldn't normally expect an object to do under static loading conditions. ("What do you mean, I need to 'properly' torque these screws?")
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
I think you're a bit confused about this. In any halfway plausible design specification, the vibration environment is specified, and is already assumed to be representative of the envelope of actual environment. Therefore, when you shake the unit under test to the specified environment and it fails, it is, by definition, noncompliant to its design specification. If there is argument about whether the test is "real," it's irrelevant, because that's what it's specified to. If the specification is too stringent, then you should lobby your customer to change the requirement.
TTFN
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RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
The test procedures are precisely specified, but there are plenty of options representing different environments.
Consumer gear would have other, perhaps less strenuous, requirements.
It's all about "Requirements".
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
VE1BLL: Thank you for that direction. I'll have a look at that standard. We've referenced a military spec before for our solder joints, and used it as a baseline for testing resilience against thermal cycling, which is of course one more way of potentially causing fatigue failure.
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
TTFN
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RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
Excessive amounts of silicon can be added for non-technical reasons also. I recall once taking a tour of an electronics assembly facility of one of the big US auto manufacturers. Knowing how cost sensitive they are I was rather surprised at one high volume but electronically simple module that was potted with an excessive amount of a pricey silicon compound, and pointed it out. The two engineers giving the tour suddenly had a dear-in-the-headlights-stare, and one quickly took me aside to explain. A corporate executive had a close friend who sold the compound to the corporation, and there were 'orders' in place to use as much of it as possible. One past engineer tasked with cost reduction pointed out the cost savings that could be had using simple acrylic coating of the circuit board and was immediately fired from on high. The engineer asked me not to mention the module and to quietly continue my tour.
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
"If there's no spec, how did [the] engineers design the board?"
I believe that the formalities of Requirements Traceability, even just writing down a spec, is a process that arose to common use far more recently than, for example, the field of circuit design. It's intended to be a value-added risk reduction process, but it indisputably adds time and labour to the schedule. It's certainly possible to completely ignore the entire process (go straight to design with only a mental understanding of requirements), provided that everyone understands and accepts the increased risk.
It's even a reasonable approach to skip it for small simple rushed projects. It may be obvious that the process would not be worth the price.
The process may be mandated of course. Even then, it can be 'tailored out' by explicit agreement to meet schedule targets.
Even invoking the entire formal Requirements Traceability process (e.g. DOORS end-to-end) certainly doesn't eliminate risks. It just allows the design engineers to point back at the requirements authors as being at fault. I've seen product specs where the revision letter had gone past 'Z' and was into the double-letters (e.g. Rev AH); each revision representing a 'new' previously-unforeseen ('Oops') requirement. Spec authors are human too, so an efficient change process is valuable.
The answer to the question is obvious...
Nike's trademark: 'Just do it.'
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
TTFN
I can do absolutely anything. I'm an expert!
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FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
RE: Are there guidelines or specifications for using silicone to secure large parts against vibration?
Z