| Paul and others | | The low voltage issue is currently under major research over here to understand the nature of the failure mechanisms. | | I can only suggest that your problem may well be inter-related to certain surfactant additives within the various process chemistries you are using. | | These surfactants and many and various but are essentially based upon either glycol or silicone. As such they have an extrmely low surface energy. In this way they are able to be "soaked" into the substrate often thru the solder mask (I prefer to call it solder resist) | | These non-ionic residues may interreact with residual chemistries from even the plating processes and wash them into the board weave - the infamous CAF influences reported by Prof. Laura Turbini at Georgia Tech. | | You are apparently running no-clean. So you probably have a higher pre-heat on your soldering? Are you also wave soldering fluxing? If so, you may be experiencing an adverse electro-chemical reaction between process chemistry residues. | | My best suggestion is that you contact Doug Pauls of CSL, Susan | Mansilla of Robisan, or Trace Labs etc., for further help and guidance. I would be pleased to proffer further help, but we are this side of the pond....unless you are too? | | Hope this helps, Regards Graham N | Hi!
IMHO, Graham and others have offered sound advice but should like to add my tuppence worth.
I don't like the idea of conformal coating over any old "no-clean" residues and, most especially, with a silicone-based one. Silicones give a false impression. Because they repel water like no-one's business, people think they are impervious. This is not true: they are like a sieve in a humid atmosphere. I did an experiment about 20 years ago, reported in my book, whereby I showed that cast diaphragms of silicone, epoxy and polyurethane coatings had humidity permeabilities in the approximate ratios of SI:EP 40:1 SI:PU 20:1. It is possible that silicone coatings have improved since then, but the fact remains that any hygroscopic materials under the coating will pump water in from the atmosphere.
What are the potential hygroscopic materials? Graham mentions surfactants, but the rheological additives of pastes are often just as bad, not to mention the active chemistry and its reaction products (carboxylic acids, possibly things like amine or amine derivative hydrochlorides and the metal salts from reactions with tin, lead and other oxides).
ANY of the types of conformal coating will not stop problems, they only defer them to a later date by delaying the time it takes for moisture to penetrate to contaminants under them. If the product is used in a mild, dry climate, this will not matter, but coating would be almost superfluous under these conditions. If used in a hot, humid climate, then the delay of real in-service problems may be only a matter of months.
There is only one condition whereby it is possibly OK to coat on top of a flux residue, but you need to carefully check the chemistry and, in particular, the stoichiometric ratios. This is where you solder (e.g. in controlled atmosphere wave soldering) with an adipic acid in alcohol flux. Because it is in a controlled atmosphere, the quantity of metal salts is reduced to the adipates from the board/leads. The conformal coating should be a modified epoxy type which cross-links using an adipic acid reactor, so that the surplus acid left on the board becomes part of the epoxy structure, having lost the -COOH radicals by reacting with the epoxide oxygen to give off H2O and CO2 as gases during polymerisation. This technique has been used but it is difficult to master.
My advice is therefore: Check whether you really need to conformally coat (i.e. what would be the failure rate of your product in practical service with and without and would either be acceptable?) If you do really need to and you wish to achieve a better reliability, then consider a cleaning process before coating. As dendrites are always an ionic function, an ionic contamination test would be a useful process control test of the cleaning. This may be complemented by an SIR test for qualification, but this is unsuitable for process control. The latter could reveal the presence of some non-ionic hygroscopic contaminants which could cause leakage currents but would probably not, by themselves, cause dendrite formation.
You mention high-impedance circuitry. I have built SIR testers capable of measuring up to 1E14 ohms. All the critical circuits were put on inner layers (ordinary FR-4) with at least 2 mm spacing (preferably 5 mm, where possible) to any other non-equipotential conductor. They came to the surface only to connect to components (through-hole only) with almost landless pads to maintain maximum spacing. The boards were wave soldered using a PEG- and PPG-free water-soluble flux (it used hexylene glycol as its vehicle, but a glycerol-based flux would be OK, as well). Cleaning was done in high-throughput batch aqueous processing line, with 10 post-wash rinses of uncontaminated DI water (>2 �S-cm at the moment of reaching the boards). The boards were tested immediately after cooling and conformally coated, using a PMMA product, with extremely high resistance, reasonable protection and good repairability but, of course, very tender chemically.
I hope this helps.
Brian
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We're getting field failures in a high impedance portion o...
- Aug 03, 1999
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