Rich,
It is an interesting topic. We had a client last fall that had replaced
a number of their ESD mats. Almost immediately they were observing
"corrosion" of metalized areas on some their IAg boards. Prior to
changing those mats they had not experienced any of those problems.
They suspected an out-gassing of sulfur based on their research and
asked that we analyze the "cleanliness" of their boards. We ran some
tests and found no appreciable contamination.
Fortunately they still had some of their old mats still in use. So what
we suggested was that they expose some of their boards (from the same
date code) to the old mat material and other boards to the new mat
material to do a comparison. Sure enough the boards exposed to the old
mat had no evidence of "corrosion". Those exposed to the new material
showed some spots of "corrosion". We then requested some samples of
the old mat and new mat material to run some tests, but the comparison
test we suggested was enough to convince them the mat was a prime
suspect. The last I heard they were going to contact their supplier to
determine what could be done to remedy the situation. I guess a lesson
here would be to always verify materials instead of just blindly placing
them into service.
Best Regards,
Joe Russeau
Precision Analytical Laboratory, Inc.
On 2020-01-15 13:40, Richard Kraszewski wrote:
> The ESD mat topic really strikes a note with me.
>
> As of late I have been involved in an investigation of sulfur tarnish on ImAg at one of our sites. Tarnishing is so severe it hampers through-hole solderability to the point of scraping some assemblies, albeit typically only < 5% of some work orders. Went down all common suspects such as handling, gloves, lotions, assembly materials, local production environment, assembly process, PCB packaging, age of PCBs and storage all to no avail. The ESD mats appear to be root cause.
>
> Type of ESD mat they have used for years without issue , uses vulcanized rubber so some degree of sulfur could always be detected with a iodine bubble spot test. Supplier claims there is no way that they can control the degree of volatile sulfur in their mat material.
>
> We have had success using silver mirrors (~ like copper mirror microscope slides) as a qualitative test to differentiate between the propensity of various mat materials/batches to tarnish.
>
> Also am testing H2S detection tubes with some limited success to provide a quick qualitative PPM test.
>
> Rich Kraszewski
> Senior Staff Process Engineer
> Phone: (920) 969-6075
>
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> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Russeau
> Sent: Tuesday, January 14, 2020 10:53 AM
> To: [log in to unmask]
> Subject: Re: [TN] [External] Re: [TN] Need advice : sulphur level on Gloves/ESD mat and reference standards
>
> [EXTERNAL] This email was sent from outside of Plexus. Use caution when opening attachments or links from unknown senders.
> .
> I would add to this conversation by saying that we have tested power-free nitrile and powder-free vinyl gloves using ion chromatography for "ionic" contaminates. We analyze the lots of gloves we use to handle customer product to ascertain the types and amounts of ionics that we may be contributing to the test results. We have typically found that gloves of those types have very low levels of ionic contaminates (less than 1.5 ppm total for a pair of gloves anions and cations combined). Couple low ionic levels with proper handling techniques and you can minimize the chance for transference to a product.
>
> That all said, I can't speak to the relative amounts of contaminates coming from ESD mats. Until this past year, ESD mats haven't really been on anyone's radar for being a corrosion contributor. Now there seems to be a lot of questions being asked about the levels of contaminates coming from mats and do those contaminates lead to corrosion products. To date, I am not personally aware of any standard that defines what contaminate levels ought to be for mats. Perhaps some guidance is needed. My 2 cents would when in doubt have those materials
> tested, if for nothing else but peace of mind.
>
> Best Regards,
>
> Joe Russeau
>
> PAL, Inc.
>
> On 2020-01-14 10:55, David Hillman wrote:
>
>> Hi team - I'll just add a couple of comments to the conversation. The
>> tarnish typically found on immersion silver board finishes is the
>> result of boards being wet/damp from processing which leads to a
>> brown/black visual tarnish due to reactions with the air. The majority
>> of the time the tarnish is cosmetic and does not impact the
>> solderability of the board. If it is not cosmetic then you will find
>> the solderability is a problem pretty easily. The gloves we use are
>> not sulfur free - they are Micro Grip nitrile gloves from VWR
>> International (part number 40101-350) and we haven't seen an issue
>> with them causing tarnishing. Of course, everyone should be handling a
>> board by its edges as much as possible too. I would look at your board
>> storage and handling practices as possible root cause sources for the tarnishing. Good luck.
>>
>> Dave Hillman
>> Collins Aerospace
>>
>> On Tue, Jan 14, 2020 at 7:42 AM Douglas Pauls <
>> [log in to unmask]> wrote:
>>
>> "Last Century"? Thanks for turning me from old to ancient Bev.
>>
>> As Bev indicated, there are no standards in our industry for this kind
>> of thing. The recommended ionic levels from Foresite, PAL, or other
>> test labs are for circuit assemblies or bare circuit boards, not for
>> amounts in gloves. We use standard latex or nitrile gloves here and a
>> lot of what we use is immersion silver. We have not seen the need for special gloves.
>>
>> In terms of functional impact, if it is just tarnish, then there is no
>> functional impact. Silver oxide conducts as well as pure silver. All
>> you have is a cosmetic issue when people collectively lose their minds
>> looking at tarnished silver.
>>
>> I suspect what happened is somebody somewhere thought "sulfur is bad
>> for silver" (which it is), "so we need sulfur free gloves" (which you don't).
>>
>> *Douglas Pauls *| Principal Materials and Process Engr | Advanced
>> Operations Engineering
>>
>> *COLLINS AEROSPACE*
>>
>> 400 Collins Road NE, MS 108-101, Cedar Rapids, IA 52498 USA
>>
>> *Tel:* +1 319 295 2109 | *Mobile: *+1 319 431 3773
>>
>> [log in to unmask]
>>
>> [log in to unmask] for all Export Compliant Items
>>
>> On Tue, Jan 14, 2020 at 7:31 AM Bev Christian <[log in to unmask]:
>>
>> First, do you like to be call GA or Tan?
>> a) & b) There are no standards. Shoot, we don't have any "official"
>> standards for sodium, potassium, fluoride, chloride, bromide, nitrate,
>> nitrate, phosphate or sulfate; unless they are in IEC standards that I
>> don't know about. We have the suggestions of Doug and Terry from the
>> last century and those from Delco and a couple of other sets.
>> Certainly the measurement of Doug Pauls, etc. are professionally
>> carried out.
>> c) Only cosmetic
>>
>> I have never heard anyone ask for sulfur free gloves. I am not aware
>> of what is available. I would start with asking a VWR or Fisher rep
>> (large chemical suppliers with extensive catalogues) .
>>
>> Regards,
>> Bev
>>
>> -----Original Message-----
>> From: TechNet <[log in to unmask]> On Behalf Of Tan Geok Ang
>> Sent: January 14, 2020 6:34 AM
>> To: [log in to unmask]
>> Subject: [TN] Need advice : sulphur level on Gloves/ESD mat and
>> reference standards
>>
>> Will really appreciate advice on the following;
>>
>> a) how much of sulphur/calcium, etc. on gloves/ESD Mat is acceptable
>> and what level is considered harmful in handling of PCBA/module with
>> sliver plated surface
>>
>> b) whether there are any "standards" or references for "compliance"
>> check for sulphur, and is there such measurements professionally
>> carried out
>>
>> c) What will be the impacts on the PCBA when silver plating become tarnish?
>>
>> There are sulphur free gloves but it is not easily accessible/available in Singapore and it is expensive.
>>
>> Thanks.
>>
>> GA Tan
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