Hi,
We use 20 thou pitch extensively and have no problems associated with
witness marks. In fact they give us peace of mind, confirming the test has
taken place. This is important when assembled product can cost as much as
$8,000. You don't want to discover shorts to ground at the end of the
assembly process, do you?
Regards
At 10:04 01/03/98 EST, you wrote:
>Hello David,
>
> I have experienced a few problems from "witness marks"...but I've never
>done a detailed study on the impact they have during assembly. I will try and
>answer some of your questions though.
>
>>>1. Are these damages causing problems at the assembly process? what
>>>kind?
>
>Yes they do on occasion. Mostly when the pitch gets below 20-mils, and
>occasionally with 20-mil pitch pads. The problem that I've seen is when the
>pin contact location hasn't been centered on the pads...it will sometimes push
>a small burr of material off to one side and cause shorts to an adjacent pad
>during reflow.
>
>>>2. Are the damages affecting the assembly yield?
>
>Yes, from the reason I stated above...
>
>>>3. If yes, is there any data about the % of savings at the assembly
>>>stage if these damages are avoided?
>
>As I said in the beginning, I've never studied the actual cost of such
>problems. It doesn't seem to occur often enough to warrant a study. On
>occasion however, I have run accross entire lots of boards that have had the
>problem, but we've always just worked through the issue, and notified our
>customer of the problem. They would go back to their board shop and have them
>make sure that they avoid the same problem on future board fabrication.
>
>>>4. Have the assebly people learn to live with these defects, overcoming
>>>the problems? what is the cost of such overoming? would elimination of
>>>these damages save money by avoiding to use these measures?
>
>Yes we have, by reworking the affected joints. Again, I don't know what the
>costs are...and of course money would be saved if the problem were to be
>eliminated.
>
>>>5. Will a contactless bare board electrical test system, causing no
>>>damage at all will be an advantage to assembly people? in what sense?
>
>That's a pretty obvious question...of course, to eliminate the problem I
>stated.
>
>>>6. What would assembly people be will to do to push further such
>>>technology of bare board testing?
>
>Probably not much...it's not something we have any control over. This solution
>is for the board manufacturers. If a board vendor can't provide a board that
>doesn't have the kind of witness marks that cause manufacturing defects, they
>will lose business plain and simple. We on the assembly side just learn to
>deal with it. If it seems to happen over and over again, I would try and
>encourage purchasing to find another vendor.
>
> I hope this helps a bit...
>
> -Steve Gregory-
>
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