John Peavoy@ISL
02/09/97 16:48


Jeff,

Just a quick note to elaborate on what Mark said:-

Certainly if a chip component is misplaced, and has a large differential in
terms of area of each termination in contact with the solder paste, when
the paste reflows, the surface tension on each end of the component is very
different.

The termination with most contact area can then actually pull the component
up off the other pad due to this force differential !

John Peavoy.





[log in to unmask] on 02/09/97 15:42:04

Please respond to [log in to unmask]; Please respond to [log in to unmask]

To:   [log in to unmask]
cc:    (bcc: John Peavoy/ISL)
Subject:  Re: [TECHNET] (TechNet): Assy: Dewetting




I believe the dewetting and tombstoning issue can be influenced by
component placement. If the part is off placement to far when the solder
reflows on the pad termination, it can either lift the part (fulcrum
action) on one end just high enough that it looses contact with the solder
on the other end or tombstone.  Keep in ming that due to the mis-placement
the one termination was not / may not have been in contact with the paste
to hold it down to the pad.

Similar situations can be found on parts such as QFP or SOIC which have
lifted /bent lead.  The inspector may classify the defect as "dewetted"
where in reality the part recieved little flux and solder contact due to
the lead being lifted.  The "lead" is actually very solderable if the
solder /paste is appropriately applied.  However, this is very difficult
when the lead is bent up in the air.

*****SNIP
>>
>>Hi Technetters,
>>
>>Could anyone give me some advise on the dewetting issue.
>>
>>One of our component supplier told me that misalignment of the chip
>>component will cause dewetting.
>>
>>I haven't heard of such thing before, and couldn't believe so. However, I
>>would like to hear from others' opinion about this issue, whether this is
>>of a true statement from my suppiler, stating "Dewetting can be caused by
>>Misaligment".
>>
>>
>----------------------------------
>Jeff L. Hempton        phone: (219) 429-7335   Fax: (219) 429-4688
>Boardshop Manufacturing Engineer                       Mail Stop: 25-31
>Hughes Defense Communications
>1010 Production Road, Fort Wayne, IN 46808-4106
>Email: [log in to unmask]
>
>"The man who rolls up his sleeves seldom loses his shirt."--Thomas Cowan
>
>
>
----------------------------------
Mark W. Spitnale                Phone: (219) 429-5992   Fax: (219) 429-4688
Hughes Defense Communications                           Mail Stop: 25-31
1010 Production Road, Fort Wayne, IN 46808-4106
Email: [log in to unmask]