Luo,
        the problem you are experiencing is called "tombstoning" in English. 
Tombstoning typically occurs because of incorrect component pad geometries, solder 
volumes or reflow heat profiles. It can be effected by your polygon connections but as 
you state it is not the only reason as not all polygon connects do it and some 
components that are not connected to polygons also do it.
        The cause of tombstoning is varied but typically it occurs because the 
viscosity of the molten solderpaste at one end of the component is greater than the 
viscosity of the molten solder paste at the other end of the component or it has 
melted prior to the other end melting. Sometimes this is because one end of the 
component reaches the solder melt point faster than the other end (which end hits the 
heat first?), usually the end that is attached to the solderpad reached melting 
temperature prior to the other end (the one stuck up in the air).
        You will need to work with the assembly facility to determine the cause of 
this tombstoning, there are many possible reasons and they are not all board design 
related. One simple test, try putting the boards through the reflow oven in a 
different orientation, 90 degrees clockwise or 90 degrees counter-clockwise. There is 
a preferred orientation for most boards, at least if they are properly laid out in the 
first place, not just willy-nilly on component orientation. The bias should see the 
heat profile sweeping across both ends of chip components, down both sides of SOIC 
components simultaneously rather than one end and then the other (one side and then 
the other for SOIC parts).
        My money would be on the assembly line running a quick temperature ramp-up, or 
they are just not adequately preheating before the components hit the main reflow 
temperature phase. This is probably resulting in the leading edge solder melting prior 
to the trailing edge solder on small chip components. What size are your components, 
0402 (or even 0201)? Lots of people running 0603 or larger parts, then changing to 
0402 or 0201 have problems with tombstoning where they had no prior problems with the 
larger parts. Do you have much history running these components through this assembly 
line?

Sincerely,
Brad Velander.

Lead PCB Designer
Norsat International Inc.
Microwave Products
Tel   (604) 292-9089 (direct line)
Fax  (604) 292-9010
email: [EMAIL PROTECTED]
http://www.norsat.com


> -----Original Message-----
> From: [EMAIL PROTECTED] 
> [mailto:[EMAIL PROTECTED]
> Sent: Sunday, March 02, 2003 6:36 PM
> To: Protel EDA Forum
> Subject: [PEDA] Components stand After reflow.
> Dear All,
> 
> I always set the polygon connect to pads directly, and it 
> always work fine.
> But now, we found there are components stand after reflow, 
> some  contain
> pads connect to polygon, some not. I wonder if this is caused 
> by the polygon
> connet style or any other reason. So my question is:
> 
> 1, what are the key points to cause components stand after reflow?
> 2, When will components-stand always occur during reflow 
> process? Preheat or
> cool.
> 3, How to slove this problem?
> 
> I am looking forward to your constructive suggestions, any 
> input would be
> great appreciated!
> 
> Best Regards,
> Luo.


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