Title: RE: [PEDA] Components tombstoning After reflow.

Subject: RE: [PEDA] Components tombstoning After reflow.


Dear Brad, Lan & others,

Thank you very much for yours kindly respond and instructions.

Since the assembly house is in another city, I could not get the exact information about this issue, Please allow me to forward your instructions to them to try and analyze. Meanwhile, they also mentioned the soldmask open on the pads which connect to copper plane, I set the slodmask override to 3mil on normal pads, So, according to my polygon connect style, The actual pad size on the copper plane will be large than the design size (3mil each side), Thus, a component with two different size pads, maybe this will cause tombstoning after reflow, So I reduce the soldmask open size as same as the pad size (override size = 0, just those pads connect to copper plane). I wonder if this is helpful.

Currently, the component size is 0402, yes, we used to use 0603 or large parts and no tombstoning occur. We found this issue most recently, but we had changed the component size for a span seldom found tomdstoning. I also notice two RF module samples, one with components on both side, one with single side components, The bottom layout of the one with both side components is same as the the one with single side component, After reflow, the single-side-component module no tombstoning , The double-side-components module with many tombstoning on the bottom side (The top side contains 4 BGA ICs, it is to say the bottom side components go through the reflow twice. ) , I will try to change my habit in next design, but I wonder if the polygon connect style is really critical, and does anyone else have got experiences on this?

Thanks again!

Best regards,
Luo.

-----Original Message-----
From: Brad Velander [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, March 04, 2003 1:22 AM
To: Protel EDA Forum
Subject: Re: [PEDA] Components stand After reflow.


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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