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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


0603 - stencil thickness

Tony

#22897

0603 - stencil thickness | 3 January, 2003

Hi, I am loading a new board with mostly 0603 packages. The pads are .5 mm apart and I am having a big problem with parts shifting towards eachother instead of centering on the pads. I believe this is caused by too much solder. I am using a 7mil stencil.

1. What is the reccomended stencil thickness to apply the proper amount of solder for 0603 components?

2. How would this change the oven profile?

3. Are there any other ways to reduce the amount of solder applied?

4. Am I looking in the right direction to solve this?

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JAX

#22898

0603 - stencil thickness | 3 January, 2003

Alot of the companies out there use Standards developed internally, centered around there paste, equipment, and reflow processes. Normally you will see either 5mil or 6mil stencils used for everything in house with the exceptions of specific boards.(this is done to simplify the stencil choice leaving adjustments to be made in other areas like print parameters and reflow profiles.) Other companies design all stencils using IPC guidelines and recomendations.(normally dependant on the fine pitch devices found on the boards.)

You need to determine what plan works best for you.

As far as the rest goes: 2. You shouldn't have to change anything. 3a. Changing blade type/material in the printer.(should be last option) 3b. Increase pressure.(watch for paste bleed-out if pressure is too high. also be careful of stencil damage)

That's just my opinion, I could be wrong!

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#22905

0603 - stencil thickness | 4 January, 2003

Q1. What is the recommended stencil thickness to apply the proper amount of solder for 0603 components? A1. The proper amount of solder is determined by: * Metal content of the solder paste. * Stencil thickness. * Aperture opening. * Other factors, such as stencil material, stencil opening fabrication methods, and the work place environment.

Initially, consider focusing on the relationship between the stencil thickness and the aperture opening. Search the fine SMTnet Archives for discussion on: * Aspect ratio * Area ration

Q2. How would this change the oven profile? A2. While your paste will probably be the primary driver to your oven profile, it possible for reflow profiles to affect the movement of components on pads. Consider measuring the temperature of the opposite pads of problem components. The temperatures should be very similar.

Q3. Are there any other ways to reduce the amount of solder applied? A3. See A1.

Q4. Am I looking in the right direction to solve this? A4. Good place to start. Other things to think about are: * While it is not necessary for the components to be centered on the pads, you are correct in trying to understand the causes behind this situation. * Pad design can cause components to float to one side of the pad or the other. Most component fabricators give good recommendations for pad layouts. [Additionally, the fine SMTnet Archives contain ample discussion in on the topic.]

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Hussman

#22950

0603 - stencil thickness | 9 January, 2003

Depends on what you mean by shifting towards each other.

Otherwise, 7 mil should be fine for 0603. I've used 6mil, 7mil and 8 mil without problems. Generally, stencil thickness won't have a bearing on "shifting" parts. Check placement. Make sure both ends are touching the pase. Check pad design. Generally this is when you see parts start to shift. We get forced into this form time to time with different flavor boards and that's when I see parts shifting. It may be board design if there is no solder resist seperating a common pad.

I doubt if it's your oven profile. Almost everyone blames the last process at which the problem occurs, which is the oven (alot of anti-oven folk out there). 99% of the time it ain't your oven. If it's in control and not going into alarm every 3 minutes, or zones 3 and 4 are out of order, there probably is nothing wrong with the oven. Unless it's tombstoning you're talking about - or you ordered the "parts shifter" option for your oven.

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Tony

#22951

0603 - stencil thickness | 9 January, 2003

Thanks for the input.... I ordered a 5 mil stencil AND used more tooling under the board. This gave me a much smaller deposit of solder. I wouldn't swear that was my problem but 96 boards later the problem is 99% gone. I believe there was just too much solder. The pads for the 0603 parts were <.5mm in some places. These were where I had the most problems. What gets me is that the guy I am building the boards for (who knows just little or nothing about SMT) swore to me and those who sign my check that I had the oven profile wrong. I guess the customer isn't always right...

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