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screen thickness

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gaz

#65773

screen thickness | 23 January, 2012

Is there a different thickness of screens we should be using for smaller components (sod523 / sot23 / 0603) or new pastes?

Thanks.

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#65775

screen thickness | 23 January, 2012

Area Ratio [PCB Assembly Guidelines for 0.4mm Package-On-Package (PoP) Packages, Part II, TI Application Report, SPRAAV2-April 2008, K Gutierrez & G Coley] The relationship between the surface of the aperture and the inside surface of the aperture walls in the stencil is referred to as the area ratio (AR). The area ratio is much more suitable for shapes such as circles than the aspect ratio. For fine pitch BGA pads, a round aperture is recommended. The area ratio of >= 0.66 aperture opening to the stencil wall has been shown to provide the best transfer efficiency and repeatability of the deposited paste. Values from 0.66 to 0.8 insure a good paste release from the stencil. The biggest impact on area ratio is the stencil thickness. The equation is: AR = Ap / Aw  0.66 Where Ap is the aperture opening area and Aw is the wall area: 0.66 >=[pi*d^2/4]/pi*d*T=d/4T Where r is the aperture opening radius and T is the stencil thickness. Table 1 shows the various values of AR for different aperture openings and stencil thickness. Table 1. Stencil||Aperture Opening (thou)||Aperture Radius (thou)||Stencil Thickness (um)||Stencil Thickness (thou)||AR 100 um||10||5||100||3.94||0.64 ||11||5.5||100||3.94||0.70 ||12||6||100||3.94||0.76 ||13||6.5||100||3.94||0.83 ||14||7||100||3.94||0.89 ||15||7.5||100||3.94||0.95 ||16||8||100||3.94||1.02 125 um||10||5||125||4.92||0.51 ||11||5.5||125||4.92||0.56 ||12||6||125||4.92||0.61 ||13||6.5||125||4.92||0.66 ||14||7||125||4.92||0.71 ||15||7.5||125||4.92||0.76 ||16||8||125||4.92||0.81 5 thou||10||5||127||5||0.50 ||11||5.5||127||5||0.55 ||12||6||127||5||0.60 ||13||6.5||127||5||0.65 ||14||7||127||5||0.70 ||15||7.5||127||5||0.75 ||16||8||127||5||0.80 6 thou||10||5||152.4||6||0.42 ||11||5.5||152.4||6||0.46 ||12||6||152.4||6||0.50 ||13||6.5||152.4||6||0.54 ||14||7||152.4||6||0.58 ||15||7.5||152.4||6||0.63 ||16||8||152.4||6||0.67

As for solder paste type selection, search the fine SMTnet Archives for threads like: http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=24034

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#65777

screen thickness | 24 January, 2012

If you failed math like I did. A 0.005 inch stencil would work fine for those parts. Most stencils are 0.004in for fine pitch 1005s 201s. 0.005in is probably the most common generic stencil. 0.006in generally is if you need extra solder for big parts.

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gaz

#65780

screen thickness | 25 January, 2012

We are using 6 mil screens, and these boards have 0603,SOT23 and SOD523.

Seems like we could use *more* paste, not less on a lot of these components, so does it make sense to get a thinner screen? Wouldn't this mean we'd get less paste?

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#65781

screen thickness | 25 January, 2012

Your uising 6 mil thick screen on 0603 and sot-23 and you think you need more paste? when i use a 6 mil screen on a sot-23 and 0603 it is almost to much paste. 5 mill does just fine on those parts but if you think you need more paste then don't go to a thiner screen as long as your paste is releasing from the stencil. if you are using 6 mil stencil and it looks like you don't have enough paste maybe your paste isnt releasing 100%

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#65782

screen thickness | 25 January, 2012

A few years ago a lot of stencils were 6 mill and 7 mills and a lot of people still use them. these days with such a hi mix on the boards 5 mill seems to be a good compromise. if the 6 mills is working for you that's fine. One thing to note as you go up on thickness 402 and 603s will get globee looking ( not sure if that's a real word) and you can get more tomb stoning. If you look at cell phones and other offshore boards you would be surprised at the lack of paste on the boards as compared to USA processes.

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#65793

screen thickness | 27 January, 2012

We're in the high rel market and use a lot of 5 mil stencils. They give plenty of paste to meet IPC class 3 and survive over 1000 cycles of -40/+125C thermal cycle testing.

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gaz

#65806

screen thickness | 1 February, 2012

Thanks very much everyone.

I'll try out a 5mil and see how that goes.

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aci

#65808

screen thickness | 3 February, 2012

> Thanks very much everyone. > > I'll try out a > 5mil and see how that goes.

If you are using 6mil stencil and you are saying that you need more on the 0603 parts then you might want to look at your squeege pressure. What kind of squeege blades are you using. You could be putting too much pressure on the blades to the stencil and causing the blade to dig in the aperatures. We actually use 4mil stenicl for 0603 and under. When using 6 mil stencil on 20mil pitch parts you should be causing your rework deprtment a lot of short removal steps.

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gaz

#65814

screen thickness | 6 February, 2012

By changing paste and making a better profile, it seems like we have enough paste. The process was just not working very well with SN100.

So far the 5mil looks good, so thanks for the input!

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#65847

screen thickness | 14 February, 2012

What is the percent solids of the paste you are using?

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gaz

#65857

screen thickness | 16 February, 2012

Is that the "general metal loading" spec?

We are using AIM ws400 sac305, so 88.5%.

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