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Old 22nd April 2011, 03:33 AM   #1
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Default via in pad design rules

I'm laying out three or four different amplifier PCBs for a panel run, mostly 0603 and 1.27mm pitch stuff with a few 0.5mm parts, and have several places where it'd be convenient to do via in pad under 0603, 0805, or 1206 packages. Not surprisingly there's not much discussion of VIP for DIY hand soldering but the one discussion I've turned up indicate it's fine for 0805 and 1206 but can cause 0603s to flip up, making soldering perhaps a bit of a hassle as the parts have to be held down. I'd like to get a bit better sense of folks' experience with VIP before commiting to it across several boards.

Anyone used VIP in their projects? How'd it work out? I'll be running the boards on Gold Phoenix's standard process so in my case the vias would be 15 mil drills in 62 mil FR4 plugged with soldermask from the bottom.
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Old 22nd April 2011, 03:58 AM   #2
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I think its still relatively easy to HAND SOLDER even 0603 (I've only done 0805) as long as the via doesnt connect to a plane (ironically capacitors directly to planes are the main reason to have a via in a pad).
However why you would want to tent these vias? Tenting would seem to only complicate the solder joint.

Hope this helps
-Antonio
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Old 22nd April 2011, 04:27 AM   #3
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The structures I'm considering are standard topside soldermask openings with a via a bottom side trace or thermally relieved annulus to a plane. Plugging from the bottom side only (Gold Phoenix's process is not tented) therefore reduces the risk of solder wicking down the via and balling on the bottom with no complication to the joint I can see other than the chance of rosin outgassing maybe tombstoning the part. Trivial to rework that so long as it's not a routine occurence.

0603 is around 1.3mm pitch (unless you're packing the pads tightly, which I don't do). Different folks have different opinions on surface mount but I've personally found such medium pitches straightforward so long as component fields are layed out for a sensible place order and thermally relieved. I could see some difficulty making an unrelieved joint to a plane if the placement didn't allow getting a decent size tip to the pad but it'd be unusual to be doing that with an 0603.
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Old 22nd April 2011, 04:32 AM   #4
CBS240 is offline CBS240  United States
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Hi

Vias are plated holes (for 2 or more layer PCB) and I have placed them inside pads often. Obviously the solder flow will fill the hole when installing the component, but usually this is not a problem. My method of soldering 0603 parts is to use a small flathead screwdiver, like the ones you use to fix glasses or slightly a bit larger. Use a 20-25W iron with a small point tip. Start by tinning one of the pads, then dip just the tip of the screwdriver in solder flux and the component will stick to the end of the screwdriver. Place the pin onto the pad you just tinned and melt it into place. When you get it into place proper, remove the iron and it will cool in short time. Then the componet is in place and soldering the other side is easy. I can solder SOT-923 transistors (0.8mm X 0.6mm), 0402, and even 0201 resistors using this method. 0201 is 0.6mm X 0.3mm. Its a good thing because my next circuit uses many 0201 and several SOT-923 and SOT-563 components.

BTW if your using 0402 or 0201, and intend to solder by hand you should leave a leader to each pad because if you bury the pad inside a copper pour, you will not be able to heat up the joint sufficiently with the smaller iron without causing damage to the surrounding mask.
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Last edited by CBS240; 22nd April 2011 at 04:37 AM.
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Old 22nd April 2011, 05:01 AM   #5
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What drills do you typically use and do you plug/tent the vias?

I've done my share of soldering over the years---my DIY rig's a Metcal SP200 with an assortment of tips---but ended up moving out of hardware work before VIP and 0201 were common. I'm no stranger to 0402 though with 0603 and up the parts are big enough I just use the solder as a push stick instead of something like a screwdriver.
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Old 22nd April 2011, 11:03 AM   #6
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twest80

Sorry my reference to tenting was triggered from your one discussion link.
I typically have used 15 mil via holes and when connecting to a plane a 20 mil relief with 2x 10mil spokes aligned with ends. I have had good luck without plugging or tenting.
It appears everyone has their preferred solder technique, in my case I flux the pads, place the part with tweezers, hold it in place with a wooden tip (shaped like a tiny flat blade screwdriver) then touch the pad with the iron and some solder. So of course the part doesnt flip etc doing it this way.

Hope this helps
-Antonio
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Old 22nd April 2011, 03:15 PM   #7
jcx is offline jcx  United States
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you may want to check power ratings throughout you design - in high quality discrete analog it is common to have noticeable power dissipation - 0603 R can't handle much

for simple engineering reliability you should derate in circuit power to <50% of the part rating - for absolutely lowest distortion you may want to derate considerably more to avoid self heating causing errors

I'm no "capacitor sound" tweak advocate but with the highest dielectric constant ceramic today you have substantial piezo coupling - I have heard the newest "high capacity" ceramic smt caps "singing" in cycle skipping switched mode DC-DC converters on boards I’ve designed and built

I have seen 0805 Zobel R burn out when an op amp output headphone amp oscillated

and I have series paralleled 4x much larger 2512 smt R in low impedance dividers to keep far enough below the power rating

Last edited by jcx; 22nd April 2011 at 03:18 PM.
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Old 22nd April 2011, 03:23 PM   #8
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Quote:
Originally Posted by magnoman View Post
I typically have used 15 mil via holes and when connecting to a plane a 20 mil relief with 2x 10mil spokes aligned with ends. I have had good luck without plugging or tenting.
Thanks. Just to be sure I'm clear, thats for 0805 and 1206 VIP right?

Quote:
Originally Posted by jcx View Post
you may want to check power ratings throughout you design
Yeah, I don't like to push the package so I usually derate by a factor of 4 or more and move to the next size up if I'm getting close to the technology limit---I'm working with 2.5 to 5V rails which is what makes the small sizes feasible. What's your experience been with VIP?
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Old 22nd April 2011, 08:11 PM   #9
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First, VIP is best avoided unless absolutely necessary. I only once had this case in many years of pcb design.
For hand soldering it shouldn't be a problem in any way. For reflow soldering the via is best plugged. Tenting seems to be a technique best for dry film solder masks. With liquid masks I observed that tenting isn't reliable and is said to be prone to cause issues with the via later. However, for this case it's better than not to tent imo.
For soldering I recommend the two solder irons method. Apply some solder to each pad, place the part, simultaneously solder both sides, remove irons -> perfect.
Pad geometry is important for good results. But with hand soldering almost every bad land pattern design can be compensated where reflow will fail for sure.
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Old 25th April 2011, 04:59 PM   #10
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Twest,Sorry for the delayed response but I wanted to see a 15mil via inside my 0603 pad.Yes, I have only used VIP on 0805 and larger.In looking at my 0603 pads and my standard 15drilled via I would avoid VIP for these, just seems to take up too much of the pad (my via annular ring spills over).I'll be interested in hearing what route you end up taking.-Antonio
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