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RoHS PCB Questions

Views: 2631

#52548

RoHS PCB Questions | 16 November, 2007

For those of you who've completely switched to Pb-Free or who're running in a mixed environment, are you also buying or specifying the "RoHS PCBs" - the ones with

* Greater Tg (glass transition temperature)

* Thru-holes with limited drill hits and minimum resharpeng of bits

* Thicker barrel plating (better for copper dissolution?)

* Largest possible via hole diameters

* Moisture-resistant packaging

Anybody buy boards like this, or are most just buying plain-old FR4?

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#52549

RoHS PCB Questions | 16 November, 2007

For RoHS assemblies, we're spec'ing at least FR-406 for the higher Tg. We're currently requesting our customers to use ENIG finish, as we've had better luck with assembly in our process; but we're currently experimenting wtih lead-free HASL.

As far as I know, we haven't changed much else in our spec for boards. Since we're not the OEM/Designer, we don't dictate via size, drill hits, etc.

We do, however, receive the RoHS boards in sealed packaging. I believe our board houses are shipping all boards to us in sealed packaging these days, without our requesting it....but that may be board house specific. Sure sounds like a good thing to me, regardless of board finish.

cheers ..rob

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#52550

RoHS PCB Questions | 16 November, 2007

For very simple boards, 2 layers lightly populated with light thermal mass, we still use FR4. For most of our Pb-Free boards we use the higher Tg material with moisture-resistant packaging. We are a CM so we don't usually get to change many of the other board characteristics.

Jerry

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#52561

RoHS PCB Questions | 17 November, 2007

Let's face it guys...in most cases (stress the MOST!) a typical FR4 board will do the job. Few layers, low copper, not too densly populated etc...no worries. Use a reflow profile which avoids the 245-250C range, use a shorter TAL, Viola you have a good board. On the other side of the coin, many layers, lots of copper, wide thermal disparity in the components...this will force you into the realm of long reflow profiles (I've seen up to SIX minutes!!) higher peak temperatures in some areas of the board just to get others to reflow (I've seen up to 265C!) THIS is when you need to start exploring different board materials. By the way, a higher Tg dosn't tell the whole story. you can have a higher Tg but if the Td (degradation temperature) is low, you're done before you start. How about Z axis expansion? Lots to think about and not one simple answer, but let common sense prevail. Remember that a salesman is just that.

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#52565

RoHS PCB Questions | 19 November, 2007

Hi Bruce,

What is your (and everyone elses)noticable defect if you reach the higher temps with FR-4? Delamination?

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#52567

RoHS PCB Questions | 19 November, 2007

Real Chunks...Yes, that's the most glaringly obvious defect in the short term (you'll probably find it on your production floor if it's really bad) but you may find that everything looks good short term only to have a high level of customer returns...even if you are using "normal" FR4 material, you REALLY want to take a look at the drilling and thickness of the copper layer in the barrels...blow holes are a very COMMON defect when waving boards. Several things combine to make this happen...crappy drills or crappy drilling, thin walls on the barrel or shoulder, longer contact times and higher copper dissolution rates can add up to a real pain in your butt....

Bruce

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#52568

RoHS PCB Questions | 19 November, 2007

Bruce,

Does the thicker barrel plating for "RoHS-Rated" PCBs alleviate some of the copper dissolution that one might experience from SAC305?

Also, for those SN100c experts out there, is Sn100c less likely to dissolve copper?

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#52580

RoHS PCB Questions | 19 November, 2007

No, the thicker plating and need for very high quality drilling and plating is DUE to the higher copper dissolution rate and higher contact time. As to the Sn100C material, their claims seem to confirm a "lower" copper dissolution rate, but so does ALPHA SACX. I would look for independent confirmation of these claims from both. There are even some claims out there that there is no need to protect your solder pot (plug & play) but if you know ANYTHING about metallurgy, you know that ANY high tin material will attack stainless even if you slow the process down with the addition of trace elements. (Nickle for instance)

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#52582

RoHS PCB Questions | 19 November, 2007

I've been running 90% lead-free (still have a couple components I can't get reliably in RoHS compliance) with a peak temp of about 238 C, on standard FR-4 boards. The only real problem is the pad adherence is compromised, making rework of the boards really tricky. Any abuse of the pads while hot and they just fall off. A couple of times I've even had larger SMT components like 5mm tall aluminum electrolytics rip the pads off when the part was knocked against, when the board was cold. That pretty much showed me that at least the boards I was using couldn't hack that temperature. These are 2-layer boards, but I had similar difficulties on a 6-layer board. The additional cost on a 6-layer board for high Tg laminate really isn't much, but it is quite significant on a 2-layer board.

Jon

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