Screaming Circuits: May 2007


Silver via problems

Here's an interesting via situation that I hadn't heard about until just recently.Silver_qfn_vias_v

In these two cases, the engineer has a QFN with a center pad that is required more for grounding then for cooling. Given the lack of criticality, it would seen that the part would give a lot more layout flexibility.

In the photos you can see the area to be soldered clearly. Surrounding that, there are fill areas, traces and vias, all masked off. Look closely at the vias though and you'll see that they aren't completely masked off. There is exposed metal on all of the vias. This creates a risk of shorts to the center ground pad. It may work, but if the QFN doesn't lay perfectly level with an air gap, its center pad can short to these vias. The fact that the intended solder points have vias in them means that it is likely that the QFN will be sucked down flush to the board.

Silver_qfn_vias_hWhat happened here? The engineer intended for the vias to be masked?

Well, it turns out that silver, the board finish in this case, can have problems with fully sealed vias. The silver surface can outgass a bit into the void and cause corrosion. The board house does not cap vias on silver boards to prevent that and without perfect registration, areas of the annular ring end up exposed, as in this case.

According to the board house, gold and HASL surfaces do not exhibit the same problem. We discussed a couple of possible solutions, but in the end, the engineer had the board remade with an ENIG finish with completely capped vias.

Duane Benson

Holiday Closing

Images Hi all. We will be closed for Memorial Day on Monday, May 28th.

This means that Monday won't be counted toward your turn times. For example, if you have asked for a 48 hour turn time and we receive you kit on the afternoon of the Friday, 25th, your 48 hour clock will start on Tuesday, the 29th.

We apologize for any inconvenience and wish you a happy holiday.

Duane Benson
By the way, this is out 100th blog post!

Microvias, Blind and Buried Vias

In a few previous posts, here, here and here, for starters, I've discussed the dreaded via in pad and hopefully, given some useful information about what to do if you must put vias in pads.

In the image, (A) shows an open via which we don't recommend at all. If you must do this, give us a call first to see if we can help you out. We have built boards like this, but we can't guarantee the work when we do and there are things you can do to make it better or worse.

Bga_microvia_in_pad

(B) shows one of our favorite options; the copper capped via, also called a blind via by some. You can have the via hole filled with a variety of materials at the boards house for mechanical durability. If this is done properly, we can't even tell that a via exists.

(C) shows a micro via. This usually works pretty good, but you should still call us first to talk about it. Sometimes the solder paste or the BGA solder ball can flow down into the microvia leaving insufficient solder to make a good mechanical or electrical connection. Call us first.

(D) shows the buried and blind via. In this case, the via is completely isolated from the BGA pad so, as with (B), we don't really care.

Duane Benson
Bury that viaspec, Bury that viaspec , bury it, bury it, bury it

Even smaller

As if QFNs and MicroBGAs aren't small enough, I ran across an article about a new package that will deliver 0.3mm or smaller pitch interconnects. We regularly place .5mm pitch BGAs and have done a few even smaller, but this is really getting down there.

The new package is primarily targeted at parts for mobile devices like DRAM, Flash and logic + memory. I don't think the package is available yet, but it is a continuation of the trend to smaller packages.

Duane Benson

If There are Questions

On May 8th,1884, Harry S. Truman, 33rd President of the United States was born. Also on May 8th, 2007, we made a few changes to our order process as described here. On the "Place you Order" page - the next one after the quote - we changed a couple of key questions.

Under the header "PARTS INFORMATION" we ask you: "If we find minor problems with your kit, such as BOM mismatches, missing parts or mis-matched pads, should we..."

This is a VERY IMPORTANT question.

Sometimes we will find small parts substitutions because the specific design, it doesn't matter. Yet, it might just matter. Read this post here. We can't tell without knowing all the details. This question is your opportunity to tell us if it matters.Parts_info_question_2

Sometimes a kit will be short a few parts but the customer still wants to get the boards built up as soon as possible regardless. We can't tell without knowing all the details. This question is your opportunity to tell us if it matters.

Icnturnkeyorn If you want us to double check you as much as we can, select the "Call to discuss..." option. If we see anything out of the ordinary, such as: BOM mismatches, parts shortages, missing parts, parts that need to be baked, pads that don't fit quire right or other similar anomalies, we will stop the build and call you. This may delay your job and will not be covered by the delivery guarantee.

Icnturnkeyworn If you just want to get the boards back ASAP and don't care if you are short or missing a few parts or have a mismatch or two, select the "Build as much as we can..." option. This does mean that if you think C2 is a 10uf, 16volt cap and you mark a bag of 100uf caps as C2, we will put those 100uf caps on the board, or if you need 100 and send us 85 we'll stuff the 85 and send the boards back to you as complete, or if you forget to send any of IC U3, we will build without it.

We want to build your boards fast and accurate, but even more than that, we want to build them the way that you need them. This question is your opportunity to tell us.

Duane Benson
Enquiring minds want to know. Or, so I've heard.

Moisture Sensitive Parts

Avoid moisture-caused delays.

Most of you have probably seen the "moisture Sensitive" components symbol. Most of you probably Msd_logo_75 understand its meaning and significance. However, in the topsy-turvy world of prototyping, details sometimes hide when they shouldn't.

From time to time, we receive moisture sensitive components that are not sealed in moisture barrier packaging. This means that we have to treat it as though it has absorbed too much moisture - and usually it has. According to the JDEDC standard, J-STD-033B.1, there are quite a few ways for components to be at risk. If the components are at risk, according to the standard, we are supposed to bake them for 48 hours before assembly.

Naturally, if you need a 24 or 48 hour assembly and we have to bake for 48 hours, something doesn't fit. If you don't want the components baked, but want them used as-is, please tell us in the special instructions when you place your order. If you don't tell us, we will try and contact you. If we can't get ahold of you, we will do our best to determine the risk and we may stop your job to bake the components. If we do have to bake them, your turn-time guarantee starts after baking.

The best procedure is to leave the components sealed in the original moisture barrier packaging when you ship them to us. If you open the packages, please let us know your preference for baking or no-baking. Take a look at our earlier MSD post for a bit more information.

Duane Benson
Baking should be for cookies, pies and cakes.

Production???

So, if you read this blog or have placed an order since Tuesday, you know that we have made some updates to our order process. If you don't read this blog or haven't placed an order since then, read the post just before this one to find out what I'm talking about.

I'll wait.

Done?

Okay, in the "Place your order" page, we now ask you if this is a prototype or a production build. We are prototype specialists, so why would we ask this question? Well, yes, we are prototype specialists, but it turns out that building small quantities of prototypes really fast, in some cases, ends up being just what the doctor ordered for small production runs. We don't actually do anything different than with our prototype builds, but a sizable number of you have told us that you use us both for prototypes and for small volume production builds.

That's where this question comes in. By asking that question, we end up better understanding what you need us to do. Eventually, we will likely change the process a bit for production orders. Whatever we do though, it will still be easy, convenient and fast for you.

Duane Benson
Is "protoduction" a word?

Web Site Update Notice

We will be making some updates to our website quote and order pages on Monday, the 7th, at the end of the day. [It went live Tuesday, the 8th]

We are expecting no more than 30 - 60 seconds site down time. Unless you are right in the middle of an order, you probably won't notice.

For the most part, we are just cleaning things up a bit, making the layout easier to read and putting more of the important tidbits of data in front of you.

  1. We've changed the tab-order for the quote page and improved the formating so that a quote will print better.
  2. We will now display your order number as soon as you select "Place your order." You can use this order number when calling us with questions. Previously, you didn't get to see your order number until you had completed the entire order process.
  3. We have changed the parts information questions. Now we ask if you want us to stop work and call you if anything is out of the ordinary with your build or go ahead and build as much as we can with what we have.
  4. We ask you if this is a prototype or a production order. The process doesn't really change, but it helps us to understand your needs a bit better.
  5. We ask for an emergency, after hours contact name and phone number so we can get those questions asked and answered even after hours and reduce delays.
  6. On your "My Accounts" page, we now list past orders and saved quotes with the most recent at the top of the list.

Unless something comes up, we will put these in place Monday at about 5:00pm Pacific time. Our site may be off line for a short period of time when the changes are turned live.

Duane Benson

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