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from the what-will-they-think-of-next? dept.
Ras Pi foundation announces a new product: the compute module.
The compute module contains the guts of a Raspberry Pi (the BCM2835 processor and 512Mbyte of RAM) as well as a 4Gbyte eMMC Flash device (which is the equivalent of the SD card in the Pi). This is all integrated on to a small 67.6x30mm board which fits into a standard DDR2 SODIMM connector (the same type of connector as used for laptop memory). The Flash memory is connected directly to the processor on the board, but the remaining processor interfaces are available to the user via the connector pins.
While not yet what I imagined by only the name i.e a unit to build a shoebox-sized Beowolf cluster of 1K-RasPi-cores the new form factor and pin-out should make this endeavour easier (ahem... for someone skilled in PCB design, EE practician, and a soldering-fu master that has achieved enlightenment... not quite my profile).
(Score: 1) by tftp on Thursday April 10 2014, @08:49AM
Here's what I found in China.... There are lots of pics of the connector here.
That sounds very reasonable. My link was about the original TE part, but this is a no-name production. I am not brave enough to enable JavaScript on that Web site, though, so I can't see much of pictures. Those SODIMM connectors also come in several variants, but I'm sure the right one is available.
As far as soldering this thing, I would use lots of rosin-type flux and solder wick.
Yes, that's what one has to do if the part has pin spacing under 1 mm. It's possible, but highly unlikely to solder the part without bridging simply because the tip of the iron is larger than several pins - and that is even if you use the smallest tip that there is. (Otherwise it won't transfer enough heat. It's already a concern with small tips.) Professionals use preheating of the board.
Another possibility is to spread low-temp solder paste onto the pads (but the amount has to be just right) and then heat the area. This is hard to do with a hot air pencil because the solder paste will produce droplets of solder that will ruin your circuit. You need IR, but for that it would be better to begin with this method. A toaster oven and a multimeter with a thermocouple will do just fine.
I would strongly advise against using any alcohol that is sold in drug stores. It contains too much water. Water does not evaporate fast enough, and it interferes with cleaning (does not dissolve rosin.) I bought a gallon of ethyl alcohol in a chemical supply store for $40. This will be enough for me nearly forever. It is denatured with a tiny amount of methyl alcohol, but it won't harm you unless you drink it in quantity :-) In professional environment a water-soluble flux is used (I have some, it's cheap) but then you need to wash it off with deionized water [puretecwater.com]. In mass production this is a significant advantage over using the expensive ethanol and then dealing with pollution.
The most important thing seems to make darned sure your part is positioned over the pads squarely, which means you gotta use your tweezers and microscope.
If you use solder paste then this problem becomes a non-problem. Solder paste is sold in small tubes for about $15; mine comes from a nearby store, but there are other sources [digikey.com]. It has to be stored in a refrigerator, but still it won't last more than a year or two - then it dries up and does not print well through the stencil.
Professional stencils are cut with laser from a sheet of stainless steel foil. They are pretty expensive ($300 for a set.) Hobbyists (and small time professionals) can use stencils that Pololu makes [pololu.com] from plastic. I had ordered some, and they were quite OK for the task at hand. Not good for mass production, of course, but they are fine for a few boards.
A hobbyist probably will not manage the two-sided SMT IR reflow assembly. This requires a machine that can maintain the exact temperature profile on each side of the board individually.
I cannot stress enough to make sure your part is centered on the pads. Once you apply solder to the whole thing, it is darned near impossible to move it around anymore
Two-pin parts, like passives, can be easily moved around with two soldering irons. I have one cheap Chinese soldering station that is not as good as Metcal, but it is handy when I need to remove a resistor. Removal of larger ICs is impossible without some sort of rework station [zeph.com]. I have one (not Zephyrtronics,) but it's very old and cannot reach the melting temperature of RoHS solders. You also need a whole bunch of nozzles for it, and they are expensive - and you need many of them, since there are so many different IC packages. If you simply want to remove a part, you can use a heat gun (that painters use to strip paint.) However its airflow is extremely high, so you have a good chance of desoldering about a hundred 0201 parts around the IC that you need to remove - and all of them will be blown away. Still, I have seen it done.
Otherwise - yes, parts need to be properly centered. This is not hard with passives. But TQFP packages, with pins on all sides, require careful placement before you commit. Do not accept any skew because it eats into the clearance between pins. Usually it's enough to tack one pin to position two sides, and then tack another on the opposite end. Once you are happy, solder the rest. But... do not neglect to check where the pin #1 is :-)
(Score: 2) by anubi on Friday April 11 2014, @01:43AM
I can tell for sure you have been around the bush a few times! ;)
I have even used your heat gun trick to remove boardfuls of components at a time, with the exact result you reported: blown away. But I did get some SMD parts recovered to use on other things.
Love your warning about pin#1. Done that a couple of times myself. Sure made me mad when I realized what I had done wrong.
About the wide tip and bridging... that's what the rosin-soaked solder wick is for. The wick will absorb the excess solder so there isn't enough material left to make a bridge, but there will be ample left to wet the area between pin and pad. With my older iron, typically the end of the iron covers darned near a dozen pins. The solder itself is several pins wide. So I run my iron and wick along the part after I have soldered it down and its full of bridges ( sometimes a solid bridge ), using the wick to remove the bridges. The solder that wicked up between the pin and pad stays there. The remaining pins hold the part in place while I am wicking up the excess.
Thanks for the stencil link. I have really got to learn about how do do this.
You are so right about the drug store isopropyl.... I do not like to use the drug store stuff for the exact reason you stated... its got too much water in it. The best stuff I got was some stuff from the hardware store that was marked I believe as lacquer thinner and came in a gallon can.
As far as the China link to AliExpress: I have used them a few times and found them to be a decent supplier, albeit you have to watch your descriptions and shipping/pricing closely. My own findings were that buying consumer items was tricky, as my experience has shown I would have come out better just going to a retail store where I could personally inspect the thing before purchase. The expense of shipping often is more than the price of the item ( even if the merchant offers free shipping from China; if you get the item and it fails to meet your expectations, you are expected to return it at your expense. Fair enough I guess. Order a couple of samples and be prepared to just "eat it" if they are not what you thought.)
I have used their dispute arbitration system ( and in all cases I called on it, they ruled in my favor ), however I used it only as a last resort before calling on VISA to reverse the charges - also knowing I would forever be barred from any future transactions through Ali. So far, I have had pretty good experiences buying parts and assemblies such as Arduino compatibles, power converters, and LED assemblies. I have presently got an order with them for a lot of IRF510, which I intend to use in building another power supply for my METCAL. I have always wanted a small 12-volt soldering iron, and I was lucky enough to get a dozen handpieces for these at a surplus house ( the power supplies were all blown ). Being I have worked in RF a lot, to me these looked just like a 13.56 MHz CCFL ( cold-cathode fluorescent lamp ) driver to me. No biggie.
A little offtopic: I understand your reluctance to enable JavaScript. I have noted that it is darned near impossible for me to go to Amazon anymore, and I cannot help but think I am infected with something that interferes with Amazon, yet my own virus scanners tell me I am clean. I hardly ever buy from Amazon anymore, as I do not know whether or not Amazon just got a new script-crazy webmaster, or did I pick up something from China that is inhibiting Amazon. For me, the AliExpress site works, Alibris works, Amazon is slow as all getout and is apt to simply lock up at the oddest moment. I am running a WIN7 box with Firefox/NoScript.
"Prove all things; hold fast that which is good." [KJV: I Thessalonians 5:21]