Rocketry Planet
| When it comes to electronics, size does matter! |
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| Tech Tips Series by James Long | |
| Friday, May 09, 2008 | |
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When you are ready to take the next step into the rocketry abyss, what method are you looking to take? When dealing with electronics, size does matter, in more ways than one. Whether you are dealing with that minimum diameter mach buster, or that WAC Corporal x2, your electronics size will play a very important part. We will give some insight as to what you should consider if you're deep into rocketry. If you would like to sell electronics for rocketry, or if you sell electronics already, how to possibly reduce the cost. Have you ever come across the do-it-yourself guy that leaves nothing to chance? He not only builds his own rocket, but his electronics, motors, parachutes, launch pad, electronic launch system, visible scope tracker, etc. He is the "Rambo of Rocketry." He wants and does it all. But is he doing it the best way? Although this is subjective material, it shouldn't be argued with the Sylvester Stalone type. I know, I can be just like that. I want to know (and control) every aspect of my rocket. If anything fails, it is no one's fault but my own (exclusively). If it all goes right, I'll need an arm sling after patting myself on the back. Not conceded, but realistic. When your project gets to this complexity, you are ready to start building electronics for the "M900 54mm Sky Crier." You should also consider where you are going to put that satellite communicating, radar altimeter, inertial navigation, triple redundant deployment system. Also, you should consider the size of that beastly payload. This is where different electronic skills can come in handy. Before diving too deep into your new piece of the puzzle, you should consider the best way to save money and space. Many people would go out and put together a standard through-hole printed circuit board (PCB) for their electronics. This is a good way to test them in the prototype stage, but to save space, it is not the most efficient way to go. Also if you want to make the same item again, you spend many hours sitting with that smoking soldering iron at the kitchen table. If you get the laser altimeter, remote control, retro landing motor electronics to work properly, what about producing them, for other people? If it worked well for you, do you think someone else may want one? This is where surface mount designs are highly desired over the standard through-hole ones. A perfect example: have you ever found an electronic gizmo design (information and files only) on the Internet? Only to find out you couldn't cram a cigarette pack-size PCB in that 29mm tube? Because of this your project has escalated to this behemoth, four inch, kit. It is a perfect excuse. Just don't let your wife (or husband) read this article. Now some readers are shaking their heads, saying, "Hey! Your not telling me anything I don't know." But others look like a puppy, when it encounters a new sound. It tilts it head just slightly and wonders what type of critter makes that noise. Don't worry we will correct that list of the cerebral cortex. I know some readers shiver with the thoughts of building a surface mount board. With the proper attitude, it is much easier than soldering through-hole components. If you are thinking of building electronics for resale it is almost a requirement. We need to cover the differences between surface mount and through-hole electronics. This is for the people with slightly tilted heads. Do not take offense, it is impossible to know everything. Through-hole electronic components have legs (or leads) that project from the body of the item and go "through a hole" in the printed circuit board (PCB). Surface mount components, on the other hand, have pins (or pads) projected from their side or on their underside that hold them on, by the use of solder. They do not go "through" the board, they are merely soldered to the adjacent side. Yep the material that produces the electrical contact, is also the "glue" for holding them to the PCB. There are surface components that are also attached with adhesives, but most of those are much bigger than what the average rocket flier will ever encounter. There are drawbacks and advantages to the different types of components. Surface mount parts will usually withstand many more G's of acceleration (physically... not necessarily electronically). But they do not stand extreme vibration well. This is not due to the vibration itself, but of the PCB flexing from vibration. The board flexing can cause components to crack or come loose from some of their solder pads (between board and part). Through-hole parts on the other hand are more tolerant of vibration (to a certain degree), but they do not deal well with high acceleration. All electronic components have a limit on how hard you can accelerate them. The tiny wires inside (the electronic chip) will break if accelerated beyond their structural limitation. They are also typically much larger in size. On average, 30-50% of board space can be saved by using surface mount parts. This statistic is probably by a designer with many years of experience, and an unlimited budget. I do not know how much room I save with surface mount, but it is worth using them. In figure 2, there are two different parts in two different variations. The pair on the left is a micro controller made by Parallax. The small version is a 44 pin QFP. The version behind it is a 40 pin DIP. The QFN package does have more pins, so there are some redundant connections on this variant. The chips on the right are both Maxim ADC's (analog to digital converters). As before, the chips serve the same function but are drastic in size differences. I have not been involved with rocketry in the last few years, but even when I was playing "Rambo" some of the timers and altimeters were surface mount. We all know this is to compress the size as much as possible. "How does this compress their size?", you may ask. Surface mount parts can be extremely tiny. Resistors (0201 size) are about the size of small glitter. Yes, we know, not something you want to try to deal with by hand. It also reduces their cost. "Wow, I didn't know that!", you say. Yes with the advent of surface mount PCB's, the use of through-hole components on boards has been reducing every year. There are many boards now with no through-hole parts on them. Although it is optimum to have total surface mount, most boards still have a mix of technology to provide the necessary functionality. The cost of having through-hole components mounted on a PCB is ever increasing. If you think you might consider having a design mass-produced, it would be in your best interest to convert your "sun spot guided, anti-prang, project" to surface mount technology. The reasoning behind the pricing is pretty easy. The cost of a machine to insert items with legs into holes is unbelievably expensive. It is like trying to throw a needle in to a hole not much bigger in diameter. The machines move and mount parts extremely fast. There are actually very few companies with through-hole placement machines, which the average small company can afford to use. Unless you are having a very large number of boards assembled, the large costs are paying for some person to stand there and put those through-hole components on the board. Through-hole PCB's are usually passed through a "Wave soldering machine." This machine fluxes (an acid that cleans the metal items, which will in turn attract solder), and passes the board over a molten solder fountain. The flux causes the though-hole pins to "wick" up the solder. Wave soldering machines require many hours of maintenance and the pumps and heaters for the molten solder are subjected to extreme heat. This is the reasons behind the prices associated with through-hole assembly. Surface mount assembly is actually somewhat simpler for mass production. The equipment is cheaper and maintenance is less of an issue. The machinery is only required to sit the parts down in the right place. The soldering for surface mount is also easy. Manufacturers/assemblers put down a solder paste (which contains a flux and solder, only placed on the part pads), which is the consistency of cake frosting. Afterward, they populate the board with all the parts. This is done with an item called a "Pick & Place". It also moves and places parts at a high rate of speed. The whole PCB (including components) is then baked to mount all the surface mount parts in one fatal blow. This is done with an oven that has high precision heat controls. If you are interested in surface mount, the number of boards to be assembled is not much different, but the prices usually are cheaper. There are small companies out there, who cater to the small manufacturer, that do not require 10,000 boards assembled in one run — our company is one such company. When dealing with items as electronics, you may notice that most of the purchased deployment/recording electronics are getting pretty small. This is mainly from the fact most if not all have moved from through-hole components to surface mount electronics. Now everyone is on the same page. Well about the differences of through-hole, and surface mount parts anyway. The drawbacks of surface mount for the "Hambo" (Home-building Rambo) The first thing to know is how to design a real electric circuit. I figure if you've read this far, you probably know something about that already. If you don't, there are tons of places on the Internet with people waiting to help. You will need some kind of software to design a circuit board (PCB) and possibly a way to have it made. Eagle CAD is what I use for small projects, but it is strange software. There is plenty of software for small projects — just search around and start using what you feel comfortable with. I recommend staying away from PCBexpress. They provide a great service, but you will be locked into their service if you ever start. It is very hard to do PCB's on a budget with PCBexpresss. The manufacturer of circuit boards or PCB's can be a long topic, and I just don't have the finger skills to cover it in this article. I personally leave the PCB's to other people. I just design the boards, and have them made by BatchPCB or BarebonesPCB. This option does cost money, but it saves a step of learning. You will need a heat source. I know some people will say you can use a soldering iron. Trust me, go buy a nice $100.00 toaster oven, you will be happy you did. There are many other methods of "reflowing" a PCB. (Reflowing is a technical term for baking till the solder is meted) They are, and not limited to, griddles, electric skillets, hot plates, heat guns, clothes irons, and yes, even candles. I do not necessarily advise some of the latter listed. But hey... if you are willing to try it that way, knock yourself out. DON"T COOK ANYTHING WITH YOUR REFLOW DEVICE. If you are using leaded solder, the lead vapors never leave the device, and you will end up adding to the lead content of your body. (So you have been warned!) Solder is next on the menu. This is another drawback. Typical solder paste expires, as it is not good for a long time. I recommend Ameritronics solder paste. It has a shelf life of 6 months, comes in small tubes (actually syringes), and they have a lead free version. (Even with lead-free paste, do not cook with the device you reflow with!) I feel it is better safe than sorry. The last thing that I recommend, for the home-based SMT project, is a stencil. When you get your board made, and want to put the solder down, you have two options. You can put it down with a syringe. It can be hard to do, and get all those pads with the same size dot. Or you can purchase a stencil. I recommend Ohararp, he does all my stencils, and they work well. The price is right too. Stenciling a PCB is pretty easy once you have the alignment right. Holding a stencil is the hard part. There are many methods of holding the stencil, like tape, clamps, etc. After you have the stencil aligned to the board and taped down, the solder is placed on the stencil in a blob, and dragged by a squeegee over the holes in the stencil. (The holes or apertures are the same size or slightly smaller than the SMT pads on the PCB.) Afterwards, you remove the stencil (leaving the solder paste in place) and place the components in their respective locations. The next step is probably the most critical, and can ruin your whole project if not performed correctly. The board must be baked to melt the solder and secure the SMT parts. The time, temperature, and speed all play a part of the process. (Speed being how fast the board heats and cools.) There are many resources on the Internet to learn these skills. Now we are to the point where you have built a prototype with through-hole, got it working (all bugs squished out) converted to SMT (surface mount technology) and have flown that successfully a few times. Now, you want to mass produce the board? There are a few companies out there that will make your boards and assemble them. Some are in China, and some are in America. The choice is yours as to the country and method. Sunstone will make the board and assemble it. Screaming Circuits will also do the board and assemble the parts on to it. If you don't mind sourcing the board (or we can), Lil Brother can assemble your boards. We do have a minimum of five boards or more. We typically only do SMT parts, and leave the through-hole stuff to the customer. We know there are companies out there already having products assembled, so if you would like to reduce some of the production cost, swing by our website, and see if our service maybe of use to you. We are old Rambo's of rocketry, and we would like to see companies servicing the rocketry community get on board with our service. Lil Brother LLC James Long is the owner of Lil Brother, LLC, a surface mount technology assembly services company from Marion Junction, Alabama. James is a long-time rocketeer from back in the day, and has recently re-entered the hobby, bringing a wealth of electronics information with him. You can contact him by email for additional information at
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Ok, we are not measuring your rocket, or your wallet, but size really does matter.
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edit: At least when I looked at the picture last night I thought it said SMC. Must be the drugs (allergy season).
The caption does say "Fig. 1: Examples of through-hole mounted components."
There are more than "a few" companies in the US that do surface mount assembly. There are literally 100's. Two motivations to go to China is for the reduced labor on the non-automatic steps, and not to incur the regulatory burden we have in the US on handling the related chemicals.
A "pick & place" machine also works with thru-hole parts. The cost and maintenance for such machines is only justified for high volumes. And setup times are only justified for at least medium volume runs. For small runs, it's still cheaper to place the components by hand.
Not all solder paste uses flux. There are water-soluble pastes for easier cleanup and low contamination over time.
For production, it is common to use a "solder paste mask" which is an acetate film with small opening where each pad is located. This is carefully aligned with the board, held in place, and the solder paste is applied similar to screen printing. Remove the acetate film (clear for re-use) and all the solder is ready for component placement and reflow soldering.
Another issue for surface mount components is heat dissipation. Smaller parts allow a shorter thermal conduction path from the internal chip die to the board for better heat sinking. This is great, but can also be a problem when soldering because you can more easily overheat the chip. Also, during normal operation of the board, the smaller parts allow a higher density of components which could be a problem for overall heat dissipation.
The next higher density in surface mount electronics requires another level of difficulty and special equipment. Standard surface mount techniques (and toaster ovens!) will not work. Ball grid arrays and other buried contacts require directed heat and carefully controlled temperatures.
Rocketry-related point: Smaller, lighter parts are less likely to get damaged under high G forces.
-John DeMar
There are more than "a few" companies in the US that do surface mount assembly. There are literally 100's. Two motivations to go to China is for the reduced labor on the non-automatic steps, and not to incur the regulatory burden we have in the US on handling the related chemicals.
That is true John, there are 100's of SMT assembly companies in the US but most are way out of reach for the small entrepreneur. If you go to China, get ready for the added shipping time, expense, and possible quality problems.
The chemicals are not a big deal in America. Most are using RoHS solder, so there is little in the process the EPA is worried about. Even the new vapor phase reflow chemicals are EPA friendly. PCB construction (the board itself) is a totally different story. They have all kinds of chemicals that the EPA watches.
Small runs are what we specialize in. Although we are new, the process is coming around. We do not place by hand. We only place by an automated machine, which speeds the process, and gives an added accuracy level. There are also some chips that are not easily placed by hand. It can be done, just not something I want to do a bunch of times in a couple of hours.
There are a large number of BGA type chips being mounted everyday with reflow ovens. You would be surprised what is being built with reflow ovens. There are companies using better technology, but there are still large numbers using the older methods.
James L
Bruce,
That was the whole point of the article.
There are a large number of people out there, that are scared of the surface mount idea. The assembly is just as easy (if not easier) than through hole parts. There are some items that require an initial investment, but after that, it get easier.
James L
Actually, pricing is very competitive because there are so many out there. I was just pointing this out because of your statement that there were "only a few". There are many SMT assembly houses with many years of experience. (I've been having smt boards job-shopped out for almost 20 years, and through-hole boards for longer than that).
This is true only if you don't "wake the sleeping giant" by having a business operation with visibility.
Do you have a list of equipment and some photos of your process? Also, a list of client references? I might be able to have you quote on some of the boards I design for my clients.
There are flux-free process and solders now. They're becoming more popular due to regulations of clean-up disposal.
Yes, a solder mask is part of the circuit board process. The solder paste mask (stencil) is a common way to apply all the solder paste quickly. Some machines will meter out a dot at each pad. Small runs are usually done by hand with a syringe. Which method do you use?
There are companies who think they can do some of these specialty chips, but the yield is poor. I've had some bad experiences with stressed devices soldered with incorrect methods.
-John DeMar
Can you post or PM me some specific URLs to study?
Specific book titles to look for?
Exactly how would one use an oven to make SMT projects?
I have had past experience, electronics; Votec in high school and one year with Itt tech, but that was back in the late '70's. Perf boards was the order of the day, and PCs were scary, SMT not even on the map. I now play around from time to time with a breadboard, but no real projects yet since the sizes started to get out of control.