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Temperature control is not critical at all. Just make sure the oven is set for "toast" which means that the top element and the bottom element are both on. A thermocouple with a Fluke meter will withstand higher temperatures than a simple meat thermometer. I think my thermometer temperature tops out at 350 degrees C but that's fine for my needs.
If I understand correctly, placing the SMT components on the board is hard for you due to failing eyesight. I too have to wear reading glasses of at least x3 magnification. Even so, I get shaky sometime and mess up a placement. Occasionally, I have had to remove the components and wipe the solder paste off and clean the board with alcohol. I then re-apply the solder paste using the stencil and start over again. I have toyed with buying a "manual pick and place station" but I can't justify the price for the small amount of boards that I build. See
I have to build at least 2 boards to test the design and correct any mistakes at the schematic level. When everything is the way it is supposed to be, I hand off the design files and instructions to a local PCB assembly shop and have them order all the parts and build production quantities.
Yes, the article you found will work just fine. Like I said before, a thermocouple sensor is nice but a simple meat thermometer is all you need. Don't worry about the recommended temperature profile included on most IC datasheets. The toaster oven will follow it's own temperature profile and that is good enough. Just watch the line of solder paste I recommended you place on a used PCB just behind the window of the oven. When you see that melt and turn into solder, you are done. You may want to shine a light through the window so that you can see what the board is doing if it is tucked further inside the oven cavity. Turn off the oven and open the door. Try not to jar the oven during the cool-down period. The best way to iron out the procedure is to do a few trial runs. Put some solder paste on a blank FR4 board and "toast it". Use different width blobs of paste. See if there are hot-spots inside the oven. If the solder paste melts unevenly, then you may need to adjust the location of the board. You will get a hang of it after a few runs.
The article you found is based on applying the solder paste by hand to each individual IC pad. This will work for small boards but soon become tedious work. A stencil is the way to go and is the method employed by the professional assembly houses. Of course, you must provide a "Solder Paste Gerber File" in order to build the stencil. They are not expensive. I think I paid $30 for a small board.
Using CAD software to design the board is a must. Although I can't recommend the best package for you, I can tell you that I use PADS and have been using it for 28 years. I lived and suffered through all the initial releases and upgrades but it stabilized and has not changed much for the past 10 years. What CAD package do you use?
I understand this is now the way to go with regard to manufacturing PCBs.
If you don't get the tested board populated and manufactured by a 3rd party what are the limitations of doing your own production runs with this technique?
I would use this technique if had to make 1 to 20 small boards. Beyond that number, it's more economical to have someone else build them for you. The professional assembly houses can leverage their buying power with component vendors and manufacturers and they usually purchase resistors and capacitors in reels. What I do is find out the values they have on hand and try to specify those in my bill of materials. Sometimes you can tweak the resistor values in your design to accommodate their inventory. They will make artwork using "step and repeat" and panelize your board. See the following photographs:
They will also program an automatic pick and place machine so that labor costs are minimum. You will have to provide a file with X and Y coordinates and rotation for each component. The price I get is usually one-half to one-third of what I can achieve in small quantities. For even larger quantities, you can provide a test jig and get guaranteed working boards. If any are found defective, you simply send them back for re-testing and repair. If you are talking 10,000 pieces or more, you could have them program a bed-of-nails tester to check each component value and operation after the board is completely assembled.
One limitation may be if you use ball-grid-array (BGA) components. The many connections under the module are impossible to see. The professionals use an X-ray machine to verify the solder links. If a board does not make all of the connections and the module happens to be expensive, they will remove it and re-ball the part to save it.
When you are an entrepreneur and the only employee, you have to wear many hats. We have been discussing how to build PCBs. That is a “Manufacturing Hat”. When we talk about buying electronic components and contacting vendors for quotations, that is a “Procurement Hat”. You wear an “Engineering Hat” when you design a circuit, create bills of material, lay out the board, write code, and debug prototypes. When writing the User’s Manual, you wear a “Technical Writer Hat”. When testing the product for UL, CSA, VDE, you wear a “Product Assurance Hat”. You wear these and many other hats when you run your own company. If you work for someone else, you may be slotted into one of these jobs and wear, for the most part, only one hat. But if you are aware of the other hats, you will have a better understanding what your colleagues are doing and how to communicate in their language.
So, let me put on my “Marketing Hat” and calculate how you should price your product. Add up all your product manufacturing costs (parts and labor) and make sure you include the manual and packaging. Then you multiply that value by 5 and that is your selling price. Remember that the salesman will want a piece of that and that promoting the product will cost you for brochures, advertising, and trade shows. The salesman will want to have some headroom to offer discounts for quantity purchases. Also, over time, competition may force you to lower your prices. So if anyone throws up when you explain that your markup is 5X, remind them of these facts.
This is tough field to be working in. The more you know about it the more successful you will be at it. Good luck.
I recently had 25 pairs of boards assembled by Elecrow in China. Communication was clear and concise, the service was excellent, and the quality was excellent. They sourced all of the parts in China. With the difference in parts cost, the cost to assemble the boards was completely reasonable.
The only difficulty was in sourcing the yellow 7-segment LED digits I needed. They couldn't find a source with the desired digits in stock; they offered an alternative of a smaller size digit in the same package size, which was unacceptable to me. We ended up purchasing the MOQ (minimum order quantity) of 1000 to get the 75 I needed for this project. They retain the balance of them for future orders. This didn't cost much more than shipping the needed 75 pieces from the US, and the digits they supplied are much nicer than the ones I had been using.
Overall, a very good experience and I recoomend Elecrow as a great supplier.
Adhesive is not required. The solder paste surface tension keeps the parts in-place. During the time the solder is in liquid phase, the components shift around to align themselves on the circuit pads. This is desirable since small placement errors can be automatically corrected. However, there are certain defects that have to be considered and sometimes fixed after the solder congeals. See http://metallicresources.com/documents/How_To_Minimize_Defects_By_Adjusting_the_Reflow_Profile.pdf
The only place you have to use adhesive is if you have components on the reverse side of the board. High density board designs have components on both sides. In that case, the components on the "solder side" of the board are tacked down by an adhesive. The "component side" of the board is soldered by the "reflow process". The bottom side is soldered by "wave soldering process" along with any thru-hole parts. For prototyping circuits, I recommend you use only one sided component placement.