Here is a quick tutorial that will show you how to do reflow soldering at home. It's not that hard as it seems. There are some up-front costs, but it's easily within the means of the average hobbyist. I have sucessfully used this process for the manufacturing of the SARK-110 Antenna Analyzer prototypes, which will the example shown in this tutorial.

 

In most of the cases the assembly of SMT components is simple and accessible for all amateurs and does not require special tools. The basic tools needed for the board assembly are a solder iron (better if it has temperature control), tweezzers, thin solder wire, solder flux, solder wick, and magnifying glasses. In some cases it may be necessary to have some additional tools such a soldering station hot air, e.g. for replacement of parts or leadless components (QFN type for example).

 

There is a great video explaining surface mount soldering techniques. You will discover that the use of the solder flux and the wick are key for the success of the work.

 

However, for the cases where it would be necessary assembling a small series of prototypes, or if we have to face a design with many small size SMD components or they are difficult to solder manually (QFN, BGA); it would be convenient doing the assembly using the reflow soldering technique.

 

The first question that arises is what would be the necessary equipment and whether it would be affordable for the average amateur. It is obvious that the professional grade equipment is far away the budget and the provided features normally exceed the hobbiyst needs. However this technique could be implemented using affordable tools, which could be even available at home. As examples, there are some references in the Internet in how to convert a convential oven or a bread toaster in a reflow oven, or even there are some tutorials in how to implement this technique using a hotplate.

In my case I didn't have time to experiment with this type of reworked ovens and my choice has been purchasing a cheap SMD oven, in this case the T-962 model available at e-Bay at $319 (including shipping), see image below.

  

 

 

 

 

 

To dispense the solder paste it will be needed a wiper of some sort, such as a paste squeege or a spatule. The paste squeeges are quite expensive so in my case I have used simple credit cards with success.

Other tools that will be needed are rubbing alcohol, lot of paper towels, gloves to safely handle the paste, and a screw driver or mixing tool. For the positioning of components you will need a set of tweezers including one with extra fine tip, some sort of magnifying glass, and good lighting.

To accurately position the stencil I decided to use a printed circuit board and cut it in the center following the contour of the board. This solution really worked well for this board but this is just an example in how to do it.

 

 

It is very important correctly aligning the stencil, therefore the magnifying glass will be of great help here, specially because the transparent stencil. Then I used tape firstly in the left side and secondly in the right side to fix the stencil. For the right side I used a tape of smaller length in order to allow lifting the stencil easily. 

  

 

Once positioned the stencil we will proceed to the application of the solder paste. Firstly, it is important that the solder paste is at the room temperature in order to have the right viscosity. My approach has been to drawn a small quantity of paste to another syringe and bring to the room temperature by heating with the hand. Once it is warmed enough, it will deposited a portion of paste on the stencil in one end of the board as shown in the image below: 

  

 

 

The paste is then dispensed using the squeege, in this case the card, at an angle of about 45º along one diagonal first and then the other diagonal. It is important to ensure that all voids are filled. Finally the paste should be removed to the sides of the stencil, driving the squeege at an angle close to 90º. Once finished, it is necessary to lift the right side of the stencil and then remove the board. 

 

   

 

The image below shows the result after dispensing the solder paste. It is important to clean the stencil with alcohol and the paper towels in order to get it ready for future applications.

 

 

Now this is the time to position the components on the board. For this project it was the more critical and laborious task because the significant number of components and its small size (most passives are in the 0402 size box). We have to be very careful to do not lose the components, avoid touching the paste areas, and correctly align the components.

 

It is very important preparing the placement process in advance, otherwise you will spend a lot of time looking for the components on the board. What I did is printing several copies of the board silkscreen. Then I use a copy for each group of components, highlighting the components with a pen and then attaching the component box to the copy. Therefore, you will have easily identifed the components you are going to place during the assembly. It will also necessary having at hand the printed silkscreen as in the image below:

 

 

Once the placing process of the components have been finished it is time to oven the boards. We need to have extreme care to avoid moving the components.

 

  

 

For the alloy used (Sn63Pb37), the appropiate temperature profile for this oven will be the Wave 2.  

 

  

 

 All the SMT components have been assembled to the board. Then we will proceed with the manual assembly of the insertion components.

 

  

 

Then we will have to visually review and rework manually the board. It is normal to find short-circuits and components that have been moved, specially the small 0402 components. For the rework I use a soldering iron, flux, and solder wick...and a lot of patient!

 

This was my first experience with this technique and with a somehow complex design. The positioning of the components have been by far the most critical and laborious process and the boards require some rework, specially for the first batches but the result has been quite satisfactory.