Thursday, March 26, 2009

DIY Reflow Soldering

I recently ordered a whole bunch of samples and high end integrated circuits from various companies such as Maxim, Austria Microsystems and Microchip (such as the PIC32). These chips are all really cool. However, there’s a problem. They’re all surface mount. It has become a trend for companies to offer high pin count and new devices in surface mount packages only. This is bad news for prototyping (especially for hobby use) because surface mount packages don’t fit in breadboards. However, it is very much possible to make PCBs with fine-pitch pads/traces for even up to 0.4 mm TQFP chips such as the PIC32 100 pin versions. However, the problem now becomes the soldering.

There are two ways that a hobbyist can attach surface mount chips to PCBs:

  • Traditional soldering iron with lots of flux and a fine tip. Or,
  • Homebrew reflow soldering, which is what today’s post will examine

For complete newcomers to the topic, the process of reflow soldering is outlined below:

  • A special type of solder known as solder paste is applied to the surface mount pads on the PCB. The solder paste very think and acts like glue.
  • The surface mount components are then populated on the PCB on top of the solder paste. Due to the viscous nature of the solder paste, the devices stay in place.
  • The entire board is then slowly heated to a certain temperature enough to get everything warm (around 100 degrees Celsius for 2 minutes). Then temperature is quickly increased to around 200 degrees Celsius for the solder paste to melt.
  • At this point, the solder paste melts and attaches the devices to the pads on the PCB.
  • The PCB is cooled slowly and is ready for use.

Reflow soldering makes it very easy to solder very fine pitch components. All you need to do is apply solder paste to the pads, place and align the device properly, and melt away. Fortunately, hobbyists have several ways to go about making their own reflow “oven”, as outlined below:

  • A conventional toaster oven can easily heat up to temperatures required to melt solder paste. Toaster ovens are also inexpensive and are commonly used by hobbyists as reflow ovens.
  • Another way to go about reflow soldering is by using the stove top or a hot plate or a skillet.
  • A third, less common way is to use a reflow station or a hot air gun. Engadget has a really good article on how to make such a device very inexpensively. http://www.engadget.com/2006/03/07/how-to-make-a-surface-mount-soldering-iron/

However, since the “temperature controls” on most toaster ovens, skillets, hot plates and stove tops is next to useless, it is important to measure the temperature of your heating device beforehand to become accustomed to the temperature at a certain setting so as not to overheat your PCB and potentially damage components.

There are also numerous videos on youtube regarding homebrew reflow soldering using various techniques including toaster ovens, stove tops, skillets and even hot air guns.

Sparkfun has a few very nice tutorials as well: http://www.sparkfun.com/commerce/tutorials.php (look in the “Surface Mount Soldering Tutorials” section).

I also asked a whole bunch of questions regarding homebrew reflow soldering on the PICLIST. Here are my questions:

1. I thought heat damages components. I was consistently cautioned not to overheat components during hand soldering. If the idea is to heat up the solder just enough to melt (and maybe a bit more so as not to form a "cold soldered" joint), then wouldn't reflow soldering do the same damage to components?


2. One of my DIP chips came with a notice saying that it should be "cooked" at 125 degrees Celsius prior to reflow soldering. I would never reflow solder a DIP chip, but this is interesting. Why do they have such requirements? Is it because of any possibility of trapped moisture in the chip that could rapidly expand in high heat and damage internals? What's the worst case for not obeying this notice?


3. I've also heard of reflow soldering "profiles". What exactly are these profiles? Is it a requirement that I adhere to them for homebrew reflow soldering projects?


4. How do I apply solder paste on pads? Do I just squeeze it out of the tube and rub it consistently over the pads so that all of it is connected? Does a very precise amount need to be applied discretely on each individual pad? How is this done?


5. Where to buy solder paste, are there different types, what brand, what type? Should I use flux? Should I pre-flux the pads or apply flux and then solder paste?


6. I've also heard that it is wise to calibrate or know where your skillet/stove reaches a certain temperature. What temperature is good for reflow soldering? How do I measure this temperature?


7. How long should the PCB be on the skillet/frying pan during the reflow soldering process?

 

Here are some other questions that I had formed based on the answers given by the PICLIST members:

8. How long does the solder paste usually keep (before it goes bad)?

9. Also, do you recommend closed ovens or open type frying pans/skillets/etc?

 

I got some very useful and interesting answers from the good folks at the PICLIST. I thank all members of the PICLIST who have taken the time to write detailed replies to all of my questions. Here are some interesting replies from the members:

Answers to Q #1:

Herbert Graf:

Almost all components are DESIGNED for reflow, so no, they won't be
damaged by it, assuming you follow a recommended profile.

Joseph Bento:

Yes, heat damages components.  It's the way the heat is applied,
however.  A solder iron may have a tip temperature of over 400c.  At
this temperature, you need to complete your connection quickly or else
risk damaging the component.  If you use an oven with paste solder for
SMT components, you will typically limit the temperature to around
200c.  Paste solder turns liquid at around 170c.   I have hand
assembled many boards with SMT components and have never seen a
component damaged in a 200c oven.

Jesse Lackey:

Parts are sturdier than you think.  Just don't be on the super hot for a
long time.

Answers to Q #2:

Herbert Graf:

Bingo. Think of a kernel of popcorn. Heat it up, it explodes. Same with
a chip. If you don't dry it out by "cooking" it, it will rupture during
reflow. The main issue is the damage may not even be apparent, very hard
to debug issues could result.

Olin Lathrop:

Popcorn.

Answers to Q #3:

Vitaliy:

Profile is basically a temp vs time graph, that shows you what the
temperature should be at a certain point in time. What it boils down to, is
you need to preheat the board at a lower temperature (preheat), then quickly
raise the temperature so the solder paste melts (ramp-up), then cool the
board, but not too quickly (ramp-down).

Answers to Q #4:

Vitaliy:

Take a small syringe, carefully file off the sharp end to make the end flat,
then use it to dispense solder paste. See this page for info on application
techniques:
http://www.seattlerobotics.org/encoder/200006/oven_art.htm
You don't need to be too precise, you just need to dispense the right
amount. Reflowing is self-correcting, to a point.

Olin Lathrop:

In real production, this is done with a stencil.  The stencil is usually
thin steel of a specific thickness.  The solder paste is sortof squeegied
accross the stencil so that it gets wiped off the stencil but blobs remain
in the voids formed by the holes in the stencil.  The stencil is then lifted
off the board and just the blobs of solder paste remain.
For one-off home use, you'd probably apply the paste manually with a
syringe, and then expect to fix a few shorts afterwards.

Herbert Graf:

The pros have a mask and basically use a silk screen technique to get
the past on the board. While much more time consuming, simply squeezing
the paste on the pads will work pretty much just as well. Don't use
alot, experience will tell you how much. If you use to much chances are
either the joint will be rubbish, or you'll get a sometimes very had to
see bridge.

Answers to Q #5:

Mark Rages:

http://www.dealextreme.com/details.dx/sku.7952

 

There are more links for where to buy the paste, but I have to carefully dig through the ever expanding thread. There are also lots more answers and questions on the thread, but these are the ones that I felt were most important.

2 comments:

  1. There is a fairly detailed description on this topic here

    https://sites.google.com/site/undergroundengineerer/projects/practical-hobby-reflow-solder

    ReplyDelete