No one of course reflows bare boards, so why would you profile one? For that same matter, no one sells bricks, so why do you put one through your reflow oven?
Profiling Bare Boards:
Today I came across CM doing exactly this. They were processing networking boards. They were just too complex and expensive to profile, so the solution instead of finding a scrap board or some other reasonable substitute was to profile it as a bare board. I guess the thinking was it is better than nothing, but can anyone honestly say that a bare board comes close to representing a true production board? After all wouldn’t you agree profiling modern boards with mixed components, higher densities and micro-BGAs are already a challenge and to think profiling a bare board would yield any reliable results is a stretch?
So if this is such a terrible solution, what about putting a brick through your reflow oven? A brick, come on Brian, who does this? Well what do you think you are doing when you take one of the many fixtures available on the market that are used for characterizing an oven and using it to profile? I bet if you melted them down (with profiler included) they aren’t far off in mass from a brick. Consider the following attributes of a large mass:
- A large mass will behave differently than a production board.
- A large mass acts like a heat sink and will cause the oven to react differently compared to when a production board is run through the reflow oven.
- A large mass will result in a change to airflow due to its larger size as compared to the production board.
Now notice I included the profiler as part of the mass. Many fixtures further add mass by adding a two pound weight to the fixture! Now don’t get me wrong, these fixture do give you a relative measurement from week to week or month to month as to changes in the oven, but they do not tell you if your product is in spec nor provide a thermal profile. Changes in the oven do not neatly correlate to changes in one’s profile. After all how can they? Chaos theory came out of the field of thermal dynamics, nothing neat about it. Just like a bare board is no substitute to a populated PCB, a brick is also no substitute.
Don’t take my word for it, hear it from the oven manufacturers themselves. http://profilingguru.com/reflow/standard-calibration-tool-for-reflow-process/
Here is a quote from Fred Dimock of BTU:
Oven manufacturers normally use custom designed test fixtures to simulate a board but their real purpose is to measure uniformity across the oven and confirm that the oven is working correctly.
….I have personally seen companies place unrealistic performance specifications on reflow oven testing with (fixtures) boards that have little to do with actual production needs. For example, we once were required to show that an oven could reproduce an inspect ramp soak spike profile on two 12 X 18 inch aluminum sheets that were 0.040 and 0.080 inches thick without changing any recipe parameters….
….From a surface mount manufacturing point of view – single board oven performance testing has little benefit. The real answers are to use actual boards with TCs placed on the critical components….
Both solutions profiling bare boards and bricks are inadequate. Make matters worse if you do both such as I saw with this CM, the results are only compounded. In other words, you are developing a profile based on an unpopulated board and afterwords taking measurements with a thermal mass that does not in anyway represent how your oven will in fact react to a production board. This is classic garbage in, garbage out.
Now there are alternative solutions that don’t require the destruction of a production board in the process. Many of the automated systems will create accurate virtual representations of production boards without the need to attach a single thermocouple. There are also some brilliant software solutions that allow you to create accurate profiles without the need to run a profile. http://profilingguru.com/category/reflow/automation/