Dry Bake Oxidation
As with any baking operation it is important to realize that the surface finish shall start to oxidize. There are many bake operations employed in printed circuit board manufacturing. When bare copper is baked at 300 deg F for an hour the copper turns color. The copper has become oxidized and must be cleaned prior to continued processing. The final solderable finish shall oxidize as well when exposed to heat. At the time of this posting the IPC and its members are currently working on a specification that deals directly with baking of printed circuit boards. The preliminary specification can be found posted on the IPC web site at…
This is a very nice work in progress. There are several supporting documents located at this site as well. Section 4 and later of the specification start dealing with surface finishes and also offers some dry bake parameters. It is important to realise that this is a working draft. For example Table 5-1 lists base line temperatures for bake cycles of boards with different finishes. Some of the temperature ranges are higher than I would like to see them. When thinking in terms of long term baking I always use the MOT rating of the material as my maximum temperature limit.
I am thinking in terms of potential field failures and following litigation. When a board fails in the field all process records are reviewed to assist in root cause analysis. If this were a catastrophic failure that resulted in litigation then the lawyers shall look for any bit of evidence to help their client. Exceeding the MOT in a dry bake would be evidence. The lawyer in me would jump at the technicality. Can the product take higher heat at the time indicated? Maybe, but that depends of the material, manufacturer and their fabrication process. UL MOT is more or less a certification of a manufacturer’s product limit to prolonged heat (under operation) after the product passes through their process.
The best way to verify if oxidation shall impact the assembly process is to use a solderability sample. Either order a few extra boards from your vendor or request an extra one for use as a solderability sample. When requesting a solderability sample you need to state if the board needs to be functional or not. Free solderability samples typically have a defect. Typically an electrical open or a short. If you plan on using it to ship to your customer then you are in for a surprise. Functional solderability samples are typically not free. Terms and conditions shall vary from supplier to supplier. The function of a solderability sample is to test the solderability of the surface finish through the assembly process. It can also be used to assist in generating the assembly profile of a printed circuit board. The more complex the design the greater the need for one or more samples. If you dry bake any boards to be soldered then dry bake the solderability sample first and try to process it through the assembly line. Look for issues. If there are none then you are good to go. If there are issues then use a non-dry baked board as a solderability sample. Look for issues. This shall help rule out the dry bake operation as solderability issue prior to assembling all of the product.