The history of NC fluxes, a method of determining if cleanliness is adequate.
In the article “Selecting the best Flux for the Job” we briefly discussed the way that “No Clean” (NC) fluxes came into general use. While there is nothing complicated in the procedures for using these fluxes, from time to time problems arise because of a lack of understanding of the factors involved. This article provides answers to most of these questions.
First of all we must always remember this is a “No Clean” (NC) flux not a “No Residue” flux. In other words there will always be some residues left on the board. By carefully setting the fluxing and soldering parameters these residues will be minimal, and will generally cause no electrical failures, However it is wise to carry out testing to assure that the material left on the assembly causes neither short term or long term failure.
If the assembly contains very high impedance circuits, or it will operate in a high temperature/humidity environment, some form of reliability testing becomes absolutely necessary. The two most prevalent forms of circuit failure are caused by ionic flux residues that pick up moisture from the environment and form a conductive path on the board surface. If this path is a very high resistance, or if the circuits involved are all low impedance then it is unlikely that any failure will occur.
However if the conductive path is of a lower resistance, or the circuits are very high impedance, then the tiny current that will flow through the ionic materials can change the operation of the circuit. If the conductive path lies between two closely spaced terminations with a potential difference of more than a few volts, it is quite possible that sufficient current will eventually flow to “plate” a metallic path from one conductor to the next, (“dendrite’ growth). This of course will produce a catastrophic failure, shorting out the power supply and often causing a fire and destruction of the PWB.
The most practical form of testing is to thermally cycle the assembly, with power applied, over the range of temperatures for which it is designed to operate. The assembly must be held at the maximum and minimum temperatures a sufficient time for the entire assembly to stabilize at that temperature. The humidity must be maintained at a minimum of 85% for the entire cycle, care being taken to prevent any liquid moisture from condensing on the assembly. The assembly must be continuously monitored for performance during the entire test. It is generally found that if the assembly is going to fail it will do so during the first few days of testing.
However we want to be sure that no long term jeopardy exists, so the testing is usually continued for four weeks. This testing will demonstrate that any similar assembly, processed in exactly the same way, will not fail in the field because of the use of the particular NC flux involved. In turn this means that every detail of the wave soldering set up used to solder the test samples, must be very carefully recorded and then used as the standard with no variations of any kind. The following are the most important items.
1). The amount of flux applied must remain constant, which in turn means that the flux density or “solids content” must not vary. With the very low solids content fluxes used with the NC process, it is impractical to attempt to monitor and make additions to maintain the solids content, and the only practical alternative is to use a “total loss” fluxer in which fresh flux is always used.
2). The foam fluxer circulates the flux and cannot be used in a “total loss” system, therefore the spray fluxer has become the standard when using these fluxes. There are several types on the market, and each have their own advantages. Remember that maintenance and cleaning are important in maintaining control of the process.
3). The flux is rapidly broken down by the heat of soldering, and therefore the preheat temperature and the contact time with the solder are both vitally important. Once the machine set up has been determined and the test sample has successfully completed the thermal cycling, then the parameters must not be changed in any way.
In setting up the wave soldering process, the amount of flux applied to the assembly must be held to the practical minimum. It is not difficult to know when this level is reached as the incidence of solder shorts and bridging will increase dramatically as the amount of flux is reduced. More flux must then be applied until these defects are minimized, and this will determine the correct amount for that particular assembly. The flux has to clean the surfaces to be joined, but it must also prevent the formation of oxides on the wave surface, and on the joints until they solidify. Any film of oxide will prevent the solder from draining from the joints and generate the solder shorts and bridges. Inevitably this means that more flux must be applied than is actually required for making the joint, and will leave some residues on the assembly.
This is where the advantages of a nitrogen blanket over the wave become obvious. Now only sufficient flux has to be applied to form the joint, the nitrogen then eliminates the formation of oxides on the wave and the solder fillet. This in turn means that much less flux has to be applied and the result is a cleaner assembly. Note that the addition of a nitrogen blanket will not improve the quality or reliability of the joints, but will often produce brighter, shinier, solder fillets as well as a cleaner assembly. Many clients claim that the improved appearance of the product and the reduced consumption of flux, more than pay for the nitrogen.