| | Looking for Suppliers of surface mount wetting balance equipment. | Bucky: Section 6 of the ANSI/J-STD-002 specification lists wetting balance suppliers. Dave F I like and continue to use the Multicore system. I have had many ours using the systems for PCBs. The introduction of regular solderability testing of surface mount components cannot be left to the component supplier, or his distributor. The introduction of this change in technology has clearly shown that solderability is an issue that requires constant vigilance if process yields are to remain high during production. The same vigilance must also be applied to printed circuit boards. With so many different solderable finishes available today, testing is of major importance. (Figure 1). It has recently been shown that the selection of the printed board finish can have a marked effect on the assembly performance. Results of a recent study are included in this paper. DIFFERENT SOLDERABLE FINISHES FOR SURFACE MOUNT SOLDER LEVELLED NICKEL GOLD FLUX LACQUER COATING SOLID SOLDER DEPOSIT ELECTROLESS TIN LEAD OXIDISATION RETARDENT FIGURE 1. The last survey of engineers working in companies using surface mount technology indicated that solderability was considered an important issue. It further indicated that the interest being shown was for automated methods of measurement and assessment. Over 50% of those surveyed were already undertaking testing, but still felt that a qualitative method was required. The interest may have been stimulated by current solderability performance of components or the concern that parts may require periodic assessment during storage. Alternatively it may have been highlighted due to the move away from more aggressive flux technology as companies eliminate cleaning. - 2 -
A further survey conducted by the SMART Group (Surface Mount and Related Technologies) indicated that a number of companies were having soldering problems which were component and board related. Much of this information has become apparent due to engineering looking more closely at the process and analysing faults. It is reassuring that careful consideration is being given to solderability as an important area of quality control during manufacture. The introduction of component and board testing can be undertaken very simply in production or preferably at goods in receipt. Two methods are currently available as draft standards which may be used as a basis for component and board assessment. The first method involves the direct visual inspection of tested parts against a criteria after a simple solder dip. This is a very subjective assessment, and a more quantitative method now being used for testing is the Wetting Balance or Meniscograph test. Over the last few years experience has been gained working in many companies helping implement surface mount projects. Using both methods of test has largely eliminated the use of materials which are unsuitable for production. Where parts have been supplied that do not meet the requirements they may be rejected at the goods in stage rather than causing considerable re-work and subsequent loss of production on the shop floor. Evidence is also available to provide to suppliers to justify rejection. Visual Examination for Solderability A component is tested in the "as received" condition, free from contamination by fingers or any other means. In the case of surface mount components, depending on the type, this does mean the loss of parts due to the method used. However, the cost in real terms, to prevent poor quality parts reaching production, is minor provided the purchasing department has included a contingency for testing. The sample is first immersed in flux which conforms to the requirements of section 4.6.2 of IEC 68-2-20. The flux in this case is a rosin non-activated solution, excess flux is carefully removed. The component is then immersed in, or floated on, molten solder of composition 60 tin/40 lead at either 215oC for three seconds or 235oC for two seconds. Assessment of the solderability is done visually using a binocular microscope of magnification x15 to x25. Photographs or diagrams may be used showing degrees of wettability, these may then be used as comparison on charts to assist in evaluation. - 3 -
The Dip and Inspect method, as it is commonly known, may be used quite easily provided that a suitably trained engineer is available to conduct the tests and to assess the results. The problem does occur during the rejection of parts back to a supplier if there is a difference in opinion on the visual appearance of the parts being tested. This method of test is not suitable for printed circuit board samples as the size of the pad, the resist thickness and coating type can affect the visual appearance. Hence the interest in an automated quantitative assessment method. Wetting Balance Method A more quantitative method for determining the solderability is the Wetting Balance test method. This method measures the time for the solder to wet the component termination or pad. In the conventional wetting balance test the leaded components, after fluxing, are immersed in a bath of molten solder at 235oC and the time taken to wet the leads and the wetting force is measured and a wetting balance curve produced. However, for specimens having small wettable areas such as chip resistors, chip capacitors, pads etc., a globule of molten solder is substituted for the solder bath. The specimens are tested using zero immersion depth, this method has so far shown to produce reproducible results. Artificial ageing of the specimens may be carried out prior to testing to gauge stock life. This is done by keeping the components in an environmental chamber at an RH of 95% and temperature of 40oC for ten days and 21 days simulating one year and two years storage respectively. Alternatively a dry ageing test may be applied at 155oC for 8 or 16 hours. Testing Surface Mount Printed Boards The existing Rotary Dip solderability test method used by the printed board industry is very limited and basically the same as the dip and inspect test. It is difficult to demonstrate the subtle changes which take place on boards as they are subjected to multiple heating operations during the manufacture of the boards or during assembly. To provide repeatable results the wetting balance may be used to assess the changes in solderability on test pads or production boards. With modern assembly processes the boards are subjected to adhesive curing and wave soldering. The board may also have been subjected to a reflow soldering process for top side mounted components. In the case of double sided products one or two high temperature reflow processes may also be experienced. - 4 -
At present there is no standard for testing solderability of surface mount pads; there are, however, proposals being discussed. To examine a suitable solderability measurements procedure, tests were conducted using the Multicore Must 2 wetting balance incorporating a micro globule to test surface mount pads. Two methods were evaluated on sample copper pads measuring 0.050" x 0.040" finished with a nickel/gold finish. Samples were removed from a circuit board featuring a series of five pads and finished so that the edge of each of the pads extended to the edge of the board. The board sample was then placed in a modified holder to allow the board sample to be lowered at an angle of 45oC to the solder pellet. The tests were conducted at 235oC using a 25mg solder globule on a 2mm globule block with an immersion depth of 0.1mm. The immersion speed for the tests were 1mm/s with a dwell time of 5 seconds. The 25mg globule was used on each pad to be tested without contacting the adjacent pad. Due to the size of the pad under test a new pellet was applied to the block after each test. It was also necessary to remove the occasional solder spike from the pads to avoid them touching the globule block and affecting the next test result. The tests were conducted using both pure rosin and activated rosin flux to assist repeatability of assessment. Samples were also tested using the above technique but with the board mounted at 90oC allowing the solderable finish to be fully in contact with the solder globule. The test sample was more difficult to align and it was impossible to remove the solder spikes after test. The second series of tests were performed using pure rosin flux. The resulting wetting curves show the first method of test with the pads presented at 45oC to the solder globule. This provides the most repeatable results and should prove to be the most effective method of test for circuit boards. Using these techniques assembly companies and printed board manufacturers can use the wetting balance to guarantee the solderability of boards and components in their drive for zero defect. The test has been used effectively as part of a solderable coating evaluation by Shipley Europe as part of their product development and to provide customers with technical support. Use of this test method will eliminate the arguments which often arise when the soldering quality does not provide the expected soldering yields in production. - 5 -
Reference: SMART Group video "Solderability Testing Made Simple" "Solderability the Key to Surface Mount Quality Control" By Bob Willis and John Buckley "Solderable Finish Selection for Surface Mount Boards" By Bob Willis and Phil Hunter, Shipley Reliability Report
reply »
Looking for Suppliers of surface mount wetting balance equ...
- Aug 11, 1998
by
