Surface Mount Technology MT
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Introduction
Surface Mount Technology has been around now for some time, and has proven that the benefits anticipated
are in fact real. The benefits of reduced space, reduced cost, and improved performance (quality, reliability, and
electrical performance) have been proved and continue to fuel the drive to increased implementation of surface
mount technology.
One aspect of surface mount technology that has been difficult to deal with is repair, touch up and replacement.
This is a complex field when all part types are considered. Many of the repair and touch up aspects of this
bulletin will be applicable to all part types, however, some of the very complex repair issues will require even further
evaluation. Use the information as a thought starter and take it from there.
Why and When?
Many of the benefits of surface mount technology are derived from the automation and process control available
with this technology. The tight spacing of parts and the different types of solder joints have caused us all to
relook at what is acceptable and what needs to be repaired. It has been proven that Printed Circuit Assemblies
(PCs) that have not been repaired will have a much higher success rate at final test and in use, compared to
PCAs that have been repaired. Failures of parts and solder joints that have been repaired, and failure of parts
and solder joints in the vicinity of repairs are common.
In General
There are many items to consider when trying to minimize repair and rework. In a very simplistic view, the
total manufacturing cycle for a surface mount product breaks down into four categories, each one of which
should be reviewed with rework and repair in mind.
1. The design and standards for the product. These are the base lines for manufacturing process, equipment,
fixtures, training, inspection criteria, and repair/rework capability in regards to the product. If the
product is designed and the standards selected well, the manufactured product will require little repair
and what repair is required will be successfully performed.
2. The second area is the manufacturing process itself. This includes whatever is necessary in manufacturing
the product to minimize repairs and rework. Design of mounting pads, selection of printed circuit
board location tolerances, placement accuracies, soldering process controls, selection of solder pastes,
operator training, etc; all of these are important to minimize rework. The “Do It Right the First Time” attitude
goes a long way in SMT Technology Assemblies.
3. The third area is the inspection process. Visual inspections are the most prevalent method today, with
“Automatic Optical Inspection” equipment being developed and fine tuned for the more difficult and the
more astute of operations. Which method to use will depend on the product being manufactured and to
some extent on the capital ($) available. It is important to recognize the limitations of each to recognize
these in the standards required of the product and process.
4. The fourth area is the actual rework and repair of the process. This is further broken down into a number
of classifications. Each of the classifications will require different levels of training and different types of
equipment to be used. These classifications are:
a. Touch-up: This is the repair of a solder joint which has excessive solder or which requires more solder.
b. Re-alignment: This is the actual movement of a part either in wet solder paste, wet adhesive, or after
soldering.
Visual Inspection Standards
The standards for visual inspection and for identifying repair candidates are also very important in
attempting to minimize repair. Inspection and repair standards should be developed under very controlled
conditions. Do not copy someone else’s attempt. Other standards could be used to generate
ideas, however the product being manufactured is unique, so it makes sense that the inspection standards
are equally unique.
a. Ask why a certain condition should be repaired.
b. Are there facts to go along with it?
c. Are the risks of the repair/replacement larger than the risks involved in not repairing?
d. Once deciding on a reasonable standard that makes sense for the product, ask whether the
process can produce product which meet the criteria on a regular basis. (If not, a relook at the
process or the inspection criteria is suggested.)
e. Do not build repair into the process. Repair only by exception and then use history to improve the
process and reduce repair.
The Manufacturing Process
This should be fairly obvious. To minimize the repair the manufacturing process should be optimized
and operated to isolate the cause of repair and correct the situation. We won’t go into a lot of areas for
concern other than to mention that Engineering Bulletins are available from KEMET which discuss the
Wave Solder Process and the Reflow Solder Process and how it can be optimized to minimize the
rework and repair. Other literature is available which will assist in improving the process. The manufacturing
process is the key to minimum repair and rework.