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SMT
Process Capability for Small Lots
A major difference between the high-mix environment and a volume assembly environment is the approach to process capability. A volume assembly does not usually move between process lines, allowing the statistical data collected for each run to be tied to a unique assembly line. “Tweaks” made to the process can be directly correlated to the results measured on the end product. In the high-mix environment, the product may move between process lines on successive runs or may be built only once. In either case, there is no direct correlation between a defect noted on the end assembly and the necessary corrections to the process. To effectively link
the process corrections to the end product, the high-mix assembler must
assess the process capability for each assembly line and then match
the requirements of a given job to the appropriate process line. Further,
assemblers must develop a generic process profile broad enough to encompass
the majority of the products they assemble. Even a beginning Algebra
student knows that you must have as many equations as variables to achieve
a unique solution. The manufacturing corollary to this math fact is
that if you wish to derive meaningful data from a process test, you
must vary only one factor at a time. There are no shortcuts. For a basic SMT
process, there are three metrics that should be quantified:
Accuracy
of Placement
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Several commercial systems are available for evaluating placement accuracy. Most use glass plates with graduations that can be read with a 10 or 20 power loop. The plate is sprayed with an adhesive. Then the placement system is directed to position a part in a mathematical location with a programmed rotation.
This process is repeated 10 times and the results are plotted. The resulting chart will give an X error, a Y error, and a Theta error, which is calculated from X and Y measurements. For fine pitch work, the placement system must be able to put at least 75 percent of the lead on the smallest pattern to be placed in production. This means that to place a 20 mil pitch QFP, the maximum placement error is .0021 inches in either axis.
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To complicate this problem, the error is not usually the same for the two axes because many systems use a lead screw driven Y-axis and a belt driven X-axis. The differing servo systems, drive mechanisms, and encoding schemes result in a mismatch. Lastly, remember the Pythagorean theorem: A one mil error in X and a one mil error in Y will result in a 1.4 mil error on the diagonal. So, even if the head rotation is perfect, a part placed at an angle magnifies the axis error.
There is no universal solder paste that can be applied in all situations. Medical companies typically require the use of an aqueous clean paste while automotive companies lean towards no-clean pastes. Further, the requirements of a high-mix environment are different from those of a volume production line.
The following evaluation process was designed to select an aqueous solder paste for a high-mix application. A test pattern on a 5 inch by 7 inch board was designed to incorporate the patterns shown in the table below.
| Package |
No.
of Placements
|
No.
of Rotations
|
| TO-252 |
4
|
2
@ 90°
|
| SOT-25 |
10
|
1
|
| SOT-23 |
48
|
2
@ 90°
|
| SO-16 |
2
|
1
|
| 304 QFP |
1
|
1
@ 45°
|
| 168 QFP |
1
|
1
@ 45°
|
| 120 QFP |
1
|
1
@ 45°
|
| 84 PLCC |
1
|
1
|
| 169 BGA |
1
|
1
|
| 46 µBGA |
1
|
1
|
| 1206 |
100
|
2
@ 90°
|
| 0805 |
100
|
2
@ 90°
|
| 0603 |
100
|
2
@ 90°
|
| 0402 |
100
|
2
@ 90°
|
The evaluation criteria was divided into three major areas. Within each area, specific inspection criteria was established. Either a 1 to 10 "rating" with 10 equal to the highest) or an "amount" equal to the number of occurrences was given.
The IPC-A-610 acceptance criteria should be used as the baseline, and all boards should be inspected and ranked by the same IPC-610 certified inspector. To make the test statistically valid, the test should be run three times with a one-hour wait between runs. Use eight boards in each run (for a total of 24 boards) and process the boards at five-minute intervals. The stencil should not be cleaned between or during the three runs, and handlers should avoid "kneading" the paste while on the stencil. The temperature and humidity must be noted and held constant. The recommended temperature is 70 °F and the relative humidity is 50%.
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|
| Printing Process | |
| Print definition | Rating |
| Aperture release | Rating |
| Wet bridging | Amount |
| "Roll" of paste | Rating |
|
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|
| Placement Process | |
| Component loss | Rating |
| Component "X" movement | Yes/No |
| Component "Y" movement | Yes/No |
| Component "theta" movement | Yes/No |
| Upside down component loss | Amount |
|
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|
|
|
|
| Reflow Process | |
| Solder balls | Amount* |
| Solder bridges | Amount |
| Insufficient solder | Amount |
| Flux residue | Rating |
| Voiding on BGA | Amount |
| Solder joint wetting | Rating |
| Appearance of solder joint (shiny) | Rating |
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|
* Use 30x magnification
For component movement, it was decided that “noticeable movement” would be noted but not measured during the paste portion of the evaluation.
Paste and hold for
30 minutes.
Populate and hold for 30 minutes.
Flip the first board for 20 minutes.
Reflow all boards.
Vacuum seal the first
board for solder ball inspection.
Paste and hold for
30 minutes.
Populate and hold for 30 minutes.
Flip the ninth board for
20 minutes.
Reflow all boards.
Vacuum seal the ninth board
for solder ball inspection.
Paste and hold for
30 minutes.
Populate and hold for 30 minutes.
Flip the seventeenth board for 20 minutes.
Reflow all boards.
Vacuum seal the seventeenth board for solder ball inspection.
Upon the completion
of this test, 19 pieces of inspection data will have been collected
on each of the 24 test boards. If this data is entered into a spreadsheet,
an easy evaluation of prospective pastes can be performed.
Too often, the role
of the adhesive in the proper attachment of bottom side SMT parts is
overlooked. If appropriately designed, the same test pattern process
is used for solder paste evaluation can employed to evaluate prospective
adhesives. This tool can also help to establish the process limits for
bottom side SMT.
The purpose of the adhesive is to retain the part in the correct orientation on the bottom side of the board until wave solder. Often, an intermediate assembly operation inserts through-hole components that will be soldered during the wave operation as well. In this event, the adhesive must not only maintain the part in proper orientation, it must also be resistant to handling as the board is moved through insertion operations.
Future articles will further explore how to establish the process limits for bottom side SMT using glue dot, placement, cure, and wave solder. We will also explore the additional difficulties that occur when adhesive glue dots are used with an aqueous clean wave solder flux.
