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Squeegee Blade Wear Evaluation

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                         Reducing Defects and Costs With Adhesive Printing
                                  by Steve Beck and Richard Lieske


         Adhesive dispensing for SMT components is a process that many manufacturing engineers wish
would disappear, but somehow it never does. Problems with adhesive dispensing include components
falling off in the wave, adhesive in solder joints and, sometimes, adhesive in places it shouldn’t be. Just
about the time an engineer thinks the process is under control, something goes astray. Very often, manu-
facturability design review teams tell designers to “get as many of those SMT parts to the top as possi-
ble,” only to hear the famous comeback, “the circuit does not allow it.”
         In an attempt to resolve this dilemma, a study of the cost and quality of adhesive printing vs. ad-
hesive dispensing was conducted on a high-count bottom side SMT printed circuit board assembly at the
Winona, Minnesota, facility of EMS provider Benchmark Electronics. For the study, a board with “high
count adhesive sites” was defined as an assembly that required more than two minutes in a typical adhe-
sive dispensing machine. This length of time calculates out to approximately 1,000 components on the
bottom side, given the dispensing equipment used at Benchmark Electronics.
         The particular assembly used for the study consisted of an 18x20-inch PCBA with more than
2,500 bottom side SMT placements, requiring only minor allowances for wave soldering. The board
was populated with 0603, 0805, SOT23, Tantalum Capacitors and 0.020” pitch QFP bottom-side SMT
components. It also featured a high mix of pin grid arrays, large connectors and various other leaded
parts.

Process Development

         Because this assembly was of the mixed technology type (i.e. surface mount and leaded compo-
nents on the same PCB), wave soldering was necessary. Also, since there were some 0.020-inch pitch
QFP devices on the bottom side, printing of solder paste was required. A complete assembly solution
was needed to address all these component types without adding hand solder as an operation. The first
attempt at processing this PCB followed the steps shown in Figure 11.

                                                 [Figure 1]

         After running a number of jobs using this process flow, it was apparent that improvements
needed to be made. The largest contributors to the defect list included:


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                                         Total assemblies run = 200
                                        Total parts placed = 522,600

                                         Open solder joints = 71%*
                                         Insufficient solder = 23%*
                                         Missing components = 6%*

                              *Defect percentages are based only on the defects listed

         Further analysis showed that the insufficient and open solder joints resulted from a layout that
had not been properly optimized for the wave soldering process, as most of the defects were confined to
particular areas of the assembly. Rotation of the PCB through the wave process was considered as a
possible solution, but further investigation showed that no other direction would put all the components
into a preferred wave orientation. A new process was needed to reduce these defects.
         Since solder paste was already being printed for fine pitch components, it was decided to print
solder paste on all SMT pads and place epoxy dots between the paste as required. This approach would
create solder joints before the assembly entered the wave, greatly reducing the possibility of insufficient
and open solder joints. A selective wave soldering process would be necessary (it was also required in
the first process), as the fine pitch devices could not be exposed during wave soldering. Since the fine
pitch area of the board was the only place where there was enough clearance to design the selective
wave pallet properly, it was decided to continue wave soldering the entire assembly and mask off the
fine pitch area with the pallet. This dictated the use of epoxy and solder paste for all discrete compo-
nents.
         An added bonus gained by wave soldering all discrete components was that no new tooling was
required; the wave pallet that had already been designed and was being used in the current process could
continue to be used. This saved Benchmark the expense of re-tooling for a non-proven process during
the development stage.
         After making the necessary changes, the revised process followed the flow chart shown in Figure
2.
                                                    [Figure 2]

                                        Total assemblies run = 260
                                     Total SMT parts placed = 679,380




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                                Open solder joints = 169x defect reduction*
                                Insufficient solder = 53x defect reduction*
                                Missing components = 1.4x defect increase*

          *Defect reduction/increase is based on comparison between the original and above production runs

       As can be seen in Figure 2, the defect levels for insufficient and open solder joints were signifi-
cantly reduced, as was expected. There was an unexpected increase in defects, however, in missing com-
ponents. This increase was attributed to the solder paste. When a component is placed directly on the
PCB without any solder paste, the component is approximately flush with the PCB. When solder paste is
added first, the component sits 2-3 mils higher, decreasing the contact area between the adhesive dot and
the component, as shown in Figure 3.
                                                    [Figure 3]

       When the dot size drifts to the smaller end of the process window, there is no longer sufficient
contact area with the component, increasing the likelihood that it will fall off during the wave process.
With the addition of the solder paste, the acceptable process window for the dot size became much
smaller, and was likely to be smaller than the machine process window. With the limited amount of time
available for the study, a capability study could not be run on the glue dispenser to verify this assump-
tion. Instead, observations were made of the remaining glue dots at several locations on the production
boards where components were missing. In these cases, a dot was present, but it was slightly undersized
and had been compressed very little.
       With the high missing component count being a cause for concern, other process and equipment
changes were considered. After many hours of brainstorming, a new process was developed: solder paste
was printed, as in the current process, and then epoxy dots were printed between the solder paste, using
the same DEK 265 screen printer for both solder paste and epoxy dot deposition. It was anticipated that
this process would provide several significant quality benefits:
       1. Much tighter process control of dot size. Printing of epoxy can be thought of as a pseudo posi-
tive displacement system, in which printing continually fills an aperture with a fixed amount and releas-
es a fixed amount.
       2. Ability to engineer the dot shape to best fit the application. A dispenser allows round dots on-
ly, when sometimes an oval or other shape would be preferred. An oval engineered glue dot minimizes



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the chance of missing components by allowing additional glue-to-component surface contact, as shown
in Figure 4.
                                                        [Figure 4]
        3. Dot placement accuracy is thought to be better with a printer than with a dispenser, since the
dispensing nozzle (the stencil) is aligned directly to the board. A dispenser aligns the glue head, not the
dispensing nozzle, to the board.
        4. The dispensing system Benchmark used in this testing was shown to have elevated epoxy
chamber temperatures. During heavy production runs, epoxy begins to cure in the tube during dispens-
ing. This temperature change causes a change in the viscosity and, as a result, the dispensing characteris-
tics of the adhesive cause the dot size to drift.
        The process flow chart for this proposed process change can be seen in Figure 5.

                                                    [Figure 5]

                                          Total assemblies run = 70
                                      Total SMT parts placed = 182,910

                                 Open solder joints = 165x defect reduction*
                                 Insufficient solder = 49x defect reduction*
                                 Missing components = 9x defect reduction*

          *Defect reduction/increase is based on comparison between the original and above production runs

        Unfortunately, a quantity of boards comparable to those used in the previous process trials could
not be run, due to schedule shifts. However, as can be seen in Figure 5, the results were very encourag-
ing. The defect levels for insufficient and open solder joints were comparable to the previous process,
while the missing component defect levels were significantly reduced.

Cost Savings Realized

        During process development, a cost analysis was conducted to see how the process of dispensing
epoxy and printing paste compared to the process of printing epoxy and printing paste.

                                          Dispensing                         Printing
                  1
Throughput cost                           $10,000/yr                         -
Tip replacement cost2                     $1,000/yr                          -



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Stencil replacement cost3                    -                                   $400/yr
Estimated adhesive cost4                     $1,469/yr                           $2,613/yr
PCB repair cost5                             $15,000/yr                          $5,000/yr
TOTAL                                        $27,469/yr                          $8,013/yr
1
  Cost is based on shifting the gating process to adhesive dispensing from component placement
2
  Tips have been found to have a limited life span due to the stand-off wearing down.
3
  Stencil replacement is for the estimated single ECO on this assembly per year.
4
  Cost of adhesive includes projected waste. If a sealed squeegee head is used, waste from the printing process would be
reduced significantly and the costs would likely be comparable.
5
  Repair cost is based on the time necessary to repair process related defects.

         On an annual basis, the total cost savings realized, for a single line on a single shift, would
amount to $19,456. In a large facility, these savings would be multiplied by the number of lines and
shifts being run. In addition to cost issues, a number of other considerations are noteworthy when com-
paring dispensing vs. printing adhesive.
         One of the benefits of adhesive printing is that spare parts inventory can be reduced, as one ma-
chine type (adhesive dispenser) can potentially be eliminated. The addition of the printer would not be
expected to add any additional spare parts, as it is assumed the spare parts inventory already in place for
the existing printers would be sufficient. The same elimination of a machine type would reduce training
time for operators, engineering and maintenance personnel. Finally, dispensers are limited to the single
task of dispensing adhesive. Printers have a much wider range of process possibilities, so the machine
could be utilized in other fashions in the future if needed.
         The benefits of dispensing include the ability to change the glue dot locations with simple pro-
gramming changes rather than with a new stencil. While this can be a concern on boards with continual
ECOs, it should be noted that an internal cost of generating these programs does exist and should be
considered. In addition, dispensing allows on-the-fly dot size changes to be made. However, having the
ability to change processing parameters easily can have drawbacks as well, specifically if unnecessary
changes are made frequently.

When to Consider the Process

         As with any process, there are products that will benefit greatly from it, and others that will not
see a change. When analyzing the potential use of this process on a particular assembly, the issues to be
considered include:




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           If the board’s SMT layout is marginal for wave soldering. Creating solder joints prior to
            wave soldering by the solder paste reflow method will almost eliminate open solder joints.
           If there is a high number of 0603 or smaller bottom side SMT components. When going be-
            low an 0805 size component, the dot size repeatability appears to deteriorate rapidly. From
            experience, this is the most common part to be lost in the wave.
           If there is a high SMT count on the bottom side of the board. Throughput can be significant-
            ly increased by printing adhesive when counts exceed 1,000 components.
           If revisions to the PCB are low. When revisions are done on the PCB, a new stencil will like-
            ly be required.

Conclusion

        The development of an epoxy printing process has been shown to have great potential for reduc-
ing defects for some products. Further data needs to be collected to verify the results, but all indications
show that significant process improvements have been made. In addition, there is a potential for cost
savings to be realized. As with any product/process relationship, a complete engineering analysis needs
to be performed and all considerations need to be factored into determining the best process for a partic-
ular product.


                                                        ###




        For more information, contact Richard Lieske at DEK USA, 8 Bartles Corner Road, Flemington
NJ 08822; Tel: 908-782-4140; Fax: 908-782-4774; website: www.dek.com; e-mail: rlieske@dek.com,
or Steve Beck at Benchmark Electronics, 4065 Theurer Blvd., Winona MN 55987; Tel: 507-453-4749;
website: www.bench.com; e-mail Steve.Beck@bench.com.




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References
1
 Figure 1 and all subsequent figures and tables are taken from: Beck, S. and Lieske, R., “A Cost and
Quality Analysis for High Count Adhesive Sites,” presented at Nepcon West 1999.




                                                                                              dkl-bart

				
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