Simutaneous Double sided reflow assy

Here is some further information on SDSRS process, but please purchase a copy of the report from t he SMTA as all the money goes to charity. In the UK the SMART Group raised over �3000 for the homeless. SMTA have been raising a lot of money for the Grant Aid.

INTRODUCTION TO SIMULTANEOUS DOUBLE SIDED REFLOW SOLDERING

INTRODUCTION

In any process there are always opportunities to improve, reduce the number of process stages or reduce the cost of manufacture. Each may not improve yield initially, may even increase manufacturing defects, but it must in time benefit some companies' processes or products. Engineering is always looking at alternative processes and Simultaneous Double Sided Reflow (SDSRS) is a possible new candidate for investigation. It is currently being considered by two Japanese companies, one large computer manufacturer in the USA and reportedly being used by Philips and one other company in Europe.

PROCESS SEQUENCE

The process of SDSRS includes the following stages:

Stencil Print Paste Side One Dispense Adhesive Side One Alternatively Screen Print Adhesive Side One (Using Recessed Stencils) Alternatively Dispense Paste & Adhesive Place Components Side One (Possible UV Curing Stage for Adhesive) Invert Board Screen Print Paste Side Two Place Components Side Two Possibly Insert Through Hole Components Reflow Solder Both Sides Simultaneously

The question has to be asked why consider SDSRS as it will undoubtedly cause problems with loss of components in the early stages of process development. If we examine the time taken for the reflow cycle of a product, typically this is 3-4 mins. By reflowing both sides of the assembly then the time equates to a total of 8 mins. By conducting simultaneous reflow then the process cycle time is reduced with time saved on the total process cycle.

It used to be the case that placement was the slowest part of the process but with increasing speeds this is not the case. Printing was the last part of the process to be attacked by the speed merchants focusing on the solder paste specification which was the limiting factor in high speed printing. The formulations of paste have now provided the speed to make other parts of the process the weak links in the chain with cycle speeds for printing being less than 10 seconds per board.

SDSRS Advantages

Eliminates one reflow process stage Only one heating process for double sided boards Reduction in capital equipment Reduced floor space requirements Reduced handling stages Increased PCB throughput speed Potential elimination of component weight issues Improved solderability of OSP circuit boards

Disadvantages

New process introduction Possible loss of components Gluing stage required New glue materials required Glue curing process Possible limits on component positioning

ADVANTAGES

Eliminates one reflow process stage

With simultaneous reflow soldering both sides of a surface mount board can be conducted in one operation. If through hole parts are also designed to be reflowed this can be done in a single operation.

Only one heating process for double sided boards

With only one heating process there is a potential benefit to the reliability of the joints with a thinner intermetalic formation being possible. The issues related to reduction in solderability of surface mount pads are also eliminated.

With only one heating process during reflow soldering, in which the board may be supported with a central wire, it is possible to reduce warpage of boards susceptible to this problem.

Reduction in capital equipment

Eliminating a reflow process means one less piece of equipment in a production line is required. The cost of power consumption and extraction systems for reflow equipment is higher than any other piece of equipment on the line and they are effectively halved.

Reduced floor space requirements

Eliminating a reflow oven reduces the floor space required in a manufacturing area. Even with the need for an additional printer or the use of a single dispenser for both adhesive and paste, floor space is still being saved.

Reduced handling stages

The number of handling stages is reduced from a conventional surface mount process, albeit only slightly reduced from a fully double sided reflow process.

Increased PCB throughput speed

The total process cycle time for each product is reduced by the elimination of one reflow soldering stage. The increased cycle time for one printing or dispensing stage is still far less than the period taken to reflow one side of a board.

Potential elimination of component weight issues

In a liquid state there is a limit to component weight during traditional double sided surface mount assembly. Large heavy components can fall off the board. If adhesive is used and cured it will increase the range of components that can be used on both sides of the board.

Improved solderability of OSP circuit boards

There has been great debate on the benefits of using OSP: improved printing and placement, cost reduction and improved yields. OSP coatings have been shown to deteriorate during two reflow cycles; with one reflow process stage the problem is eliminated.

Note: It is the opinion of the author that most of the problems seen on OSP coated boards stems back to the quality or preparation of the original coating. Potential users of OSP should understand some of the simple process evaluation techniques or goods receipt tests.

DISADVANTAGES

New process introduction

With any new process there is a learning curve, particularly with new materials and processes. There will also always be a degree of reluctance on the part of some company departments to invest time on a new process. Always ensure that a justification has been well thought out prior to starting tests.

Possible loss of components

Component loss may occur if the adhesive is not cured or if products are incorrectly supported during the different process stages. Trials have shown that if a curing process is used them loss of parts on a normal process are eliminated.

Gluing stage required

To allow this process to be conducted successfully a gluing process is required, which requires either a printer or a dispensing system. It is one additional stage at the start of the process.

New glue materials required

The use of UV curing materials has declined for surface mount application in favour of thermally cured products. Most development of materials has focused on either their high speed dispensing or printing characteristics. Examination of the material properties of UV curing products will require further engineering time.

Glue curing process

If the process is to be successful the curing cycle or part curing will need to be achieved in <10 seconds without heating the board surface. The UV lamp would need to be placed over a conveyor suitably screened to meet all health and safety requirements. If the time to fully cure was insufficient then it would also be possible to place a UV source under a conveyor section after board turnover to supplement the cure. This may also allow a lower intensity source to be used.

Possible limits on component positioning

When using glue to hold parts on the base of the board there will be some limitations just like with the use of double sided reflow. BGA, CSP and most four sided devices may not be useable. The access for positioning glue spots may be restricted. This is dependent on the capability of the adhesive application process.

SDSRS PROCESS ISSUES

The most obvious problem with this process is the potential loss of components during flipping of the board, transportation or during the reflow cycle. Side one components may only be held by the paste tack characteristics and the green strength of the adhesive. If the process of adhesive dispensing is not controlled it can lead to contamination of the solder paste print. Adhesive may be mixed with the paste contaminating the joints either during dispense or during the reflow stages.

Process trials have shown that chip components, SOT23, SOT89 and SOIC16 parts will be held in place with the paste and adhesive holding the part in place without any curing stage. The limit has been found to be PLCC devices of 40 pins and above. Further trials are being conducted on small QFPs, TSOP and fine pitch SOIC devices. Although the adhesive volume can be achieved to secure the parts initially the glue will not hold heavy components on the board during reflow unless it is cured first. If a curing stage were possible without causing the paste to slump it would reduce the speed of throughput making the use of SDSRS of little value. The following table provides a list of common surface mount components and selected component weight; it also includes the number of terminations.

Component Weight Terminations Chip 0805 0.007g 2 Chip 1206 0.009g 2 Chip 1210 0.012g 2 SOT23 0.008g 3 SOT89 0.0976g 4 V/Resistor 0.124g 3 MELF 0.1325g 2 Mini MELF 0.031g 2 SOIC8 0.102g 8 SOIC12 0.123g 12 SOIC16 0.142g 16 TSOP20 0.212g 20 PLCC28 0.688g 28 PLCC44 2.21g 44 PLCC68 4.67g 68 PLCC100 9.97g 100 LCCC44 5.32g 44 PLCC84 5.21g 84 PLCC100 9.97g 100 QFP100 4.24g 100 BGA225 2.65g 225 BGA313 6.32g 313 CBGA256 24.21g 256 CCGA625 29.53g 625 CCGA1089 34.12g 1089 Tape BGA360 2.695g 360

Like traditional double sided surface mount the surface area of the lead forms is different on gull wing and "J" lead parts. The approximate surface in contact with the gull wing is 0.040" x 0.018" with the "J" lead having less 0.010" x 0.014". Video sequences have been taken which show successful reflow of SOIC devices on the base of the board. The PLCC devices fall off during the pre heat stages of reflow as any tack strength is lost. Part of the problem here is the weight of the component and the other is the surface of the lead in contact with the solder paste.

If the adhesive can be cured very quickly during transportation between placement and the board turnover unit then the problem of component loss is reduced. Unfortunately surface adhesives used today are primarily thermally cured unlike the original UV materials used in the early days of SMT. Most suppliers do, however, have materials which can be UV cured but that in turn requires the adhesive to be placed outside the component body footprint so the material is exposed to UV radiation. In SMT today the majority of the adhesive dots are placed under the component where the UV light would not be seen to start the curing process.

Recent trials on components have shown that cure strengths in excess of 400g can be achieved on components with very short UV exposures of less than 10 seconds. The trials were conducted using a product supplied by Loctite.

Full reports on the use of double sided reflow soldering and the use of pin in hole intrusive soldering are available from the SMART Group offices. Both reports should be read in conjunction with this report to fully understand the process issues involved with SDSRS.

To improve the process capability a lot of process engineering work needs to be done to achieve successful results with this process. Consider each of the manufacturing stages in a little more detail.

PCB SOLDERABLE FINISH OPTIONS

The board finish is now becoming a major issue in fine pitch printing; conventional tin/lead coatings do not provide the ideal surface for printing. The selective tin/lead solder levelled surface on most circuit boards is uneven across the board. This will result in inconsistent thickness of paste deposits. In the worst case it will not allow direct contact between the stencil and pad surface. Adhesive dispensing is also affected due to contact of height probes on the tin/ lead surfaces. The graph below shows the results from some of the author's trials and the defect levels in terms of PPM (Parts Per Million) defective.

The results were obtained after running mixed batches of boards through a production process. No attempt was made to optimise the process for any one finish, which of course is possible. The different batches of board were mixed together to make the comparison more revealing.

The graph shows results after inspection at screen printing for different printed board surface coatings. The flat surfaces provided the best performance for printing and Hot Air Solder Levelling the worst.

For fine pitch printing the ideal surface is a flat pad which will assist in providing a consistent paste thickness by allowing the stencil to gasket on to the pads. Some of the available alternative coatings are silver, copper preservative coatings like Entek, palladium, immersion tin and nickel/gold.

Printed Board Solder Finish

The following is a brief review of the solderable and protective finishes which are available and being used in the industry. The finishes are all currently used for printed boards which will contain both conventional and surface mount components. Although a mixed technology process exposes boards to heating during reflow and glue curing double sided reflow exposes the board to two high temperature cycles, care needs to be taken that the intended soldering process is compatible with the solder finish.

A more detailed introduction to evaluating solderable finishes and the introduction to a company is covered in the SMART Group/Shipley Europe reports or on the SMART Group video on Alternative Solder Finishes.

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