An Introduction of Strip Chopping Cut Method to Establish a Robust Strip Based Dicing Process on Tape Dicing Concept

Transcription

1 An Introduction of Strip Chopping Cut Method to Establish a Robust Strip Based Dicing Process on Tape Dicing Concept Ibn Asyura Zainuddin (Author) Discrete Unit Process Development Infineon Technologies (M) Sdn. Bhd. Melaka, Malaysia IbnAsyura.Zainuddin@infineon.com Abstract Tape dicing concept is most commonly used in miniature QFN package singulation process due to its low operating cost and high throughput [1]. Processing with strip based material, starting from saw singulation to unit transfer into reel tape, while sawn unit still intact on dicing tape, robust process is needed as well as short cycle time for productivity. Currently, there are 2 common tape dicing methods. Firstly, strip cutting method, and secondly, strip to panel cutting method [2]. Both methods have pros & cons respectively. Nevertheless, in order to improve cycle time, extreme processing method is needed without jeopardizing the processing quality. Introducing this strip chopping cut method enables singulation process of maximum unit density per 12 inches wafer ring and processing abnormal lead frame design with thick metal side rail and moulded-only layer on panel. This method needs dicing blade to directly touch down on top of moulded package during cutting without fully cut metal side rail. In this paper, will further discuss in details how it works and what are the pros and cons, as well as its benefits. processes such as auto vision inspection and electrical test before the single unit being transfer into tape and reel for final packaging process. By going through several processes, it is very important to maintain all diced unit on dicing tape, otherwise, there will be interruption on subsequent process if unit drop or missing along the transportation between processes. With these requirements, dicing process has to be robust enough to maintain the quality and productivity within the process flow. There are two common mechanical tape dicing method. First of all is full strip cutting method and secondly is panel cutting method. Refer to Fig. 2. Full cutting method is required the cutting line beyond and exceed strip physical dimension. Therefore a huge gap between strips has to be maintained. While the panel cutting method is required to separate from one strip to several individual panels (based on respective strip design), then, the individual panel will be full cut one by one. This method have extra process which is separating one strip to several panels and this process can be done by using mechanical jig dicing method. The purpose of separating into several panels is to remove unnecessary non active part of leadframe and the cutting process can be focused only at active unit area. Fig.1 Miniature QFN package cutting methods I. MECHANICAL TAPE DICING Mechanical dicing (also called blade dicing) is one of the chip separation methods in semiconductor industry. Others method such as laser dicing, stealth dicing, plasma dicing and et cetera, was being developed to meet certain quality criteria of the product. Nevertheless, mechanical dicing still the most common die separation method nowadays. Mechanical tape dicing is referring to separating the panel of high density matrix units into single unit which left on dicing tape after diced. Dicing tape is the medium to hold every single unit firmly so that it can be processing through several Fig. 2 Strip and panel mount layout

2 II. SCOPE OF STUDY In this paper, the scope of study is pre molded QFN leadframe which is also called molded interconnect substrate (MIS) design leadframe. Please refer to Fig. 3. Fig. 3 MIS leadframe design before and after assembly process This leadframe design enable high density unit matrix to be filled in one panel with narrow dicing saw street capability. But, due to active panel area only consist of pre molded layer from supplier. Strong carrier holder is needed to maintain leadframe flatness and co planarity to ensure smoothness flow of subsequent process. This strong carrier holder is referring to side rail of the leadframe. III. STRIP CHOPPING CUT INTRODUCTION Strip chopping cut is the combination of two processing methods in one. By referring to Chapter I, strip chopping cut is unit separation inside panel while panel still in strip form. This method without required side rail to be fully cut, therefore the cutting quality can be focused on active unit instead of focus on inactive part cutting quality. Fig. 4 shows strip chopping cut method. Red dotted line representing the cutting line. Fig. 5 Spindle movement during strip chopping cut On the other hand, current conventional strip cutting method required blade touch down and go up outside from strip physical area. Therefore high loading on blade surface will occur due to many layers of material need to be removed which are leadframe side rail metal layer, mold compound layer and tape dicing layer. Refer to Fig. 6. Fig. 4 Strip chopping cut method This process ideally for MIS leadframe design, where the leadframe side rail is hard metal material and the active unit area is fully filled by only mold compound material. The thin dicing blade not capable to do fully strip cut as it will damage during cutting the metal side rail. Fig. 5 shows the movement of the blade spindle. Spindle blade has to touch down, moving and cutting along the saw street and go up within active panel area. Fig. 6 Strip full cut method

3 IV. STRIP CHOPPING CUT REQUIREMENT There are several identified requirement to enable strip chopping cut process. The identification method was studied based on three major items which are material, machine and process parameter. A. Material requirement Leadframe has to be metal free on the saw street in order for smooth penetration and cutting. This is because the blade is directly hitting the work piece surface during touch down and the first layer of removal has to be mold layer instead of metal layer. Blade clogging will cause high blade loading and difficult to penetrate into saw street due to ductile metal material covering the surface of blade diamond grit and fills the chip pockets. Sharp diamond grit is needed to remove the saw street material smoothly. By designing only mold compound on the saw street, the blade loading can be reduced and there will be always sharp diamond exposed due to self-cleaning feature as the mold compound will become a dust during mold material being removed by blade. Please refer to Fig. 7 for illustration details. The difference of high and low loading could be identified by monitoring current supply to the spindle. The A design shows lower current indication whilst B design shows higher current indication. The current value will spike high in few milliseconds then become stable when moving along the saw street. Refer to Fig. 8. Fig. 8 Chart of spindle current monitoring with different leadframe design. Leadframe design of outermost saw street line must be at the tangent of the blade tip. This is the line where the point of blade touches down. If the point of touch down is too near to the side rail, the blade will cut it and causing high blade wear. By using drawing software, the x value could be calculated. t1 is referring to package thickness while t2 is referring to cutting depth. This gap is the minimum value to ensure that the package is completely sawn. Longer gap is better to avoid blade always hitting side rail edge that could cause higher blade wear. Fig. 7 A design with metal free on saw street while B design with metal tie bar on saw street. Fig. 9 Touch down point based on top and side view. Low strip smiling warpage desired in order to get consistent blade touch down point and also to avoid high blade wear when it hit and cut side rail edge. In case on warpage level is not consistent, blade wear has to be monitored frequently and

4 touch down point distance could be adjusted by compensating cutting depth area. The maximum adjustment of blade touch down should be L millimeter farther than side rail edge. With this adjustment, the first row and column of unit still fully sawn. Refer to Fig. 10. Machine is capable to adjust blade touch down speed before start cutting. If the blade touch down on panel surface too fast, it will cause high loading to the blade and blade broke. This is due to the amount of material to be removed during penetration was not fast enough. Slower touch down speed is better but it will reduce productivity as well as fewer units per hour produced. Fig. 10 Adjustment of touch down point position B. Machine requirement Machine is capable to perform cut based on coordinate of multiple panel layout. This feature is important due to machine has to cut each panel completely before cutting the next panel. For instance, with 12 inch wafer ring, maximum 4 strips can be fitted with 212 centimeter length and 57.8 centimeter width of leadframe dimension and consist of 16 panels in total. Refer to Fig. 11. V. BENEFITS OF SRTRIP CHOPPING CUT TO MANUFACTURING INDUSTRY Time-to-market is very important to manufacturing company for future success. That s mean to be in the market at the right time and right product without losing focus on product quality, productivity and stability. Nowadays, semiconductor development is very rapid and steep. Assembly process has to find the correct balance to produce the product in short cycle time as early as development stage. This will ensure the company success factor for future market competition. Below are the benefits when implementing strip chopping cut method. A. Saving on dicing tape usage In mechanical tape dicing concept, tape dicing is the indirect material use for processing. Therefore, full strip cutting method only manage to fit 3 strips per wafer ring, but with strip chopping cut method, maximum 4 strips can be fitted in one wafer ring. As a result, more units can be produced and tape cost per piece down by 25%. B. Shorter processing time Dicing processing time includes material loading, saw street alignment, chip dicing, cleaning and unloading. Only saw street alignment and chip dicing is the effective processing time compared to other process steps. Therefore the movement from one wafer ring to another will reduce when the number of wafer ring reduce. Example, 12 strips production lot need 4 wafer rings to process. With strip chopping cut, only 3 wafer rings are needed. Refer to Fig. 12. At the same time, productivity will increase too as unit process per hour increase. Please refer to Table II, 3 wafer ring processing time reduced by 25%. Saw street alignment time and chip cutting time not included due to both layouts are processing same amount of unit quantity. Fig. 11 Maximum 4 strips mounting on 12 inch wafer ring

5 Process steps TABLE I. Processing time per wafer ring (s) STEP BY STEP PROCESSING TIME 4 wafer ring processing time (s) 3 wafer ring processing time (s) Loading Cleaning Unloading Total D. Maintain next process stability after dicing End of line assembly process generally will end at unit dicing process. The next process will be auto vision inspection, electrical test and pick and tranfer into tape and reel before send to customer. Refer to Fig 13. All those procesess, singulated unit still intact on dicing tape for robust processing. Therefore, it is important that singulated unit must maintain remain on dicing tape especially special area called fiducial mark. This fiducial mark is needed for each processess either for pattern recognition or alignment setting or unit pitch auto adjustment process. The convesional dicing method does not guarantee the fiducial stability as it could be drop or missing due to fully cut. By implementing chopping cut, this fiducial portion still connected to leadframe, therefore it is impossible to drop or missing. Refer to Fig. 14. Fig. 12 Strip mount layout C. Simplified process flow Currently, panel cutting method needs two different machines to complete the process. Jig saw dicer is to do side rail removal and then tape saw dicer for complete unit cut process. With this method, in 1 wafer ring capable to fit in 16 panels which are 4 strips in maximum layout. But, need longer time to complete due to additional process of side rail removal. Therefore by implementing chopper cutting method, process flow will be shorter and productivity will increase. Please refer to Table III. TABLE II. PROCESS FLOW COMPARISON Fig. 13 Process flow after chip dicing Dicing method Side rail removal process Strip or panel mounting Unit cut process Panel cut Chopping cut

6 time. Units that going to be collected inside bowl must be all the same unit size to avoid track jammed during sorting. Therefore, strip chopping cut method is the best dicing method due to this process only dices active unit and not dice unnecessary part surrounding units. If using full strip cut and panel cut method, all unnecessary sawn part with different dimension will mix together with sawn active units. Refer to Fig. 16. Fig. 16 Chopping cut strip shows unnecessary part will remain on dicing tape and not mix with active unit Fig. 14 Fiducial point sawn condition E. Enable different leadframe size with MIS leadframe design MIS leadframe design has thin pre-mold layer and supported by thicker side rail metal. This support is needed as pre-mold layer too fragile and also to maintain process stability at front of line assembly. This type of leadframe can be designed in any shape and size to utilize unit density per frame. With chopping cut method establish, no more difficulties as the side rail is not part of the cutting element, it just only to support fragile premold layer for active unit area. VI. LIMITATION OF STRIP CHOPPING CUT METHOD A. Material limitations High unit density per leadframe will reduce unit cost per piece and all active area has to be utilized effectively. But, with chopping cut method, the clearance area which needs to maintain minimum by x value, refer to Fig. 9, will reduce unit density per panel. This matter can be solved by increasing leadframe size to occupy more unit density per strip. Saw street width has to be narrow as possible but not too narrow in order to achieve high speed cutting. If using wider saw street, wider blade width have to apply and will cause slower cutting speed. Based on the study, optimum blade width which can run at 300 millimeter per seconds cutting speed is millimeter blade width. Besides, if using millimeter blade width, maximum speed observed is at 50 millimeter per seconds. If exceeds the speed limits, high loading on blade causing blade broken. No metal layer design at saw street. This is to avoid blade clogging and resulting blade damage. Fig. 15 Examples of leadframe design F. Enable bowl feeder process system Bowl feeder process means the sawn unit randomly collected into a bowl with spiral track inside and the machine will sort unit by unit for vision, electrical test and packing. The unit per hour produce is very high due to fast processing B. Machine limitations Accuracy of touchdown point for blade spindle is very critical. When blade spindle touchdown offset near to side metal rail, blade will cut partially side metal rail and causing blade damage or higher blade wear.

7 VII. EXTENDED STUDY In this paper, the scope of study is based on MIS design leadframe which only mold compound element at saw street. But extended study also had been done with others type of leadframe. A. QFN leadframe (leadframe thickness is millimeter) Low speed cutting at maximum 50 millimeter per seconds due to full metal layer at saw street design. Furthermore, lower touchdown speed is needed to reduce high blade wear during blade hit the work piece surface B. Laminate substrate without mold layer (thickness is millimeter) Capable to run at high speed cutting at 300 millimeter per seconds without any issue. implement. Below are the summaries of benefits as discussed in this paper. a) Saving on dicing tape usage b) Shorter processing time c) Simplified process flow d) Maintain next process stability e) Enable different kind of leadframe design f) Enable bowl feeder process system Conclusion, with the right design, right material and right processing parameter at the first place, chopping cut is not an impossible process. VIII. SUMMARY AND CONCLUSION Not many industry implement strip chopping cut method due to its complexity of leadframe design and machine processing capability. This also adds in the sentiment of rough cutting will cause blade damage, therefore high risk to [1] Nayan, N.A., Taib, A., Saw Singulation Characterization on High Profile Multi Chip Module Packages with Thick Leadframe, Electronics Packaging Technology Conference, EPTC Proceedings of 6th, December [2] Butsoongnoen, S., Jittabut, Y., Sombatklung, P., Chainok, M., Sriprasert, P., Method and Apparatus for Semiconductor Testing at Low Temperature, Patent Application Publication, September 2014.