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Elimination Mold Flash by Mold Design Enhancement and Leadframe Process Control on Flat Power Package

Kow Siew Ting & Vinod Kumar a/l Nanta Kumar, Innovation & CPE department

siewting.kow@, vinodKumar.nantakumar@.

Abstract

This paper will explained detail of challenge to eliminate mold flash in mold assist film technology on flat power package. Film assist molding uses a film is coated with an adhesive layer on one side which works as a soft cushion to cover of the heat sink / paddle. This thin film also holds the lead frame paddle for prevention the paddle to tilt. Tilting of the paddle causes the mold to flow towards the paddle area during molding process and causes a reject known as “Pad Bleed”. The flash layer on the heatsink is unable to be removed by typical deflash setting, thus it prevents tin plating coverage during plating process. As a result, the solder-able pad is unable to be mounted at end customers’ application circuit. The hypothesis of mold flash occurrence is due to paddle deformation caused by over-clamping during mold process. This paper will emphasis on effectiveness of combining mold design enhancement and lead frame incoming quality control to eliminate mold flash on exposed heatsink flatpower product using film assisted molding system. Distribution clamp force evenly in mold tool is major area to investigate for mold tool design enhancement. Furthermore, process control on supplier to minimize paddle tilting is another main contributor factor need to evaluate in order to maintain incoming quality. By identifying and establishing a control on a critical lead frame dimension and redesigning the mold tool, mold flash or resin bleed was able to be eliminated. The verification run showed paddle tilting is the most critical factor that needs to be controlled in order to minimize resin bleed issue, these thin resin bleed or flashes are able to be removed by the subsequent “Deflashing” process.

Introduction

Transfer molding is a common used molding process in semiconductor field compare to injection and compression technology due to cost effective and stability molding process control. An enhancement of technology was introduced for molding process by applicable molding film during molding process due to hanging tie bar design introduce on flat power package. Design of paddle without tie bar connected to leadframe is a challenge for mold and leadframe manufacturing control due to hanging design ease to cause paddle tilted. Severe mold bleed on the paddle area would then acts as a mold flash that would be too stubborn to be removed in the subsequent deflashing process. This then leads to expose copper being created on the paddle, causing the unit to be unable to be soldered on the board at customers side. Thus the reduction of mold bleed on the paddle is a high necessity required for flat power package due to its surface mount application. Flat power packages requires a package design without a tie bar holding on lead frame sides. Customer’s specific requirements states that there should not be any exposed copper on the sides of the package, to meet the solderability performance.

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Figure 1 with and without tie bar design for flat power package.

In order to cater to this need, film assist molding was incorporated and the package was design to be without a tie bar. Mold assist film was introduced to cover the paddle area, to make the film more efficient, adhesive layer plays a role as paddle holder and at the same time as a mold bleed blockage to stop the mold compound to flow into the paddle area during molding process. Mold assist film works flow is firstly to load a substrate to mold by uses one or two sides film with an adhesive layer. Prior to the lead frame or substrate being loaded onto the cavity, this layer of plastic is sucked by vacuum placed on the top mold cavity. Then continuous transfer molding is taken place. After molding cycle is completed, old film is release with the vacuum drop and roll back to empty film roll. A new film with certain amount of predefined length which same length of mold tool is released from the film roll to mold cavity. Film assistive molding offers a number of advantages compared to conventional molding. One of which is an easy releasability of the molded product from the top cavity, stickiness issue of runners and cull can also be eliminated with the use of 2 layers of mold film. The service life of the tool also significantly increases with the use of 2 layers of mold film which fully covers the molded parts and this has a much more wear and tear rate. Film assistive molding also provides an additional advantage, this film actually acts as a soft cushion barrier to hold or cover the hanging design product which prevents the resin bleed issue. Flat power package design has an exposed paddle or heat sink on the bottom of the package, conventional Flat power package has a tie bar holding on two side of paddle and which is connected to lead frame’s tie bar.

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Figure 2 Top FAM Concept

Methodology

In the initial mold tool buyoff, it was observed to have a serious over clamping issue which was implemented into film assistive in transfer molding. This is gauged by using a depth gauge, a 40% of additional clamping was applied on the paddle. Additionally imprint paper was also used to check and determine if the mold is balanced well, using this it was observed that the mold was not balanced evenly across, since imprint mark was less prominent on the center to the side of the mold tool for both left and right side. Thick mold flash was observed on the molded shot for both big and small die sizes, dummy frames also showed a similar behavior. Film assist mold was checked and found to be in good condition. Film wrinkling was also checked but none was observed, showing that the film vacuum was functioning well with no abnormalities. Equipment issue was ruled out after a precise and thorough check was done on the equipment. As result of no abnormality on equipment, mold shot-shot was perform additionally other than mold imprint paper checking. During mold filling process it was observed that only certain portion of die paddle covered on the top mold cavity by the film. As showed as below figure 3 the film contact area is the portion above the red line. As soon as the transfer is started the EMC starts to fill the cavity area, the EMC will flow and fill all the empty space between the top film and bottom cavity.

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Figure 3 Mechanism showed mold flash into paddle due to paddle tilted which partial area not cover by mold film.

Under high magnification scope it is visible that the die paddle has been tilted downwards thus creating a gap between the top film and die paddle area. This gap is then filled by EMC, since the gap is larger than the filler size, which then results in mold bleed or flash on the paddle area. Mold bleed will become thicker upon application of high packing pressure. There are 2 main contributors to this issue based on a thorough analysis that was done. The first contributor is heavy clamping or over clamping on the paddle area by the mold tool and the second contributor is the co-planarity of the incoming lead frame paddle itself. The hypothesis has been proven through a series of trials that was done. Both heavy or over clamping by mold tool and co-planarity of the incoming lead frame paddle are the major contributors to the issue. To validate further, additional investigation was also done on the type of film that is used. This is done via dry clamping where 2 different type of films was used, each with a different amount of thickness on the adhesive layer. Both film show result that portion paddle are not full cover by molding film which allow mold compound to flow into bottom paddle.

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Figure 4 Paddle was tilted away and create a gap between film and paddle to allow EMC flow in for both type thick and thin film

3.0 Result and Discussion

3.1 Over clamping issue

The first action that was done were performing optimization and balancing the mold tool evenly then verified on the imprint paper, where the imprint is supposed to have an even distribution of the press mark and also the marks has to be prominent with not too heavy press mark at low clamp pressure 50 Tons. Evenness is achieved by rearranging the support pillars and also by increasing the height of the pillars by 30µm on the low press mark area. This action to create balancing and evenness in each mold cavity surface for both left and right side. Hence, additional 20um thickness gauge was added on center cull sleeve to increase center height of tool to reduce over clamping on center cull block. However, an even imprint could not be achieved after several trials. Then, the second step was to increase the process delay mid clamping time from 5 to 20 sec. EMC would start filling the cavity at a lower tonnage and after 20sec the clamp tool would reach the final tonnage when EMC had fully transfer into the cavity. Thus minimizing the pad tilting due to paddle had supported fully by EMC which compacting the cavity to form as supporter or holder to paddle prevent tilted. This helped to reduce the bleeding on the paddle but it was not significant.

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Figure 5 unevenly imprint mark on tracing paper to showed unbalancing mold clamp.

Investigation was done continuously to check the clamping mark on tab and lead side. Based on mold tool design rules and clamp depth calculation by design is supposed to be lesser then 10um of the observed value in actual. Data distribution over rows and columns shows that high clamp mark depth on tab area and the clamping was distributed unevenly across the rows. High clamp mark showed 10 to 20um across the row. The data distribution had similar pattern with uneven imprint pattern which had high clamp on center and low clamp on side cavity. Heavy clamp mark depth on tab area had violated supplier design rule in mold design. As mold tool design clamping area is focusing on tab area to cause have heavy clamp and tilted the paddle on mold cavity. It allow mold bleed or flash fill up on paddle area as illustrated as figure as below.

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Figure 6 heavy clamp mark to cause pad tilted to have mold bleed

An improvement design was recommended to expand the clamping area from tab area to further lead with tie bar area. As the clamping area was increased, the force distribution on the lead frame was distributed in more evenly manner. As per calculation for the clamping area base on design rule, smaller area is not meeting clamping area over tonnage ratio. As new design which bigger clamping area are more evenly and better ratio. Increase further clamping to tie bar from 3um to 40 um and this action was significantly able to minimize the pad and lead tilting. Clamp imprint taken at 50-100 tons clamp force was found to be even and balance with and without film. Imprint paper showed good result after mold balancing. No any paddle tilted was found when empty clamp run on leadframe.

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Figure 8 improvement clamping area design on tab to tie bar

3.2 Incoming material checked

Others than over clamping on mold tool issue, verification showed that mold bleed was inherited from pad paddle tilting of incoming material. Any die paddle tilting that produce gap in between top film and die paddle which is more than 30 um (average compound filler size) will result to mold bleeding. Verification checking on incoming lead frame found 94% of units in row 1 to 3 have pad titling issue before mold clamping. The tilting of the die paddle was observed to be more than the filler size of the EMC, around 20µm. The higher the pad tilting the heavier the bleed on the paddle. Base on 3D scanning, paddle was tilted obviously from surface from 40 to 100 um which bigger than compound filler size. This allow mold compound flow in to create flash. Hence paddle will have a tendency to further tilt in negative direction after mold clamping. Base on scanning result, the serious titled row was found on first 3 rows and row no 7. After a series of brainstorming, a root cause was able to be identified. The plating process in the lead frame manufacturing was the root cause for the tilting of the paddle. Since there is no tie bar, the paddle has no support thus causing it to tilt at the 3rd and 7th rows, which are the weakest points of the lead frame. To counter react with this effect a mechanical mask was introduced and used by supplier during plating process. Further understanding of the process shows and confirms that this is due to the Ag-plating, but this was difficult to control. A new counter reaction was introduced to minimize the effect of this tilting by the mechanical masking process, at the end of the process a spanking process was incorporated at the opposite direction. As a precaution and to ensure a sustainable quality, co-planarity control was introduced. This action ensured the flatness of the lead frame or the paddle has a positive direction tilt. Positive direction of paddle which can press down by top cavity and ease film to cover fully paddle area. This caused the paddle of the lead frame to completely be covered by the film thus preventing the mold flow from filling the top cavity. As solution, incoming control had setup paddle, paddle with lead and lead titled measurement. Batch to batch leadframe need go through checked on coplanarity.

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Figure 9 Paddle titled Vs normal paddle in 3D scan.

4.0 Summary

With the both improvement of the lead frame paddle tilting and optimization of the mold tool, verification results shown was positive which mold flash had been eliminated. Zero mold flash was found on paddle in all rows.

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Figure 8 Before and After improvements

Main reason to have bleed or flash on paddle area due to tilted paddle which cause by incoming material and over clamping by mold tool. Positive direction of paddle is needed to minimize the bleed level.

Acknowledgments

The author would like to acknowledge the mold tool designer ASM and lead frame manufacturing Enomoto for supporting the experiment.

Bibliography/References



Cliff J.Scribber, James H.Knapp “REPLACEABLE COVER PLATE TO REDUCE MOLDED PACKAGE RESIN BLEED”, Number 000007203

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