THE METHOD OF MAKING LOW-COST MULTIPLE-ROW QFN

Mary Jean Ramos, Rico San Antonio, Lynn Guirit, Anang Subagio and Hadi Handoyo Unisem Singapore Pte Ltd

1 Maritime Square, #09-80 HarbourFront Center Singapore 099253

jramos@aitemail.com, rsantonio@aitemail.com, lguirit@aitemail.com, asubagio@aitemail.com and hphandoyo@aitemail.com

Abstract

QFN demand has dramatically increased during the past 3

years. It has been replacing older packages like SOIC and TSSOP mainly because of the added thermal enhancements from the exposed pad, as well as the elimination of coplanarity issues. QFN is also being chosen for next-generation packages because it has a relatively bigger pad size, allowing more flexibility in accommodating bigger die size, die integration and the added advantage of thermal enhancement. However, as the package requires more I/O in a smaller footprint, current QFN structure will have some limitations. Currently, there is a solution for two rows of I/O but any higher than that, the available solution becomes more expensive – one solution requires laser cutting to isolate each row and another requires use of photo-imaging, develop and etching process to define the I/O.

This paper describes an alternative methodology for making a low-cost QFN with multiple rows. The leadframe raw material that will be used will be solid copper, half etched on one side, with selective silver plating. The difference from existing QFN leadframes is that this does not need tape during molding.

There will be two new processes introduced: selective copper etching and electroless plating. Although these processes are commonly used in other industries, the challenges and proposed solutions encountered in adopting them in IC packaging assembly will be discussed.

Apart from the cost advantage, manufacturing and performance advantages and disadvantages related to these new methods will also be tackled.

Keywords: multiple-row QFN, Cu etching, electroless/ immersion plating.

Introduction

Leadframe-based packaging is a low-cost IC solution.

However, conventional leadframe proves to have limitation in the ever increasing complexity of the product. Moving to a substrate-based solution is inevitable since it offers a lot of flexibility through routing and multi-layer metal construction. However, the drive for a low-cost solution is still commonly preferred. Multiple-row QFN has brought the package to another step of satisfying the increase in I/O. The inner leads could be produced by using a narrow tie bar attached to the connecting bar that is holding the outer leads (refer to Figure 1, Type A). The inner leads are very challenging to make and the narrow tie bar lacks rigidity, making the wire bonding process more difficult. This results in lower productivity and

yield. On the other hand, dual-row QFN can also be produced by having the inner leads connected to the die pad (refer to Figure 1, Type B). This is more robust, but it requires a two-pass sawing process, whereby the first pass is using partial cut to isolate the inner lead from the die pad and the second pass is the final cut to singulate the units. Due to this two-pass process, the saw capacity is reduced 50 percent and is likely to produce rejects (i.e., burr, smear and metal shorting). Refer to Figure 1 to illustrate the two designs. Type A illustrates the former method and Type B illustrates the latter method.

With the current challenges on the existing dual-row QFN, a new multi-row leadless packaging was developed to offer not only dual-row but also higher I/O low-cost packaging with the advantage of better wirebond stability with patronizing standard manufacturing processes such as the single pass sawing process.

This paper will discuss in detail the methodology and construction of etched leadless package (ELP) as the new alternative multi-row packaging.

Type A Type B Figure 1. Dual-row QFN options. In Type A, the inner leads are attached to the same connecting bar that holds the outer leads. In Type B, the inner leads are connected to the die pad.

Comparison Between Multi-row QFN and ELP

Leadframe Design and Bondability

A standard multi-row QFN utilizes the conventional leadframe-making process: the majority of the leads and features are created by etching through the top and bottom of a copper stock, which is about 0.20mm thick. For Type A multi-row QFN, the inner leads are attached to a half-etched connecting bar. This makes the structure weak and prone to bent leads.

A common problem for a typical QFN with tape is the stability during the second bond formation. The use of Type

Die Pad

Connecting bar

A QFN makes this even more difficult due to longer lead lengths and thinner connecting bars.

Dual-row Type B QFN provides better stability of the leads during wirebond since the inner leads are shorter and connected to a more rigid structure (i.e., connecting bar and die pad). However, this solution requires a dual-pass package saw singulation process that results not only in low yield and productivity, but also in design limitations such as a smaller die pad and a smaller number of leads to allow path for a saw blade during inner cutting. Currently, both solutions, Type A and Type B, allow for a maximum of two rows of leads only.

ELP is a multi-row leadless package that utilizes a partially etched leadframe. The top portion is half-etched which defines the desired features such as leads, power, ground rings and die pad. The bottom portion of the leadframe is solid. Due to the rigidity of the leadframe, multiple heavy wire bonding such as 2mil Au wire, aluminum wedge/ribbon bonding and copper wire bonding, are viable. Figure 2 illustrates examples of design flexibility for ELP. Figure 2. (A) Wire-bonded multi-row. (B) Isolated power and ground rings. (C) Flip-chip (1. Peripheral, 2. Array). (D) Multi-chip module/system in package. Thermal Performance

The thermal performances of QFN and ELP are comparable because they have similar features. Refer to figure 3 for a theta ja comparison between QFN and ELP.

THERMAL PERFORMANCE

Figure 3 Thermal dissipation comparison between QFN and ELP.

Post Mold Cleaning Typical QFN comes with tape underneath to prevent mold

flash. Depending on the tape used, this will require an additional cleaning process after mold to remove the tape residue and mold flash. Current cleaning methods available are laser cleaning, chemical deflashing and mechanical buffing. These additional processes and the tape required add to the cost of the overall package by as much as 15 percent.

ELP has a big advantage in this area because it does not require tape, therefore no extra cleaning is required and the tape cost is eliminated.

Package Saw Singulation

Package saw singulation for a QFN requires metal cutting between units. The metal thickness can range from full to half the copper leadframe thickness.

There are several issues associated with this. First is low productivity: Saw speed is typical at the range of 35-60mm/sec, which is very slow compared to a standard punch singulation process. Second is high blade consumption due to metal cutting. Third is unavoidable cosmetic concerns such as copper burrs and smear.

ELP undergoes back-etching process after mold. At this stage, the copper in between units is fully etched out, leaving only 100 percent mold compound left for sawing. This results in an improved saw speed up to 120mm/sec, longer blade life and absolutely burr-free units.

Refer to Table 1 for a summary comparison of multi-row QFN and ELP.

Multi-Row QFN Features A B ELP

Leadframe Full etched Full etched Partial etched Taping Yes Yes No

Number of Rows 2 2 3 Pwr & Grd Rings No No Yes

Back Etching No No Yes Standoff No No Yes Plating SnPd / Sn SnPb / Sn Immersion Sn

Plate Thickness 7.6µm-20µm 7.6µm-20µm 1µm MIN Saw Singulation 1 pass 2 pass 1 pass

Metal Cutting Yes Yes No Typical Cut Speed 35-60 mm/s 35-60 mm/s >120 mm/s

Table 1. Basic comparison between multi-row QFN and ELP. ELP Process Flow

Generally, ELP has similar processes and applicable controls as an existing QFN package except for the etching process and immersion plating process needed to isolate and plate the leads respectively. For QFN, the isolation is done by either sawing or punching. Figure 4 shows the typical ELP process flow.

(A) (B)

(C)1 (C)2

(D)

(A) (B)

(C)1 (C)2

(D)

0

5

10

15

2 0

2 5

3 0

0 .0 0 .5 1.0 1.5 2 .0Velocity (m/s)

Thet

a-Ja

(deg

.C/W

)

ELP DP soldered to PCB VQFN DP soldered to PCB

Figure 4. ELP process flow. Backside Etching

In backside etching, the features are pre-defined by a pre-plated mask. The process of masking can be done during leadframe assembly or after the mold assembly process. Since masking is an established process in leadframe assembly, it is preferred to perform the masking during leadframe preparation for process simplification and better control such as placement accuracy.

The mask is either NiPdAu or Ag and can be the same material plating used on the leadframe bonding side. Ag masking is used because it employs straightforward stripping before final plating. For NiPdAu, the stripping process is very complex and costly due to the multiple stripping processes needed (i.e., separately stripping Ni, Pd and Au). For this reason, Ag masking is selected, as the process is simple and takes only one stripping procedure.

To isolate the leads, rings and pads, the leadframe material needs to be back-etched. This is done after the mold process. After etching the backside to define the features, it is optional to leave the Ag mask as a final finish. However, the Ag mask will have overhang after etching. Since the mask is very thin, it will tend to deform at any direction, which is cosmetically unacceptable. Stripping off the Ag mask and re-plating it with the desired finish is suggested.

There are two industry standards available for copper etching: acid-based and alkaline-based. So far, there is no known process that uses Ag as a mask. In selecting the etching process, it is important to understand the chemical’s effect on the Ag mask.

Ferric chloride and cupric chloride are the most commonly used acid-based copper etchants, and ammonium chloride is used as an alkaline-based copper etchant. Based on the study, both chemistries were able to etch the copper, but acid-based etchants easily attacked the Ag mask. After etching, the Ag mask turned a dull color and was difficult to strip off.

Ammonium chloride slowly etches the Ag when exposed to it for more than 1 minute. The etch time can speed up by increasing the temperature to about 55 degrees Celsius to make the chemical more aggressive. However, it shortens the life of the bath due to the high loss of ammonia that results in

inconsistent etching. Increasing the pH will also increase the etching rate of the chemical. This eliminates the Ag attack and can operate at a lower etching temperature which stabilizes the chemical. Another observation is that ammonium chloride does not affect the Ag mask and thereby enables the Ag stripping process. For reference, a starter makeup has 8.2-8.6 pH. The pH can be adjusted by the supplier and is maintained by adding ammonia liquid or gas during the process.

Further evaluation was done to fine-tune the process using alkaline-based etchant. There are two challenges: etching consistency and attack on the Ag mask. Etching consistency is defined as the etching rate uniformity on every strip processed, which can be measured. The attack on Ag mask is a visual criterion evident in the surface of the pad. Once the Ag mask is attacked, the texture of the copper will be rough or, in the worst case, etched. Refer to Figure 5.

Figure 5. Attack on the Ag mask. There are four components in the alkaline etchant that

may affect etching consistency and Ag mask attack. These are pH, copper content, temperature and chloride content. A process guideline study for alkaline etching was used as reference for the alkaline etching optimization.2 It was determined that all of these factors must be analyzed and controlled in order to find the fastest etch rate and the least amount of undercut. The pH is a measure of the relative amount of free ammonia (NH3) that is available to the etching process. The lower the pH, the less the undercut. However, in order to not attack the Ag mask, there is a need to bring up the pH. On the other hand, if the copper content increases, the amount of undercut decreases. Therefore, to compensate for the high pH effect on undercut, the copper content must be increased. The recommended range is between 140~165 gm/liter Cu. The chloride concentration indicates the amount of ammonium chloride (NH4Cl) present in the system. As the chloride concentration increases, more copper metal can be held in the solution, allowing a decrease in the amount of undercut. The chloride component also acts as a buffering agent in the etchant, permitting a narrow pH window. The etch rate will increase with the increasing bath temperature, but so will the undercut. The best compromise between etch rate and minimum undercut exists when the alkaline etch baths are run at temperatures between 43~48 degrees Celsius with 55 degrees Celsius as a maximum. Lower temperature drives off less ammonia, making control of pH much easier. A good balance between these four main factors must be maintained to obtain the best and consistent etching. Table 2

below summarizes the effects of these four factors and the recommended controls.

Further experimentation has been conducted to confirm the impact on etching speed and time to the etching quality. In the experiment, it was noted that lower speed increases etching rate due to an increased in etching dwell time. For a higher temperature setting, etching dwell time can be set shorter. while longer dwell time is needed if lower temperature setting is used. Main Factors Process Effect Test Method Control

Method Copper Concentration

Affects etch speed Too high = risk of precipitation Too low = slow etch

Titration (e.g., with thiosulfate, weekly)

SG controller High = dump Low = dissolve copper salt

Chloride Concentration

Use to hold copper in the solution Too high = solubility limit Too low = slow etch

Titration (e.g., silver nitrate, weekly)

High = dilute with concentrated ammonia Low = add ammonium chloride

pH Free ammonia is needed to complex and keep cuprous and cupric salts in solution Too high = lowers etching rate due to copper dilution Too low = attack on silver mask

pH meter (handheld, daily)

High = evaporate Low = add aqueous ammonia or ammonia gas

Temperature Affect etch rate High = higher etch rate but rapid loss of ammonia to ventilation Low = low etch rate

Thermometer (weekly)

Heater/cooler on/off; adjust set point

Table 2. Effects of etching factors and recommended controls. Electroless/Immersion Plating

After backside etching, surface finishing is applied on the isolated circuitry to prepare the material for board mounting. While the leads and pads are already isolated, conventional electrolytic process is not applicable and electroless/immersion process is the suitable process. There are two designs in the market: horizontal and vertical immersion plating processes. The difference between the two is that for horizontal process, the material is fed through a conveyor, while for vertical, racks or trays are used to carry the materials and dip them from station to station. Immersion or electroless process is a longer process compared to electrolytic. To achieve the plating thickness required for electroless/immersion plating, the materials must be submerged in the chemical for a long period of time. This is where the decision must be made to either go vertical or horizontal. If there is a space constraint, the vertical process is recommended since a horizontal process could be as long as 40 meters. If there is no space limit and the capacity requirement is high, the horizontal process is the best choice. Both processes should have similar output in terms of quality. In this paper, the process used was vertical

There are many immersion processes that can be used for ELP: electroless Ni and immersion Au (ENIG), immersion tin (ISn), organic solderability preservatives (OSP) and immersion Ag (IAg). Each has its pros and cons. Refer to Table 3 for a basic comparison.

Factors ENIG ISn IAg OSP Solder Joint Ni-Sn Cu-Sn Cu-Sn Cu-Sn

Plating Thickness

Ni 3-6 µm Au ~ 0.08 µm

>1 µm >0.127 µm 0.01 µm

Contact E-test E-test E-test No Whisker No Yes No No Shelf Life 24 mo 6 mo 12 mo 6 mo

Cost $$$$ $$$ $$$ $

Table 3. A basic comparison of electroless/immersion plating chemistries in the market.

ENIG is a very stable plating solution. However, the cost

is high and the process control is challenging since there are two metal components involved, nickel and gold. Also, waste treatment must be considered due to the cyanide component of Au. OSP is the cheapest solution but there is concern of visual in-process control since it is colorless. There is also a concern for electrical testing since it is non-conductive. Immersion Ag is a cheaper solution than ENIG. The plating time is very quick and low-temperature with plating thickness only above 0.127 µm. However, IAg is very sensitive to the environment and is easily tarnished. Good material handling and environment control must be employed. Immersion Sn is comparable in cost to immersion Ag. The thickness of ISn is also relatively thin but doesn’t easily tarnish compared to IAg. In IPC -4554 section 3.2.1, the immersion Sn thickness shall be 1 µm minimum at 4 sigma. To achieve ISn thickness, the plating process must be extended. Typically, it requires about 30 minutes dwell at 70 degrees Celsius in the main plating chemistry. This will define the length of the plating system.

The plating thickness of ISn being only over 1 µm may affect solderability due to quick intermetallic growth. It is then suggested to have a 6-month shelf life to ensure solderability. There is also a question on whisker growth common to Sn plated products. ELP is considered as leadless package and is exempted from the whisker growth category based on JEDEC JESD201, as the leads will be fully covered by solder during board mounting. However, the chemistry selected has a whisker-limiting agent.

Studies on the kinetics of whisker growth state that whisker is produced by compressive stress coming from the buildup of Cu-Sn IMC during storage.3 This stress is relieved by diffusion of Sn atoms resulting in whisker growth. This whisker can occur along the Sn grain boundaries or near lattice defects (see Figure 6).

Figure 6. Whisker growth along the Sn grain boundaries and near lattice defects.

Whisker can be controlled by thermal excursion or what is

commonly called baking. This process increases the oscillation of atoms within a crystal and at the same time heals the lattice defects. However, since ISn is very thin, thermal excursion or baking is not ideal since the temperature will speed up the IMC growth that affects solderability. With this as limitation, change in the chemistry must be made. The chemical identified for ELP has a proprietary additive that changes the Sn crystal structure to control diffusion. The additive blocks the pathway along the grain boundaries and heals the lattice defect, thus preventing whisker growth. With solderability still a question due to the thickness, the chemistry process requires pre-plating of a dense Sn layer prior to final plating. This was done at a low temperature of about 25 degrees Celsius. The purpose of this pre-plating is to minimize IMC growth, ensuring better solderability after long storage and multiple reflow processes. Refer to Table 4 for information from the whisker growth study.

7-months Data Collection Test Chemistry

26.04 06.05 17.05 31.05 09.07 07.09 22.11

Standard 0 4 7 7 9 9 9 Ambi-ent

temp. Additive 0 0 0 0 0 0 0

Standard 0 1 1 1 1 1 1 2X Reflow Additive 0 0 0 0 0 0 0

Table 4. Whisker growth study. Rating @ 100x, where, 0=0, 1=<10µm/<5pcs, 4=<50µm/<5pcs, 7=>50µm/<5pcs, 9=>50µm/>10pcs. ELP Package Construction

The etching process makes ELP construction different, as

shown in Figure 7. Since the leadframe is half-etched at the beginning, a conventional locking feature is not applicable, such as “lip” on lead tips and pad edges. A different approach can be applied, such as slots on pad and irregular lead shapes and profiles. Also, the external leads will be tapered as a result of the etching process. The lead will have higher standoff to about 2-4 mils (50-100µm). Refer to Figure 8 for an ELP and QFN lead comparison. This feature will have stronger soldering to the PCB and at the same time will provide helpful space for flux cleaning when required.

Since the lead has a narrow tip compared to conventional QFN and has more leads, test socket manufacturers need to design good and reliable contacts. Several suppliers have reviewed the design requirement and suggested that it can use conventional socket design. So far, two sockets were tested using 228L 12x12 ELP with triple-row, 0.5mm pitch. The test insertion is only one pass with 100 percent yield.

Figure 7. ELP package construction. Figure 8. ELP and QFN lead comparison.

Second-level reliability studies were performed to define the limitations of the new lead design. These were done at an independent lab in Hong Kong. The tests included package pull test, mechanical drop test, mechanical bending test and temperature cycling test. Refer to Table 5 for test references and conditions.

Test Reference Testing Condition Failure Criteria

MechanicalDrop Test JESD22-B111

Condition B g-level: 1500g Duration: 0.5 ms

20% resistance increase

MechanicalBending

Test IPC/JEDEC-9702

Load Span: 70 mm Support Span: 90 mm Strain Rate: 5000 micro-strain/s

20% resistance increase

TC JESD22-A104-C

Condition G Max. Temp: 125oC Min. Temp: -40oC Ramp Time: 15 mins Dwell Time: 15 mins

> 10 kΩ (not specified in

the standard)

Table 5. Second-level reliability reference.

The package pull test was done in comparison with 10x10 72L QFN package. To have one-on-one comparison, only the outer leads of ELP were soldered to the PCB and both die pads were unsoldered. The resulting pull forces were very close to each other, with ELP showing two failure modes (lead and pad). Lead failure mode is observed when the pull force is <150N. Therefore, ELP lead pull strength is comparable with QFN. Refer to Figure 9 for a package pull data comparison between 10x10 QFN and 10x10 ELP.

Remarks: 1: Overall package 2: Leadframe 3: Die attach epoxy 4: IC die 5: Gold wire 6: Mold compound 7: External leads/ pad profile 8: Plating/surface finish

1

6

54

3

2

87

HALF-ETCHED

QFN

QFN

ELP

FULL LEAD ELP

Figure 9. ELP vs. QFN package pull data. In the mechanical drop test, a high-speed oscilloscope was used to monitor the daisy chain resistance in real time. The oscilloscope can only measure voltage, not resistance. Therefore, an extra bridge box setup is needed. The voltage across the 100-ohm resistor was measured. If the package failed during the drop test, the resistance of the daisy chain would increase and the voltage across the resistor would drop to zero. The units passed the mechanical drop test since no failures were observed on all samples after dropping each PCB up to 30 times. Refer to Figure 10 for the mechanical drop test setup and results.

Figure 10. Mechanical drop test setup and results.

In the mechanical bending test, the crosshead speed was 2.54 mm/s. A strain gauge was installed in the middle of the PCB to monitor the load, displacement and strain reading in real time. The samples passed the mechanical bending test since no failure was observed in all the test boards. Refer to Figure 11 for the mechanical bending test setup and results. Figure11. Mechanical bending test setup and results.

In the temperature cycle test, each unit in the board was

monitored in real time inside the chamber. The ELP samples tested has reached >1,300 cycles @ -45/+125 degrees Celsius without any failure and was still an ongoing test. Refer to Figure 12 for temperature cycle test setup.

Based on board-level reliability design studies, an 8x8 QFN package was able to reach 1,126~1,356 cycles in -45/+125 degrees Celsius conditions.4 Table 6 shows that ELP performance is comparable with QFN package. Figure 12. Temperature cycle test setup.

140N 150N 156N 270N

QFN

ELP

136N 230N QFN (Break @ Pad)

ELP (Break @ Pad) ELP (Lead Failure)

140N 150N 156N 270N

QFN

ELP

136N 230N QFN (Break @ Pad)

ELP (Break @ Pad) ELP (Lead Failure)

-40

-20

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140 160 180

Time (min)

Tem

pera

ture

(C)

Temperature Profile

Table 6. QFN lifecycle reference. Package Reliability

Internal features such as pad and lead designs promote interlocking with the mold compound and contribute to package integrity during reliability tests.

The ELP internal feature design is different from the QFN internal feature design, but it offers the same interlocking strength as shown in its reliability test results in Table 7.

QFN interlocking has defined corners and extended lead lengths while the ELP interlocking is on the curved half-etched portion of multiple leads.

MSL 2 @260C

TCT -65C/150C 500 cycles

Autoclave 121C, 15 PSIG,

168hrs

HTST 150C

1000hrs Lot 1 0/77 0/77 0/77 Lot 2 0/77 0/77 0/77 Lot 3 0/77 0/77 0/77

Table 7. Reliability results on 68L 6x6, 2row ELP @ MSL2 and 260C IRR. Assembly Challenges

As the construction of ELP limits routing of bond post, it

is expected to have long wire lengths, especially for triple-row applications. The die sizes are typically small with fine pitch bond pads requiring small bonding wire diameter. This impacts the wirebond and molding capability. Wirebond Process

The wire length is increased by 30-50 percent and requires multi-row bonding. Standard loop or square loop type for QFN is practically not adequate for ELP, as the latter package requires good vertical clearance between different loop tiers and extra kinks to support the longer wire lengths. It is therefore recommended to use a loop profile capable for >3 kinks for better loop stability. Refer to Figure 13 for an optimized multi-tier looping profile evaluated on two wirebond platforms. W/B Platform A W/B Platform B Figure 13. Looping profile from two wirebond platforms using 0.9mil Au wire diameter with 160 mils span.

Mold Process With complexity in wire layout, it is necessary to

determine wire sweep performance after mold. Parameter optimization was conducted to get the best mold response to eliminate risks of wire short and wire sagging. Also, a new mold compound type was considered to ensure robustness in the molding process.

The property of this new mold compound is such that it puts less pressure on the wires during the mold transfer process. It has lower narrow gap flow pressure (NGFP) that is most suitable for longer wires with multiple wire loop tiers. As shown in Figure 14, the new molding compound shows better wire sweep response than the standard mold compound.

0

1

2

3

4

5

6

7

8

9

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24W

iresw

eep

Standard New Figure 14. Wire sweep response between standard and new mold compound. Conclusions

ELP is a solution to bring leadframe-based packaging

closer to substrate-based packaging. It offers flexibility that a standard leadframe package cannot support such as heavy wire bonding, split pad, SIP, etc. Its design improves yield by eliminating known issues at assembly such as second bond stability, mold flash and tape residue, all of which has a direct impact to packaging cost.

Though the ELP package-locking feature is different from QFN conventional locking design, the package- and board-level reliability test results are comparable.

ELP introduces new processes that need to be considered, such as back-etching and immersion plating. Back-etching uses alkaline-based chemistry and plating uses immersion type. These processes are not new but were used in different industries. It is shown that with fine-tuning, these processes will be able to support ELP requirements.

With the limitation of routing and with the higher I/O design, the wire length will be longer and bring challenges to wirebond and molding capability. It is needed to optimize the looping parameter and mold parameter to ensure good wire clearance. Using a low-pressure mold compound also helps improve the manufacturability of ELP.

The simplicity of the process and the advantages at saw singulation and mold processes makes ELP the best available alternative solution for low-cost multiple-row leadless packaging.

Acknowledgements The authors would like to thank Wei-Leong Lee of

Atotech for support in the ISn evaluations, Sharon Loh of MacDermid for support in the etching evaluations, Lowel Begonia of KNS for W/B evaluation support, Raymond Koh and CS Lim of ASM also for W/B evaluation support, Benedict Yuen of ASM for support in mold evaluation, and Clarence Loh and Chen Ping of Sumitomo for support in mold evaluation. The authors would like to thank AIT’s QA-FA team, Denny Muharyadi and Tanti Rahayu for the SEM and DECAP support. References

1. ELP patent, U.S. patent Nos. 6,777,265, 6,812,552 and

7,129,116 2. Chemcut, “Process Guidelines for Alkaline Etching”

(Technical Report for Alkaline Etching) 3. Atotech Deutschland GmbH, Sven Lamprecnht, Carl

Hutchinson, “Immersion Tin - Kinetics of Whisker Growth” (technical paper for whisker-free ISn plating)

4. Advanced Packaging PennWell with title “Board Level Reliability Design by Tong Yang Tee,” reference for QFN board-level reliability

5. JEDEC MO-239, “Thermally Enhanced Plastic Very Thick, Quad Flat No Lead Package,” reference for dual-row QFN

6. IPC -4554 section 3.2.1, reference for minimum thickness requirement for ISn

7. JEDEC JESD201 section 1 scope stating QFN type package is excepted to whisker growth criteria