35th International Electronic Manufacturing Technology Conference, 2012

Combined Molding Package (Proximity sensor)

YF Lee, CC Kum, Carsem Technology Center

Carsem (M) Sdn Bhd S-Site (Ipoh/Perak/Malaysia) Lot 52986, Taman Meru Industrial Estate, Jelapang.

cckum@carsem.com.my yflee@carsem.com.my

Abstract Proximity sensors, which are designed to detect the

presence of nearby objects without any physical contact. Proximity sensor basically employ an Infra Red LED transceiver and Light receiver to achieve proximity detection purpose.

Existing proximity sensors comprise of 1 Infra red LED chip and 1 light receiver chip. This 2 separate chips setup has disadvantages of occupying more space and high assembly cost which does not meet the requirement for future application in smart phone and tablet PC

Many current processes produce optical proximity sensors with poor optical isolation and unfavorable noise (e.g., light leakage) between the light emitter and light detector dies, among other poor performance characteristics. Furthermore, current molding processes utilize slow and inefficient human-operated manual casting methods that produce low and inconsistent manufacturing yields.

This paper will present an innovative method of combining Infra red LED chip and receiver chip into 1 package with enhanced infra red shielding capability that keeping cross talk to extremely low level. The paper also elaborate importance of using FEA method in material selection and series of evaluation to accomplish development of combine molding package (patent filed by Carsem) 1. Introduction

Proximity sensor is a device commonly used for detection of present of object and also able to measure distance of object from the sensor, it has become an essential device use in every smartphone and tablet PC today. Its primarily functions include to disable the touchscreen when answering call and adjust screen brightness to conserve battery power.

Proximity sensor packaging solution for excellent transmission and detection characteristics are most sought after to meet ever growing market demand. Similarly, the new packaging solution must meet manufacturing requirement of producing optical proximity sensors in faster, cost efficient and high volume production mode.

There are many existing packaging solution available in the market, which involves combining two individual Infra red LED and Light receiver IC in a module. This method is relatively more costly, require bigger foot print on print circuit board ,besides, longer assembly cycle time and lower production yield. There are other packaging solution which is using premolded cavity package that requires gel filling, some are come with lens attached on top of the package, there are some using cast molding to form clear molding package , and followed by metal lid attached. The

metal lid form as barrier between the IR Led and IC light sensor. Obviously, these methods incur higher cost but produce lower output, require complex human dependent manual operation.

This situation formed a gap between packaging solution and product demand. Therefore, it is necessary to develop a packaging solution that able to address all the current short coming in existing product.

To take hold of this opportunity, Carsem has developed and patent filed an innovative packaging solution which is called Compak that combine 2 chips (IR Led and IC sensor) into 1 common platform. At the same time it can prevent cross talk (false triggering) between the 2 chips. Ref Figure 1. [1]

Figure 1 Combine molding Package Construction. This Compak technology uses the unique idea of

assembling package using mutiple types of molding compounds. This Compak molding processes utilizes transfer molding technology, both IR-LED and IR receiver IC dies will be encapsulated, while forming a solid barrier to prevent cross talk effect, as such the signal produced by IR LED die can be filtered by the barrier from reaching the receiver die prematurely as to prevent cross talk effect.

2. Combine Molding Package Conceptual Development

The placement of Infra Red LED and Light receiver die onto 1 package posted issue of cross talk effect. Either this is the effect which infra red ray penetrated through internal package and reached the light receiver or the material used is not opague to IR ray.

This cross talk effect will degrade the performance of proximity sensing. Thus, it is important that construction of the package design must provide excellent infra red shielding between the 2 dies, likewise, the IR ray from IR led can be directed and transmitted to target object, the target object will reflect back the IR ray and received or detected by the light sensor die.[2]

Object

Infrared LED

IR Receiver / Ambient Light

Sensor

Opaque Side Wall

Opaque Middle wall

Object

Infrared LED

IR Receiver / Ambient Light

Sensor

Opaque Side Wall

Opaque Middle wall

35th International Electronic Manufacturing Technology Conference, 2012

The IR ray can be easily reflected and scattered by any object, this scattering effect can also produce significant signal disturbance that eventually become the unwanted noise or cross talk effect. In the design stage of the Compak package, proper IR ray isolation within IR LED and prevent it from scattering must be careful considered

To achieve this requirement, at first, the bottom stucture of the package must be studied, as such whether both the dies to be attached on laminated substrate frame or copper based lead frame. This is due to existance of any gap in between leadframe or substrate can act as a light transmittance path for infra red to pass through, thus, creating possibility of cross talk effect.

Secondly, careful study of how to encapsulate both dies that provide a medium for light transmittance and at the same time provide good protection for dies and wire from mechanical damaging and moisture ingression that could caused premature reliability failure.

Thirdly, barrier wall that able to isolate both dies and it does not allow any infra red ray to penerate through. The process of forming the barrier wall with infra red ray filtering capability required complicated process. It is a process of forming clear compound to protect each individual dies and allow formation of opaque barrier that can block infra red ray to pass through. In term of structure, it form a mechanical barrier wall between two dies.

The subsequent section will present in detail the method used for material selection, evaluation and validation of this new semiconductor packaing solution.

3. Mold Compound Material Selection

Clear molding compound is the best candidate to encapsulate IR LED and Ambient Light Sensor devices that allow IR ray to transmit out from package and at the same time protect the chips from moisture ingression or achieve better moisture sensitivity level.

As clear mold compound does not contain silica filler particel in order to maintain high transparency and high light transmittance capability. The mold flowability issue can be addressed by using vacuum molding. However, clear compound with materail nature of high CTE, it tends to cause higher warpage behavior, thus s caused frame warpage after molding process. [3]

3.1 Clear Compound Warpage Simulation Study

This section shows methodology to select the best clear compound type to use for this package. There are two major compound material characteristic are considered, ie. Tg and CTE value.

Compound properties for warpage FEA simulation as

follow:- Table 1. Clear Molding Compound Technical Propertise Run Clear CTE 1 CTE 2 Tg (°C)

Compound (ppm/k) (ppm/k) 1 Type A 68 170 100 2 Type B 65 172 130 3 Type C 71 180 143

Compound Type A Strip Warpage ( mm )

Compound Type B Strip Warpage ( mm )

Compound Type C Strip Warpage ( mm )

Figure 2.Warpage Simulation Result

Figure 3. Frame warpage by compound type

Based on simulation result, figure 3, clear mold

compound with higher Tg point exhibits lower frame warpage.

Based on the result derived from the warpage simulation, compound Type C to be selected for this package.[4]

Figure 4, Mold compound shrinkage with different compound Tg

Compound type C exhibited lowest warpage

0

20

40

60

80

100

120

Type A Type B Type C

Maximun Strip Warpage (mm)

Frame warpage reduced ~30% with high Tg compound

T1Tg

Tota

l S

hrin

kage

Room temp Mold tempT2Tg

Tota

l S

hrin

kage

High Tg compound

Low Tg compound

T1Tg

Tota

l S

hrin

kage

Room temp Mold tempT2Tg

Tota

l S

hrin

kage

High Tg compound

Low Tg compound

35th International Electronic Manufacturing Technology Conference, 2012

In order to explain the reason of the high Tg good for warpage control. Referring figure 4, it is a graph showing two different compound with different Tg value, by comparing cure shrinkage for both compound with different Tg, the graph display clearly that the level of compound shrinkage can be reduced when Tg point of mold compound increased. Therefore, clear compound type C with higher Tg is selected to be used in this package. [4]

There is no significant difference in CTE value between three compound types, the simulation result shows that the CTE difference could also make improvement in warpage. 3.2 Clear Compound Stress Simulation Study

Apart from warpage simulation, compound stress is also a crucial measurement point to find better reliability performance. Compound with relatively low stress will enhance the overall reliability. [5]

Mold Compound Type A

Max. S1 : 89.248 N/mm2

MMoolldd CCoommppoouunndd

Mold Compound Type B

Max. S1 : 64.929 N/mm2

MMoolldd CCoommppoouunndd

Mold Compound Type C

Max. S1 : 63.501 N/mm2

MMoolldd CCoommppoouunndd

Figure 5. Compound Stress Simulation Result Figure 6. Stress by compound type graph

Compound stress simulation result, figure 6, shows that compound type C has the lowest stress which will improve in reliability performance. 4. Frame/ Substrate Thickness Selection

Frame or Substrate thickness is a crucial factor that will determine overall package warpage. The purpose is to find out the most optimum substrate thickness that produce minimum package warpage. This section will discuss using FEA method to perform warpage simulation to analysis the degree of package warpage with various substrate core thickness.

For this simulation, 2 assumption will be used which is mold cap of K mm and 1 type of clear compound properties.

Figure 7. Package construction

The warpage simulation used 4 frame/ substrate core thickness as follow:-

Table 2. Substrate Information Run Design Core

Thickness Subtrate Thickness

1 Dual Layer 1x 1y 2 Dual Layer 2x 2y 3 Dual Layer 3x 3y 4 Dual Layer 4x 4y

Thickness 1x, 1y Thickness 2x, 2y

Max warpage = 5554um Max warpage = 4562um

Thickness 3x, 3y Thickness 4x, 4y

Max warpage = 4113um Max warpage = 4013um

Figure 8. Warpage Simulation Result

0

10

20

30

40

50

60

70

80

90

Type A Type B Type C

Maximun Stress in Compound (N/mm2)

Compound stress reduced ~29% with high Tg compound

Compound type C exhibited lowest stress

Mold Cap = K mm

Core Thickness= x Substrate Thickness= y

35th International Electronic Manufacturing Technology Conference, 2012

Core,Substrate Thickness vs Warpage

0

1000

2000

3000

4000

5000

6000

1x,1y 2x,2y 3x,3y 4x,4y

Figure 9. Warpage result by substrate type

Based on simulation result, figure 8 & 9, the core and substrate thickness reached stabilized and saturated warpage with 3x,3y thickness. This indicated that further increase in substrate thickness will not produce significant warpage reduction and thicker substrate will increase cost.

Thus, from designing with cost in mind, the substrate

thickness of 3x,3y was selected in this project. [5] 5. Barrier / Canal Forming Evalution

The role of Canal Forming is a process that provide a shape for infra red filtering mold compound to form and eventually constructed a barrier wall surrounding clear mold compound. Black compound barrier wall is the most important part of the package to eliminate cross talk effect and at the same time improve signal to noise ratio performance.

One of the proof of concept studies is to evaluate different saw blade grit size that able to produce smooth and even canal and follow by barrier wall dimension to achieve minimum cross talk effect.

Blade grit size selection to achieve even and smooth canal formation. 2 type of blade grit size were used for this evaluation.

Figure 10. Canal Photo with Different blade type

Canal forming blade type B produced even and smooth finishing and also passed dimension buyoff meeting 1.67 cpk. Figure 10, Thus,this blade will be selected to use in this project.

Barrier wall dimension figure 9, will focus on altering wall height(H) and width (W) that capable of shielding IR from penetrating through that caused cross talk effect.

Figure 11. Cross Section of package Table 3. Barrier wall dimension

5.1 Functional & Proximity Testing Methodology

Assemble sample unit with predefined barrier wall width and height dimension. The functional test with fixed current 100mA supply to infra red LED. The first step of the test is to determine noise signal, it is tested with no detection object placed in front of the proximity sensor, amount of proximity sensing output signal was recorded as noise prox count. Then, place object at 50mm away from proximity sensor and measure proximity sensing output and record the detection prox count. Figure 12, Next, calculate signal/noise ratio, the higher the number, the better is the proximity sensing sensitiveness. Repeat the test on unit with different dimension of barrier wall height and width and collect data for prox count for noise and detection signal.[5,6]

Evaluation result, table 3, with various dimension of barrier height(H) and width (w). From the proximity functional test result, the combination o f W4 and H4 able to provide best IR shielding and attained highest S/N ratio. Thus, barrier dimension of W4 and H4 will be applied in this package.

Blade Type A Blade Type B

Blade Type C

Run W (mm)

H (mm)

Noise

Signal S/N

1 W1 H1 2763 3250 1.2 2 W2 H2 1450 3152 2.2 3 W3 H3 865 3241 3.7 4 W4 H4 78 3234 41.5

IR LED Light Sensor

H

Highest s/n ratio

W

Frame warpage stabilized

Observation: Wavy and jagged canal Failed canal dimension <1.67cpk

Observation: smooth and even canal Passed canal dimension >1.67cpk

35th International Electronic Manufacturing Technology Conference, 2012

Figure 12, Proximity testing setup

6. Summary of Selected Material and Configution

In summary, upon completed series of material and process study with both FEA simulation and sample build. The list of selected material and configuration are as follow:- Table 4.Summary Table of Selected Material

7. Assembly Validation

By combining the most optimum material based form previous study, actual unit was assembled and key data was measured to validate against the warpage simulation result.figure 13. Figure 13. Photo of molded substrate Table 5. Warpege Comparison

The result in table 5 revealed that the difference between warpage prediction through FEA simulation and sample build are comparable with less than 10% difference. 8. Functionality Validation

Upon finalized material set and configuration that able to meet proximity detection functionality. Actual sample was build with the optimum material set and subjected to functionality test.

All units were subjected to functional test to measure proximity detection (prox count) with various current setting of 25mA, 50mA, 100mA & 150mA.

Based on test result, figure 14, it able to pass functionality requirment and achieved acceptable s/n ratio over the current range. [5]

Figure 14. Proximity Sensing Test Result 9. Reliability Validation

The next development phase is to validate reliability performance on these combination of material.

As for reliability testing, the units will be subjected to MSL 3 @260°C relfow test and measure level delamination before and after stress. Figure 15. CSAM result Before and after stress

CSAM result figure 15 revealed that no measurable delamination was detected at all interface. This shows that the combination material set able to withstand and passed Jdec MSL 3 requirment.

Further long term reliability test with temperature cycle was carried out to assess the operation life span of this package. Figure 16. Temperature Cycle Test Result Summary

Temperature cycle test of -40°C/+85°C passed 1000 cycle. This shows that this package able to pass operating life test. Figure 16.

Substrate Clear Compound

Blade Type

Barrier wall

Dual Layer 3x,3y

Type C Type B

W4,H4

Simulation Warpage

Actual Warpage

Diff%

4.1mm 4.3mm +4.9%

Top scan Thru Scan Top scan Thru Scan

Post Assembly Post MSL 3

Top scan Thru Scan Top scan Thru Scan

Post Assembly Post MSL 3

0

500

1000

1500

2000

2500

3000

3500

4000

10 mm 30 mm 50 mm 70 mm 90 mm 120 mm 150 mm

Distance

Pro

x. c

ount 25mA

50mA100mA150mA

Prox Detection Functional Test

0

500

1000

1500

2000

2500

3000

3500

4000

10 mm 30 mm 50 mm 70 mm 90 mm 120 mm 150 mm

Distance

Pro

x. c

ount 25mA

50mA100mA150mA

Prox Detection Functional Test

0/750/750/750/750/75Eval 1

1000 cycle

750 cycle

500 cycle

200 cycle

TC (-40°C / +85°C)o/s

TestLot

0/750/750/750/750/75Eval 1

1000 cycle

750 cycle

500 cycle

200 cycle

TC (-40°C / +85°C)o/s

TestLot

Warpage Measurement

Prox Count100mA

Detection Object

IR LED Receiver

Intended object detection signal

50mm

Detection Object

IR LED Receiver

Intended object detection signal

50mm

35th International Electronic Manufacturing Technology Conference, 2012

10. Result and Discussion The construction of this package with 2 molding

compound with great difference in technical properties especially for CTE and Tg point has raised critical concern of mismatch of thermal mechanical expansion. As a result, this combination of 2 mold compound will induced stress into the package and may cause premature failure in reliability test.

However, with detail study of the material through FEA simulation, it has successfully identified material that will produce minimum warpage. When warpage reduced, the overall mechanical stress that generated and induced to the package will be reduced proportionally.

From the first FEA study to identify the most optimum substrate thickness, the result has proved the selected substrate thickness able to control frame warpage within assembly process capability

Clear molding compound warpage and stress simulation has identified that Type C clear mold compound able to achieve both low warpage and compound stress. The simulation was confirmed with actual reliability result that passed MSL 3 and long term reliability test 1000cyc temperature cycle. [5]

Barrier wall dimension has been proven to provide perfect IR shielding capability; this was shown in functionality test that able to perform required proximity sensing, and at the same time, improved signal / noise ratio.

10. Conclusion

The concept of combining 2 molding compound into one package has provided a cost effective and flexibility solution to package proximity sensor into one package.

Extensive material study was required in order to select the most optimum material set that capable of achieve manufacturability, functionality and reliability performance.

With this innovative combine molding package concept materialized, it provided a packaging solution for proximity sensor that capable of mass production, flexibility of changing package size to meet market requirement and accelerated time to market.

Acknowledgement:

The authors like to thank LW Yong, BM Chan, CW Chow, SL Liew, KM Yong, SS Liew, Ravinder for their valuable contribution to realize this project.

Reference 1. Suresh Basoor, Peng Yam Ng, Deng Peng Chen “Infra

Red Proximity Sensor with Reduced Cross Talk” US2099/0159900 A1, Jun 25 2009.

2. Dr. Bernhard Stojetz, Osram, “Ambient Light Sensor and Proximity Sensor SFH7770 Application Note”

3. W.H Zhu “Cure Shrinkage characterization and its implementation into correlation of warpage between simulation and measurement.

4. Ernst, L.J., et al (2006) “ Fully Cure-Dependent Modeling and Characterization of EMC with Application to Package warpage Simulation”. In Proc.

Of IEEE CPMT Int. Symp on Advance Packaging Material 2006, Atlanta, March 2006.

5. Mohammed Wasef & Mike Anderson“Evaluation of High Temperature Overmold Compounds for Manufacturing of Laminate Based Leadfree System in Package ”

6. Dr. Hubert Halbritter , Osram, “SFH7773 (IR-LED + Proximity Sensor + Ambient Light Sensor) Application Note”.