Temperature Cycle-Induced Die Metal Shearing In Surface Mount Devices After Infra Red / Vapour Phase Reflow.

S. Kumar, Goh Jing Sua, David Lam Tai Wai

Texas Instruments Singapre

ABSTRACT

Moisture-Induced "Popcom" effect in Surface Mount Devices during Infra Red or Vapour Phase Reflow manifests itself as visible cracks originating from the edges of the die or the leadframe mount pad. More robust packages often give rise to only microcracks after re@w which subsequently require further thermal stresses to induce device or package failure. This paper analyses the secondary effect of die metal shearing in SOJ Packages subjected to Temperature Cycling after an Infra Red Reflow. This phenomenon has signijkant bearing on the current practice of preconditioning packages using Infra- Red or Vapour Phase Reflow before subjecting them to reliability tests like Temperature Cycling. A model is developed to explain the relationship between package microcracks after reflow and subsequent metal shearing when subjected to Temperature Cycling. The metal shear lines are characterised using failure analysis techniques to validate the model. Various methods of improving the SOJ package resistance to microcracking during rejbw are discussed.

1. Introduction

Moisture induced package "popcorn" cracking in surface mount devices during solder reflow is a big concem in the industry today. The conventional method employed by semiconductor manufacturers to ascertain the moisture sensitivity of surface mount packages involves subjecting the parts to varying levels of moisture (85% RH, 60% RH and 30% RH are common) followed by infra-red or vapour phase reflow. Severe intemal package cracking and severe delamination of the mold compound from the die surface or leadframe is used as the criteria to classify a surface mount package as moisture sensitive. JEDEC test method A112 uses a Si level moisture classification system, the highest of which (level 1) requires packages to exhibit robustness to 168 hours of moisture soak at 85% RH and 85 deg C followed by infra-red or vapour phase reflow at a peak

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temperature of 220 deg C. Robust packages, even under JEDEC level 1 moisture test conditions, may only exhibit internal microcracks after reflow and require further thermal stresses like temperature cycling or thermal shock to induce device or package failure. The different rates at which the various components of a plastic package - the die, leadframe, mold compound - expand and contract in a temperature cycling test creates intemal package stresses which may cause package damage. This paper discusses the role of Infm Red reflow preconditioning and Temperature Cycling in causing stress related die metal shearing in plastic SOJ packages.

13% failures were observed when a sample of memory devices in 300MIL SOJ package was subjected to 3 cycles of infra red reflow followed by 500 cycles of Temperature Cycling at -651150 deg C. Preconditioning samples to a series of infra-& reflows prior to reliability stressing is a common industry practice [9] and is intended to simulate end customer use. Other reliability tests like Operating Life test at 7V/125 deg C, Temperature-Humidity-Bias test at 85%RH and 85 deg C and Autoclave test on samples from the same batch of material did not show any failures.

2. Failure Mechanism

The failures were slot decapsulated with fuming nitric acid at 70C after electrical failure analysis. Inspection with a 200X optical microscope showed the polyimide (PIX) overcoat at the comers of the die to be peeling. Removing the PIX with ethylene diamine showed s W a r damages to the Protective Overcoat (P.0).

The P.O. was removed with Plasma etching and Common Oxide Etch to reveal the metallisation below. The metallisation at the affected areas were observed to have jagged cracks. Extensive metal shearing was observed. The failing device has a two metal system - AlSi over TiW. The AlSi metal layer on the failures were observed to be displaced inward from the TiW. This observation, together with the direction of the cracks, suggest a shearing force acting from the edge on the metallisation causing the cracks CFigl).

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Liquid Crystal technique was employed to find the existence of current leakages. Most of the leakages were found to occur at areas where there were cracks. The electrical failure signatures were found to be consistent with the physical fail mode.

infra red reflow process and the subsequent temperature cycling. This would explain the metal shearing at the corners of the die. A degradation in the adhesion of the AlSi metallisation to the underlying TiW metal layer or a change in the AlSi metal grain structure could also explain the metal shearing during the Temperature Cycling test. These 2 factors, together with P.O./PM overcoat stress and mold compound stress, formed the basis for further analysis.

Possible Csosea Unitol Measure

P.OlPix DynesCM2

AlSiMetal TapeTestl Adhesion Scratch Test

overcoat stress

A commonality study of the 4 factors was conducted between lots which exhibited metal shearing and lots which did not show this failure mechanism. The results showed no s i ~ c a n t differences in observed values of these factors between the two groups of material. This ruled out the four factors as possible causes of the metal shear failures.

Measured Values

Lots LotsWitbNo Condusion Showing MetalSbear

2.58Eo9 2.59EO9 NoDiffexence

Good Good NoDiffmnce

Metal Shear Fig 1: Metal Shearing at the comer of the die

ALSi Grain Size

MoldCompmi stress

Another group of failures were analysed with a Scanning Acoustic Microscope (SAM). Package/die delamination was observed at the centre of the package. Cross-section analysis on a sample of failures with delamination showed cracks originating from the die attach interface and from the corner of the mount pad (Fig 2). Cracks on the package were observed to be parallel to the package's length.

Microw 1.13 1.13 NoDiffexence

Stresshdex 3.51 3.60 NoDifference

Fig 2: Cross section of package showing cracks

3. Diagnosing The Problem

It is clear from the physical failm mode that the failures are caused by stress on the metal. The crack patterns on the metallisation support this conclusion. In diagnosing the problem, 2 stress factors were initially idenflied as possible causes of the metal shear problem - the protective overcoat/PM (polyimide) overcoat stress and mold compound stress. An increase in either one of the stress factors or both, due to drifts in the respective processes, would result in a larger stress on the metallisation during the

Table 1: Commonality study of possible causes between lots showing metal shearing and lots with no metal shear

Failure to establish a relationship between the package stress factodmetal properties and the failure mechanism caused a shift in focus to the package microcracks observed on the failing units. Further analysis and evaluations showed: e Package microcracks were observed on units

immediately after 3 cycles of Infra Red reflow. No evidence of metal shearing were seen on these failures. Microcracks were noted to propagate from the die attach area or mount pad towards the lead fmgers during the temperature cycling process. All failures which showed metal shearing after Temperature Cycling also showed package microcracks.

e

A model was developed to explain the above phenomenon.

4. Modelling the Failure Mechanism

It is evident from the failure analysis findings that package microcracks is the primary failure mechanism and that the metal shearing is a secondary effect caused by temperature cycling of units with microcracks. The following model is proposed to explain the relationship between package microcracks and metal shearing during temperature cycling.

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Plastic packages absorb moisture when exposed to the atmosphere. Moisture absorption studies on 300 mil SOJ package show that the package becomes fully saturated with moisture within 96 hours (4 days) in a 3040% relative humidity environment. Package microcracks develop along the die attach area or from the edges of the mount pad due to moisture expansion during Infra Red reflow or Vapour Phase reflow. This is a well understood phenomenon. Cyclical expansion and contraction of the various components of the plastic package during temperature cycling causes the package microcracks to propagate from the die attach area or the mount pad to the lead fingers and eventually to the package surface. The package “popcom“ effect and the subsequent crack propagation during temperature cycling is shown in Fig3.

moisture absorption

die - - mount pad

t t (a)moisture absorption

mici ...~. :ing :h area

The secondary failure mechanism of metal shearing requires as a pre-requisite, at least a partial separation of the die from the mount pad. The package microcracks, with their origin at the die attach area, cause the didmount pad separation required for the onset of metal shearing. The silicon die, once partially or fully separated from the mount pad, is fjree to undergo thermal expansion and contraction during temperature cycling, as shown in Fig4. The coefficient of thermal expansion (CTE) of the mold compound and the polyimide PIX are much higher than those of the protective overcoat (P.O.) and the silicon die in a composite plastic package. This difference in the CTEs of the various components of the package explain the nature of the thermal deformation shown in Fig4.

At -65 DEC C

die attached die

mount pad

At 150 DEC C

die attached die

ro-cracks from die

mount pad

(b)3 cycles of Infra Red Flow

(c)Temperature cycling (-65 deg C to 150 deg Cl

f ig 3:Propagation of package microcrack during Infra- Red reflow and Temperature Cycling

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F’ig 4: Thermal Expansion and contraction of components of an SOJ package during temperature cycling

The interfacial stresses that occur between the various layers of the package, namely, the mold compoundpolyimide PIX interface, the P W . 0 . interface and the P.Olsilicon die (metallisation) interface, during temperature cycling, are shown in FigS. The adhesion between the mold compound and the poyimide P E layer and between the PIX and P.O. is very much higher than that between the P.O. and the surface of the die (metallisation). Adhesion measurements confirm this conclusion. The poorer adhesion between the P.O. and the surface of the die causes this layer to shear during temperature cycling, The shear action of the P.O. over the metallisation on the surface of the die during temperature cycling causes metal shearing and metal cracking to occur. The edges and the corners of the die which are freer to deform during the temperature cycling process has the highest probability of exhibiting metal shearing and cracking. Physical failm analysis of the fdures confirms this conclusion.

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6. Improving Package resistance to "POPCORN" Cracking

At -65 dee C At 150 dee C

- silicon U

Fig5:Stresses acting on the components of an SOJ package during temperature cycling

5. Confirmation of Findings

Samples of 4Meg DRAMS in 3oomil SOJ package from different assembly lots were subjected to a split evaluation Half the samples were subjected to Temperature Cycling (-65/150 deg C) with 3 cycles of Infra Red reflow preconditioning while the rest of the samples were subjected to Temperature Cycling (-65/150 deg C) without the 3 cycles of preconditioning. Electrical and physical data were taken at 3 readpoints - 100,200 and 500 cycles. At each readpoint, all the devices in the sample were subjected to an electrical d.c./functional test and a small sample pulled for cross-sectioning and deprocessing. The cross sectioning was intended to detect package microcracks while the deprocessing, to detect metal shearhg. The results of the evaluation (Table 2) showed package microcracks and metal shearing in the split with 3 cycles of Infra Red reflow. The split with no preconditioning of samples showed no signs of package microcracks or metal shearing.

Table 2: Comparison of 3x Infra Red preconditioning vs no preconditioning on 3OOmil SOJ samples. Data points show sample size and no. of fails.

Extensive research and studies have been done to prevent pop-corn cracking and develop more robust packages. Some of the techniques used include radical changes to the leadframe and package design, addition of adhesion promoters and buffering material between the mold compound and the die/mount pad and migration to very low stress mold compounds. All the techniques used aim to reduce the enormous stresses experienced by the package during solder reflow by providing better thermally matched interface materials, stress buffering and better adhesion . Some of these techniques ace discussed below.

One of the most commonly used method of preventing package cracking during reflow is to use leadframes with slots or openings in them. This allows the mold compound better adhesion to the backside of the die and reduces the amount of thermally incompatible material between the die, mountpad and the mold compound. Another modification to the mount pad involves making dimples or roughening the backside of the mountpad . This improves adhesion of the mold compound to the mount pad thus preventing mold compound/mount pad delamination that occurs before package cracking . Techniques like these have the disadvantage of introducing extra processing and hence extra costs. Dimples and roughening, for example, require chemical processing of the leadf"es,thus increasing the cost of the leadframe and introducing chemical impurities.

A solution for "popcorn" cracking developed by Texas Instruments Singapore involves depositing polyimide on the backside of the mount pad to promote adhesion to mold compound and act as a buffer for stress. This technique works well in preventing popcorn cracking but incurs additional processing in the coating and curing of the polyimide.

Another solution developed by Texas Instruments Singapore is to radically change the shape of the mountpad. The relationship between the die size, mountpad size and package size are important factors in l ' p ~ p ~ ~ r n ' ' cracking. Evaluations [l] have shown that the width of the package has a bigger impact on package integrity than it's length and a reduction in stress occurs with reduced pad size. Using these guidelines, 9 different mount pad designs (Fig 6) were developed and packaged in 3OOmil SOJ package using 2 different low stress mold compounds - Shinetsu 165VA and Shinetsu 180VA. These designs, when subjected to stringent 85C/85%RH + WR/IR REFLOW and -65/150 deg C Temperature Cycling [2] showed excellent resistance to "popcorn" cracking. Die pad #lo, which shows extensive internal package cracking failures, is the control split using a non-optimised pad.

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7. Conclusion

(*) - conwol split Fig 6: Optimised Mount Pad Designs and

corresponding moisture resistance performance

kad-On-Chip (LOC) package is designed primarily for real estate savings and electrical considerations but it has a secondary advantage that it does not have a mountpad. Mold compound adheres directly to the backside of the die, improving adhesion and reducing the number of interfacial materials. This improves the package’s resistance to “popcorn” cracking.

Wg 7! Lead-On-Chip (LOC) package

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Die metal shearing in plastic surface mount packages during temperature cycling was found to be a secondary failure mechanism. The root cause of this phenomenon is package “popcorn” cracking which occurs during Infra Red preconditioning of the devices prior to temperature cycling. Thermal expansion and contraction of the corners and the edges of the die, separated from the mount pad by package cracks, create shear stresses in the weak protective overcoat, resulting in metal cracks. Numerous methods of preventing package “popcorn” c racbg were discussed in the paper. Leadframe designs with optimised mount pads and slotted mountpads offer good margins to package ‘popcorn” cracking.

8. Biography

S. Kumar received his Bachelor of Engineering in Electronic and Electrical Engineering from the National University of SingaporeNanyang Technological Institite in 1986 and his M.Sc. in Industrial Engineering from the National University of Singapore in 1990. He joined T.I. Singapore in 1986 as a Quality and Reliability Assurance engineer. At present, he is the QRA Operations Manager in T.I. Singapore with responsibility for reliability engineering activities.

Goh Jing Sua is the Advanced Package Development Manager in Texas Instruments Singapore. He joined T.I. in 1988 as a backendsymbol engineer and worked through the whole packaging process - cleanroom assembly, mold and trimform. His current responsibilities include new product/package introduction in MOS Memory DRAM and technical support to MOS memory Non-Volatile plastic products. His work interest is to explore packaging technology and to match technology development to T.I.’s business objectives and goals.

David Lam Tai Wai is a Senior Failure Analysis Engineer in T.I. Singapore. He graduated from Nanyang Technological University of Singapore in 1989 and has been working in T.I. for the last 6 years. His working experience includes a year as a QRA engineer and 5 years as a Failure Analysis engineer for 1 Megabit, 4 Megabit and 16 Megabit DRAM. His interests include Electrical Failure Analysis, Fault Isolation and software programming.

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References

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Lim T.B., Shim K.C. and Teo B.C., "Popcorn Package Integrity without PIX-On-Pad", 1st Memory Packaging and Test Conference, 14-19, (1993)

Michael Raj Marks, "New Thin Plastic Package Crack Mechanism Induced by Hot IC Die", Proc Rel. Phys. Sym., 190-197, (1992)

Richard L. Shook, "Moisture Sensitivity Characterization of Plastic Surface Mount Devices Using Scanning Acoustic Microscopy", Proc Rel. Phys. Sw., 157-167, (1992)

Lim T.B., Tan G.L., "A criterion for predicting delamination in plastic packages", 1st Memory Packaging and Test Conference, 31-45, (1993)

Terry Steiner and David Suhl, "Investigations of Large PLCC Package Cracking During Surface Mount Exposure", IEEE Tran. on Components, Hybrids and Mfg Tech., 209-216, (June 1987)

Walter H. Schroen, "Challenges of LOC Packages and Overview",Proc SEMICON Singapore Technical Conf, 255-264, (1993)

JEDEC Test Method A l l 2 Moisture-Induced stress Sensitivity for Plastic Surface Mount Devices

9. JEDEC Test Method A113: Preconditioning of Plastic Surface Mount Devices Prior to Reliability Testing.

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