Previous Page  37 / 68 Next Page
Information
Show Menu
Previous Page 37 / 68 Next Page
Page Background

35

Chip Scale Review March • April • 2017

[ChipScaleReview.com]

Optimization of die attach to surface-enhanced lead

frames for MSL-1 performance of QFN packages (part 1)

By Senthil Kanagavel, Dan Hart

[MacDermid Performance Solutions]

This article is part 1 of a two-part series. Part 2 focuses on MSL-1 evaluation of the material combination.

uad flat no-leads (QFN)

semiconductor packages

represent one of the steadiest

growing types of chip carriers, and they are

predicted to continue growing as original

equipment manufacturers (OEMs) strive

to put more signal handling into a smaller

space. Owing to their low-profile, condensed

form factor, high I/O and high thermal

dissipation, they are popular choices for chip

set consolidation, miniaturization, and chips

with high power density, especially for the

automotive and RF markets. As with any

package, reliability is critical, and due to their

widespread acceptance, OEMs, integrated

device manufacturers (IDMs) and outsourced

semiconductor assembly and test suppliers

(OSATS) demand continued improvements

in the reliability of QFNs.

Chemical processes that treat the

surface of copper lead frames to enhance

mold compound adhesion and reduce

delamination in chip packages deliver

improved reliability in QFNs. These

chemical processes result in micro-

roughening of the copper surfaces, while

concurrently depositing a thermally robust

film that enhances the chemical bond

between the epoxy encapsulants and the

lead frame surface. Typically, this type

of process can reliably provide JEDEC

MSL-1 performance.

While this chemical pretreatment process

provides improved performance with respect

to delamination, it can create other challenges

for the lead frame packager. Increased surface

roughness magnifies the tendency for die

attach adhesives to bleed (epoxy bleed out,

or EBO), causing the silver-filled adhesive

to separate and negatively impact package

quality and reliability. Additionally, any

epoxy resin that bleeds onto the lead frame

surface can interfere with other downstream

processes, such as down-bonding or mold

compound adhesion (

Figure 1

).

Anti-bleed or anti-EBO coatings have been

developed to control the amount of bleed, but

different adhesives can have different physical

properties (surface tension, percent solids,

viscosity, etc.) that impact the interaction

with the anti-bleed coatings. Consequently,

the selection of die attach adhesive can

be critical to package performance. This

article examines the appropriate methods for

optimizing both die attach adhesive chemistry

with state-of-the-art lead frame technology.

P e r f o r ma n c e a t t r i b u t e s f o r

achieving MSL-1

MSL-1 performance is typically

attributed to a number of factors in the

semiconductor package. The various

ma t e r i a l s s u c h a s e poxy mo l d i ng

compound, die attach material, lead

frame alloy type and surface chemistries,

as well as the die type and size, all

influence the performance of the package

a s a who l e . The pe r f o rmanc e and

interaction of the individual materials is

important in preventing delamination in

the package during MSL-1 testing. This

article focuses on the key material

interactions and their effects on

MSL-1 performance.

Conductive die attach typically

will undergo stress during the

MSL-1 exposure and reflow so it is

important it maintains its properties

and does not initiate delamination

with the lead frame surface or die

back side (

Figure 1

). The other key

factor that contributes to the failures

is epoxy bleed out or resin bleed out.

The resin from the epoxy will bleed

onto the lead frame surface. This

can cause loss of adhesion to epoxy

molding compound and result in

delamination during MSL-1 (

Figure

2

). In addition, as the epoxy bleeds

onto the lead frame, the composition

of the adhesive under the die

changes—less epoxy and more

silver. This can impact the adhesion

of the die attach to the lead frame

or the die, and result in an adhesive

failure, as opposed to the desired

cohesive failure mode. So, it is very critical

for the die attach to not cause any significant

bleed out on the lead frame surface.

With the challenges driven by the move

to lead-free electronics components,

reflow temperatures have increased

significantly. This move triggered

a reduction in reliability at MSL-1,

specifically delamination of epoxy molding

compounds (EMCs) and die attachment

from the lead frame surface. To improve

MSL performance, many semiconductor

packagers have turned to different methods

for adhesion improvement. The most

popular of these is generically termed

“brown oxide” or “alternative oxide,”

which roughens the copper lead frame

surface while concurrently applying an

organometallic coating.

The brown oxide mechanism comprises

an intergranular etching process that

selectively etches small gaps between

copper grains of the lead frame alloy. The

etching composition includes organic

Q

Figure 1:

Typical construction of a QFN package showing EBO

from die attach material.

Figure 2:

Delamination observed due to EBO from die attach.