ull Wing

Whitepaper

Side-Wettable Flanks
enable AOI on Leadless SMD (DFN) packages
By Hans-Jürgen Funke, Nexperia

Quad flat no-lead (QFN) and discrete (or dual) flat no-lead (DFN) packages provide
a welcome increase in component density on a PCB area by eliminating leads.
However, the solder connection quality cannot be tested by automated opticalinspection. This paper explores solutions to this often costly problem.

Many applications are space-limited which has led to thedevelopment of QFN (Quad Flat No lead) packages withconnection pads only on the underside, thus increasing
component density on a PCB area by eliminating leads.

Figure 1: Example packages: DFN1006D-2, DFN1110D-3, DFN2020MD-6

Advantages of Leadless Packages

lat Lead

No space required for connections• Less boardspace for comparableelectrical function• Or more performance at same boardspace

FNEnables heigth reduction because of ﬂatleadframe (no leadframe for DSN)

SN

Optimized thermal performance• Large heatsink under the package• Short disctance from active Silicon to solder connection

Package dimensions

Function
per PCB areah:0.9Gull Wing

h:0.63

Flat Leadh:0.37DFN

h:0.12

DSNMorefunction per
PCB area

Figure 2: Advantages of leadless packages

Such packages are used in high volume for discrete
semiconductors, termed as DFN - Discrete (or Dual) Flat No
leads – characterized by small size and low number of I/Os.
(Figure 1). A huge variety of DFN packages is now available.
As well saving space, the internal construction of DFNpackaged devices leads to a reduced thermal path (Figure 2).
However, QFN/DFN packages suffer the disadvantage thatthe solder connection quality can only be fully inspectedby costly x-ray processes, rather than Automated Optical
Inspection (AOI), because the solder connection is only
underneath the plastic body of the package.
The automotive industry, in particular, wishes to use AOI,
so companies such as Nexperia have taken a long look atsolutions to this challenge.

DFN packages are assembled in a similar manner as leaded
packages except that a group of several products are moldedwith epoxy plastic in one shot. All QFN / DFN package
leadframes consist of Cu alloy base material. Many of themare plated with a NiPdAu layer stack which is pre-applied by
the leadframe supplier and which guarantees an oxide freesurface for chip attach, wire bonding and, on the connection
pads, for wetting with solder. Optionally, the NiPdAu layer may
be additionally plated with tin. Cutting into individual devices
Advantages of Leadless Packages
is done after electro galvanic tin plating. Of course, this makes
No space requiredfor connections
tin platingof the side flanks of the bottom pads which are• Less boardspace for comparableelectrical function
exposed after sawing impossible. The material of the side
• Or more performance at same boardspace
flanks of the DFN package pads is Cu alloy (leadframe baseEnables heigth
reduction
becauseﬂat and so a wetting with solder in thematerial)which
may ofoxidize
leadframe (no leadframe for DSN)
reflow soldering process depends on storage conditions anddurationand therefore cannot be guaranteed.
Optimized thermalperformance• Large heatsink under the package• Short disctance from active Silicon to solder connection

Side-Wettable flanks guarantee solder wetting of sidepads for low I/O DFN packages
To overcome this challenge, a solution has been developed
which covers the side flanks with plated tin in the same
electro galvanic plating step as used for the bottom pads.
This technique is only applicable for DFN packages with up
to four pads (more if multiple pads are fused together) andthe pads need to be on opposite sides of the package. Platingon all four of the pad´s side flanks on a DFN package is notpossible with this method. Figures 3 and 4 show detail of a
DFN package with wettable flanks.

With the full tin-plated side-wettable flanks it is guaranteedthat the complete side pad surface will be wetted withsolder during the reflow soldering process. An important
advantage is that the plating layer on the side flank is as
thick as on the bottom pads - around 10µm. This guarantees
a wettable surface even after long periods of storage.
Examples of the optical appearance of side flanks after
soldering are shown in figure 5 for a DFN2020-6 packagewith and without side-wettable flanks and in figure 6 forthe two-pad DFN1608-2 package.

100% solder wetting solution
with new 2x2 mm leadlesspackage
DFN2020MD-6› No complete wetting on sidepad› Quality of solder connectiondifficult to determine› Optimal visual solder inspection›Very limited options for› High-quality solder connections

optical solder inspection

Figure 3: Detail view of side-wettable flanks - DFN2020MD-6 package

Figure 5: AOI example comparison of a DFN2020-6 package with SWF
versus bare Cu side flanks after soldering

Figure 4: Cut away view of DFN2020MD-6 package

Figure 6: Appearance of side-wettable flanks (DFN1608) aftersoldering2Whitepaper, Side-Wettable Flanks

AOI capability and proof
The main purpose of the side-wettable flanks, of course,
is to facilitate a reliable AOI capability for DFN packages.
Thus, costly x-ray inspection can be skipped.

AOI

AOI

Additional benefits of low pin count DFN packages withside-wettable flanks
An additional benefit of DFN packages with side-wettableflanks is that the mechanical robustness of the bond to thePCB is improved when compared with devices without sidewettable flanks.
Improved mechanical robustness
Maximum board bending
Very high board bending capability
for designs with ﬂexible PSBs

Minimum tilting angle
Reduced tilting angle for ultra ﬂat
PSB designs

Shear bit

Maximum shear force
Optimized for high shear forces
for robust soldering

pcb

45mm

45mm

Solder ﬁlet (meniscus) formation

Figure 7: AOI enabled DFN package with Side-Wettable Flanks (SWF)

Figure 9: Board level robustness improvements of DFN packages withside-wettable flanks

A condition that must be considered is that the PCB solder
pad size must be extended to be larger than the package
dimension to allow space for the solder to build a meniscus
or filet. The solder footprint recommendations of suppliers
which offer packages with side-wettable flanks include thisextra space.

As shown in Figure 9, the shear force required to dislocate thepackage off the PCB is increased due to the meniscus thatis formed after soldering. Shear force data for a DFN2020-6
package with and without side-wettable flanks have beencollected. Overall, 80 samples each had been sheared on thePCB after soldering. The results show that the shear force isimproved by about 10% with side-wettable flanks: also thestandard deviation is improved (see Figure 10).

DFN2020-6 with SWF

DFN2020-6 without SWF

Shear force average and standard deviation
1210
Shear force in kg

To examine the suitability of Nexperia’s side-wettable flanks
for AOI inspection, multiple test boards were built with solder
footprints modified to accommodate the SWF package. The
printed solder paste volume has been modified deliberately on some PCB solder pads no solder was printed (see example
figure 8). Working with a leading AOI equipment vendor, it
was confirmed standard AOI techniques are able to reliablyidentify soldering failures using the DFN packages with SWF
after reflow soldering.

89.03

8.16

6

Number of samples:
80 per variant

420

DFN2020-6

DFN2020-6_SWF

Figure 10: Shear Test on PCB for a DFN2020-6 package with and without side-wettable Flanks

Solder paste printing without fails (a)Solder paste printing without fails (a)Board bending tests also confirm an increased robustness
for DFN devices with side-wettable flanks which is a result ofadditional features at the package solder pads that achieve abetter anchoring to the plastic body. Summarizing the data, it
could be proven that board bending depth for the DFN10062 package with SWF is up to 14 mm. For some passive chipcomponents of same size the bending depth is often specified
with 1mm.
Solder paste printing with failure (b)Solder paste printing with failure (b)Figure 8: Example of solder failures on test board

3

Whitepaper, Side-Wettable Flanks

Side-Wettable Flank solutions for DFN packages withmore than six I/Os
For DFN/QFN packages with multiple I/Os (above 6) andleadframe thicknesses of 200µm and above, one alternative
is to use dimples on the side pads. Dimples are pre-etched
and NiPdAu plated together with the bottom pads by theleadframe supplier. Device separation is done at a point
between two adjacent packages in the middle of the etcheddimples. The wettable feature size formed by the dimples issmaller than for the galvanic tin plating solution described
previously. Usually packages with side-wettable flanks madein this way will be delivered with NiPdAu pad plating, i.e.
without additional tin plating on the pads. Figure 11 gives anexample of such a package without, on the left, and with thedimple feature, on the right picture.

Figure 11: Example of multi I/O DFN/QFN package with dimples toachieve wettable flanks

Another alternative is partial separation of the DFN packages
after the molding but prior to tin plating, also known as the“saw plate saw” method. Sawing is performed just to a depth
to partially expose the side flank. This means that the pads arestill connected by the remaining metal part of the pad flanks
ensuring that the continuity of the leadframe is maintained
for the galvanized plating process. Full device separation,with a thinner saw blade, is done after tin plating. Due to thenecessary sawing tolerances, this method is - like the dimples
alternative - only suitable for leadframes greater than 200µm
thick. Note that the complete height of the side flank is notcovered with tin.

Figure 12: Example of multi I/O DFN/QFN package with step cut
approach to achieve wettable flanks

Nexperia offers leadless packages with the sidewettable flank option across its full product portfolio,including Logic and ESD protection devices, MOSFETs,
diodes and bipolar transistors. Today, 11 package
versions are available, and the portfolio is growing.
A video at www.youtube.com shows how Nexperia’s
leadless packages are meeting the requirements of
the automotive industry. By featuring side-wettableflanks they allow a visible solder joint to develop
enabling automatic optical inspection. At the sametime, they help save space in vehicles with increasing
semiconductor content due to more electronic
functions while maintaining the high safety andreliability standards needed in automotive applications.
For more information visit: www.nexperia.com© 2020 Nexperia B.V.
All rights reserved. Reproduction in whole or in part is prohibited without the prior written consent of
the copyright owner. The information presented in this document does not form part of any quotationor contract, is believed to be accurate and reliable and may be changed without notice. No liability will
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imply any license under patent- or other industrial or intellectual property rights.

nexperia.com

Date of release:
February 2020