GQFN Application
Notes
APPLICATION NOTES
Purpose
PCB Design
This document provides guidelines for Printed Circuit Board
Assembly (PCBA) for Surface Mount Assembly for Grid Array
Quad Flat No-Lead Package (GQFN).
PCB Land Pattern Dimensions
Prior to assembly, PCB's shall meet the solderability requirements
in ANSI/J-STD-003 and be flat to within 0.1 mm per linear cm.
PCB's shall meet the design requirements in IPC-D-275 and must
conform to Solderability Test for Printed Circuit Boards in ANSI/JSTD-003. The carrier surface geometry will have the following
characteristics:
The PCB land pattern should match the GQFN terminal and
exposed pad dimensions, as indicated in the UTAC package outline
drawings. The PCB terminal should be approximately a 1:1 ratio of
the package lead used on the PCB land pattern for the following
GQFN dimensions:
• Vias in the PCB lands are allowed on the condition that the
through vias are plugged and top metallized, for thru-hole mother
board design, or the micro-vias are plated up for build up mother
board design. This is to prevent solder from leaking through.
• e = terminal pitch
• Thermal vias should be used on the PCB thermal pad to boost
thermal performance, the number depending on the package
thermal requirements, as determined by thermal simulation or
actual testing.
• Øb = terminal diameter
• PCB terminal pads are recommended to be non-solder mask
defined.
• The solder mask opening dimensions are recommended to be as
tight as possible to ensure that some solder mask remains
between the PCB pads.
• Solder mask misregistration shall not reduce the effective length
or width of the PCB pad by more than 0.05mm.
• No residues (e.g. undeveloped resist) shall be visible on solder
pads at 7X.
Stencil Design Guideline
Stainless steel stencils are recommended for solder paste
application. A stencil thickness of 0.075mm – 0.127mm (3 –5 mils)
is recommended for screening.
Stencil aperture sizes can be 1:1 with pad size, however certain
parts may require reduced apertures to reduce solder ball defects.
Stencil thickness and aperture openings should be adjusted
according to the optimal solder volume.
Solder Paste
“No clean” paste is recommended for assembling the GQFN to the
PCB due to its low standoff and small pad openings. For fine
terminal pitch, such as 0.40mm, type 4 solder paste is
recommended. Upon receipt from the supplier, solder paste shall
be stored and handled according to the paste manufacturer’s
recommendations.
Component Placement
QFN/DFN packages are small, have fine pad pitches, and typically
require 0.050 mm package placement tolerance and low placement
pressure to prevent paste smearing or squeezing out of the solder
joint. Because of these considerations, high precision component
placement machines are recommended.
Reflow Profile
The packages are attached to the PCB by convection mass reflow
techniques as part of standard SMT processing.
Thermal profiling of the convection / IR reflow machine is required
for each product design.
• For 6 3 /37 eut ec t i c s ol d er , t h e r ef l ow t emper at ur e s ha l l
not exceed 240°C with time above liquidus temperature
(183°C) of 45 seconds minimum.
• For Pb-free solder, the reflow temperature shall not exceed
260°C w i t h t i m e a b o v e l i q u i d u s t e m p e r a t u r e ( 217°C) o f
4 5 seconds minimum.
• Standard CDA atmosphere is acceptable but the use of nitrogen
is recommended.
• The airflow may need to be reduced in some cases to prevent
the lightweight parts from shifting or being blown off. The
temperature would have to be adjusted accordingly to maintain
the reflow profile.
• Do not exceed the maximum motherboard temperature
recommended by the supplier.
TEMPERATURE, °C
Eutectic Solder Reflow Profile
Board Rework
GQFN rework processes are very similar to, and in some cases a
simplification of, ball grid array package rework processes. The key
components of this process are:
• Board preheat to ~120°C to avoid warpage
• Solder reflows of component to be removed
• Vacuum removal of component
• Cleaning and preparation of PCB lands
• Screening of fresh solder paste
• Placement and reflow of new component
• Inspection of solder joints
Several automated rework systems a r e a v a i l a b l e off-the-shelf
which address the previous steps in several ways. The rework
steps summarized previously (except inspection) can be
accomplished with high precision in a single machine under either
automatic or manual mode.
Closed-Loop, computer-controlled time, temperature and airflow
parameters help ensure repeatable and reproducible processes.
The system software controls the reflow profile (preheat, soak,
ramp, reflow and cooling). In addition, the board temperature can
be monitored closely to prevent warpage during the rework
process.
Miniature solder stencils may also be used for reapplying solder
paste after package removal.
Solder Reflow
TIME (SECONDS)
TEMPERATURE, °C
Pb-free Solder Reflow Profile
TIME (SECONDS)
A heated gas nozzle surrounds the device to be removed.
Conductive heating may also be used for small body sizes. The
GQFN is heated from the top side with hot gas while residual heat is
exhausted up and away from adjacent components. The entire
assembly is also heated from the bottom side with an under-board
heater to help prevent warpage. Succeeding photos were taken from
Air-Vac Engineering showing 0201 components for illustration
purposes.
Component Removal
Once the solder is completely reflowed, the component to be
removed is typically picked up by a vacuum nozzle, which slowly
lifts the component off the pads. Automated machines disengage
the nozzle if the solder has not fully reflowed to prevent lifting the
PCB pads. The application of flux is recommended.
Cleaning and Preparation of PCB Lands
Residual solder must be
removed from the PCB
pads after the component
is removed. Due to the
small PCB pad sizes used
on the GQFN, the site
redress process must be
performed very carefully
to avoid PCB damage.
No-clean paste flux is
applied to the site after
component removal. Using a temperature-controlled soldering iron
fitted with a small flat blade, gently apply solder braid that has
been presoaked in flux over the PCB pads. Residual flux is
removed from the site with alcohol and a lint free swab. The site is
then inspected prior to the replacement process.
Component Replacement and Reflow
A component insertion tool is
used
to
ensure
proper
registration of the GQFN
package in the nozzle. Optical
systems are used to assist the
operator in ensuring proper
alignment.