IPC-D-279 EN.pdf - 第121页
GUIDELINE AREA OF QUALITY/RELIABILITY IMPROVEMENT Eliminate Engineering Changes on Released Products Fewer errors due to changeovers & multiple revisions/versions Lower assembly error rate Make Assembly Easy and Fool…
Appendix K
Design for Manufacturability and Assembly Checklist
K-1.0 SUMMARY
Definition of manufacturability: a measure of the ease or
simplicity with which a product can be manufactured or
assembled.
These guidelines deal with Design for Manufacturability
(DfM) primarily at the detailed component and process
levels and the focus is on doing things right, particularly
with respect to improving quality and reliability; if the
DfM process is used early in the design process, the focus
can be on doing the right things in terms of system organi-
zation and setting testability expectations. More benefits
are derived from early use of conceptual design and DfM
processes.
Section Topics:
• Minimize number of parts
• Minimize number of part numbers
• Design for robustness (Design of Experiments)
• Eliminate adjustments
• Design for efficient and adequate inspection and test-
ing (testability)
• Eliminate engineering changes on released products
• Make assembly easy and foolproof (Poka-Yoke)
• Use repeatable, well-understood processes
• Choose parts that can survive (are compatible with)
process operations including rework, repair and main-
tenance
• Choose or design process for compatibility with sus-
ceptible parts
• Layout parts for reliable process completion
Table K-1 Checklist for Design for Manufacturability and Assembly
GUIDELINE AREA OF QUALITY/RELIABILITY IMPROVEMENT
Minimize Number of Parts
Fewer part and assembly drawings
Less complicated assemblies
Fewer parts to hold to required quality characteristics
Fewer parts to drift or fail
Fewer solder attachments to make or fail
‘Design Guidelines for Quality Improvement’
Fewer documents to control
Lower assembly error rate
Higher consistency of part quality
Higher reliability
Minimize Number of Parts
Fewer variations of like parts Lower assembly error rate
Design for Robustness (Design of Experiments)
Low sensitivity to component variability
PTH, via aspect ratio (AR) < 5:1 temperature
or use blind/buried vias
Use standard or preferred parts (EIA/JEDEC registered)
Use compatible SM land patterns (IPC-SM-782)
Higher first-pass yield and less degradation of
performance over time
Less sensitivity to cycling and thermal shock, lower failure rate
Fewer new failure modes and mechanisms
Fewer suppliers to manage
Widen process window
Increase assembly yield
Eliminate Adjustments
No assembly adjustment errors
Eliminate adjustable components with high failure rates
Eliminate change in adjustments under vibration and shock
Provide adjustments and compensation through software
Consider digitally switched resistor network
Higher first-pass yield
Lower failure rate
Design for Efficient and Adequate Inspection and Testing (Testability)
Less mistaking ‘‘good’’ for ‘‘bad’’ product and vice versa
Less effort to locate defects
Better control over rework/repair
Truer assessment of quality, less unnecessary rework
Faster diagnosis to root cause
Less service/maintenance time
More ‘‘up’’ time
Less part damage
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GUIDELINE AREA OF QUALITY/RELIABILITY IMPROVEMENT
Eliminate Engineering Changes on Released Products
Fewer errors due to changeovers & multiple revisions/versions Lower assembly error rate
Make Assembly Easy and Foolproof (Poka-Yoke)
No ‘‘force fitting’’ of parts
Parts cannot be assembled wrong
Obvious when parts are missing or wrong orientation
Assembly tooling designed into part (self-aligning/securing)
Less damage to parts, faster and better serviceability
Lower assembly error rate
Use Repeatable, Well-Understood Processes
Part quality easy to control
Assembly quality easy to control
Higher part yield
Higher assembly yield
Choose Parts that Can Survive (are Compatible with) Process Operations including Rework, Repair and Maintenance
Less damage to parts
Less part degradation or latent damage through prior
evaluation
Plastic Encapsulated Surface Mount Components -
IPC-SM-786
SM Connectors - IPC-C-408
Other SM Components - IPC’s ‘‘Solvent Compatibility’’
No silver termination finish
No nickel termination finish
No thick gold printed board or termination
Minimize number of TH components
Maximize printed board Tg (glass transition temperature)
Printed board thickness compatible with placement machine
Dry film solder mask and solder paste compatibility
Components and solder mask result in adequate clearance
to printed board
Higher yield
Higher reliability
Avoid silver leaching, weak solder joint
Increase solderability, joint strength
Increase solder joint visual yield finish
Lower defects due to solder bridging wider TH and
SM process latitude
Decrease hand loading, manual soldering
Decrease patent and latent printed board damage at high
process temperatures
Minimize solder balls
Enhanced cleaning. Higher SIR PWA robust to dendrite
formation
Choose or Design Process for Compatibility with Susceptible Parts
Less part damage or degradation
Ceramic components thermal shock < 4°C/second
Sensitive components preheated so that ∆T < 100°C
Susceptible Plastic Encapsulated Surface Mount Components
handled per IPC-SM-786
Don’t impact ceramic parts with pick and place tooling
Reflow process adjusted for thermal unbalance due to
thermal masses of parts (PGA, Heat Sinks)
Dry susceptible substrates before reflow
Component terminations not used for in-circuit testing
Higher yield
Higher reliability
Layout Parts for Reliable Process Completion
Less damage to parts during handling and assembly
Orient parts for non-interfering single axis insertion
Sequence parts for insertion for easy disassembly
Orient similar parts similarly
Parts not hanging over solder quality (own or neighboring
parts). No shadowing
Parts can automatically be placed or inserted
Higher yield, higher reliability
Less part damage
Easier rework, repair and maintenance
Fewer orientation sensitive bridging or solder joint failure
modes
Consistent solder joint
Lower assembly error rate
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110
Appendix L
Corrosion Basics and Checklist
L-1.0 CORROSION BASICS
The result of corrosion is material loss due to corrosion of
the metallic conductors, permanent or intermittent continu-
ity loss due to build up of non-conductive corrosion resi-
dues (particularly between contacts) and permanent or
intermittent shorts due to build up of conductive corrosion
residues and conductive metal dendrites. Corrosion accel-
erates the failure of components under cyclic fatigue con-
ditions. Corrosion can also disrupt painted or plated prod-
uct coatings. Surfaces roughened by corrosion are less
effective as sealing surfaces. Water increases the oxidation
rate of oxidants such as SO
2
,SO
3
and O
2
. Water greatly
increases the corrosion rate and metal migration growth
rate of halides such as chloride and fluoride. Water
enhances galvanic corrosion in the presence of dissimilar
metallic finishes; this issue is critical for EMI gaskets, EMI
seals and brazed joints to electroplated structures in
ceramic packages. In the presence of nutrient materials,
water increases fungus growth; the fungi release organic
acids in their waste products.
The oxides of tin, nickel and copper are not good conduc-
tors. Low interfacial pressure contacts to these metals can
become resistive or intermittent.
Salt atmosphere/spray and flowing corrosive gas atmo-
sphere are excellent sources of hydrolyzable, conductive
contamination + water + oxygen. Salt atmosphere/spray
stress is required in military systems but is not commonly
encountered in commercial situations; it normally results in
the detection of plating porosity, but is also known to result
in loss of hermeticity in sealed packages as well as loss of
legibility of component marking.
Office and factory dusts have been found to contain high
levels of chlorides; water pastes made with dust from the
tops of benches and fume hoods are highly conductive. The
atmosphere of paper mills contains acidic sulfide and sul-
fate compounds. Unfiltered forced cooling air can contrib-
ute to premature failure of peripherals and system.
The common halogenated cleaning solvents decompose at
high temperatures or in the presence of catalytic metal sur-
faces. Extremely high halide levels have been found in
droplets of water floating on the solvent surface in the cold
sump of vapor degreasing systems; in these cases, the
water absorbing cartridge had failed or the solvent stabili-
zation additives were exhausted. The halides deposit onto
assemblies which are ‘‘cleaned’’ in the cold sump.
A typical source of hydrolyzable or ionizable contaminants
which result in corrosion in electronics is human finger-
prints, spittle, and food. Fingerprints also contribute oily or
greasy residues which keep the conformal coatings from
fully protecting the conductors and lands from electro-
chemical corrosion.
L-2.0 CORROSION OF THE PWA
The common insulation system in a PWA is the printed
board, its solder mask and any conformal coating. Adsorp-
tion of water or condensation of water vapor on the surface
of insulators with dissolution of hydrolyzable contaminants
results in the subsequent loss of Surface Insulation Resis-
tance (SIR); this effect is seen particularly on porous sur-
faces such as uncoated printed boards which have been
contaminated with hydrolyzable materials and have not
been scrupulously cleaned and can lead to electrochemical
corrosion effects such as metal migration and dendrites.
Metallic dendrites of the common electronic metals (silver,
copper, tin, lead, gold) have been found on the surface of
PWAs contaminated with chlorides and operated under
high humidity.
Dendrites have also been found within the bulk of the
printed board where voids allowed entrapment of conduc-
tive solutions and within delaminated areas of IC’s where
flux residues were found. See ‘‘A Review of Corrosion
Failure Mechanisms during Accelerated Tests.’’ The pres-
ence of water, DC bias and ionizable contaminants at the
interface between the resin matrix and glass fibers between
PTHs, vias and conductors lead to interfacial electrochemi-
cal corrosion and dendrites or Conductive Anodic Fila-
ments (CAF). See Appendix C. See also IPC-TR-476.
L-3.0 CORROSION IN COMPONENTS
Common halogenated solvents can and have diffused
through the rubber seal of aluminum electrolytic capaci-
tors; the result is the dissociation of the solvent inside the
component, the release of HCl, the corrosion of the alumi-
num foil, and failure of the capacitor. A solution is the use
of capacitors where the elastomeric seal is augmented by a
hermetic or epoxy seal.
Absorption in the bulk of insulators with dissolution of
hydrolyzable contaminants results in the subsequent loss of
bulk Moisture Insulation Resistance (MIR) particularly in
printed boards, dielectric film capacitors and plastic encap-
sulated electronic components such as integrated circuits,
networks, and hybrids. Dendrites have also been found
within delaminated areas of IC’s where flux residues were
found.
L-4.0 OTHER EFFECTS OF WATER AND WATER VAPOR
Absorption of water in the bulk of the insulating film of
capacitors results in increased dissipation factor and
increased leakage current. Absorption of water by specific
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