Tombstoning, Beading & Ultra-Fine Pitch Issues. Harry Trip Cobar Europe BV, Holland

Transcription

1 Tombstoning, Beading & Ultra-Fine Pitch Issues Harry Trip Cobar Europe BV, Holland

2 Tombstoning What is it? The partial or complete erection of passive SMT-components Picture: Courtesy Fraunhofer ISIT Institute

3 Observations (1) Microscopic Image of a Micro-Section of a tombstoning resistor at the soldered side Picture: Courtesy Fraunhofer ISIT Institute More frequent occurrence of tombstoning: 1. When passive components become smaller 2. In vapor-phase reflow processes 3. In N 2 -Reflow systems 4. Suddenly with new batches of - components - pc-boards

4 Probable Cause The differences in initial wetting of the solder, between both joints of a passive component. These differences are influenced by differences in temperature and differences in the wetability of the surfaces in both joints. Microscopic Images of a Micro-Section of a tombstoning resistor Top: The soldered side Bottom: The unsoldered & erected side Pictures: Courtesy Fraunhofer ISIT Institute

5 Wetting The mechanism of wetting consists of 3 important parameters: 1. Time of initial wetting 2. Wetting force 3. Time of complete wetting 2 3 1

6 Thermal Mass of the Solder Joint (1) Differences in the thermal mass of both solder joints of a passive component are caused by: 1. A tolerance in dimensions of pads 2. A tolerance in the component metallization 3. A tolerance in the volume of printed solder paste 4. The difference in heat dissipation through via s or inner layers

7 Thermal Mass of Pad C B A F D E Recommended Footprint [mm] Placement Type A B C D E F accuracy , / / / / / Recommended dimensions of pads. Tolerances not specified! Placement accuracy in nominal values. Should be relative!

8 Thermal Mass of the Terminals H W T L Component Outlines [mm] L W H T1 Type Spec Tol - Tol + Spec Tol - Tol + Min Max Min Max Tolerances in nominal values. Should be relative!

9 Thermal Mass of Solder Paste 3D imaging of the printed deposit helps to monitor the thermal mass of the solder paste.

10 Probable Mechanism (1) A smaller thermal mass will heat-up more rapidly. Registration problems may cause significant displacement of the component terminal relative to the pad, causing disruption of the thermal mass resulting in larger? T s. The solder paste will melt a fraction of a second sooner.

11 Probable Mechanism (2) The cleaner & oxide-free the surface of the pad/terminal: The lower the interfacial surface tension The sooner initial wetting will occur The stronger the wetting force The sooner wetting will be completed When surfaces in both joints are oxidized to the same degree, some oxidation will delay initial wetting. Delayed initial wetting provides more time for the temperature in the larger pad/terminal to rise, so the? -T is minimized. The smaller the? -T, the smaller the time-difference of initial wetting.

12 Wetability of Surfaces Metallization of component terminals can be damaged, incorrectly plated or contaminated, reducing the wetable surface area. Microscopic image of a micro-section of a tombstoning resistor. The unsoldered and erected side shows a missing part of the tin-plating. Pictures: Courtesy Fraunhofer ISIT Institute

13 The Role of N 2 & Vapor Phase N 2 prevents re-oxidation of surfaces, so it helps in fast initial wetting. The Vapor Phase process should include a controlled ramp of the temperature. Fast initial wetting does not provide more time to reduce the? -T s.

14 Surface related issues Temperature related issues Pads Terminals Paste deposit Contamination or mechanical damage of specific pad or component terminal Component placement Heat dissipation through via s and inner layers Placement force Displacement Reflow equipment N 2 blanketing?-t too high, Variations in heat flow (accumulated contamination) Solder paste Thermal stability issue of flux Alloy choice Registration issues

15 Probable Causes Pad Solder Particles Comp. Terminal Oxidation Surface Area Virgin Surface Properties Surface State Temperature Surface Wetting

16 Recommendations (1) The reduction and elimination of tombstoning is supported by: - The use of solder paste with a thermally stable flux system providing and maintaining tackiness. - A paste with metal particles with 2 different eutectic points: 50 % melting at 179 C and the balance at 183 C. After reflow the following alloy is formed: Sn62.5/Pb36.5/Ag1.0. The angling effect caused by the force of faster initial wetting in one joint is mechanically hindered by the solid particles of the alloy that melts at 183 C. This provides the alloy that melts at 179 C on the other pad with a fraction of a second more time to wet as well, and restores the equilibrium forces.

17 Model for Tombstoning* H (* Klein Wassink et all) ß d L a W e g γ α ß ε M Surface tension liquid solder Angle of erection Angle center of gravity/component conforming to arc/tan (H/L) Angle perpendicular/direction of force on component by surface tension of solder S-H sin α e =arc tan H cos α Mass of component G Movement of earth W Active forces: Upward: T 1 =M*g*d*cos (α+ß) Surface tension of solder on pad: T 2 = γ*w*cos(α/2) Upward: T 3 =γ*h*sin(α+ε) S D H W L Distance center of gravity/angling point of component = ½ (L 2 + H 2 ) Height of component Width of component Length of component

18 Tombstoning & Mixed Alloy Normal alloy Mixed alloy

19 Recommendations (2) - A minimized? -T - Controlling the tolerances on boards, components & component placement - Controlling the O 2 ratio in N 2 -systems (preferred level: 1000 ppm)

20 Beading What is it? A spherical mass of solder formed upon reflow on the side of the body of a component with a tight capillary space between its body and the solder mask (primarily chip components, sometimes SOT23, SOT89 and LCCC s).

21 Observations Beading occurs as long as surface mount components are reflowsoldered. Beading became a topic in SMT since the introduction of No-Clean technology, as the beads are no longer removed by the post solder cleaning operation.

22 Probable Cause The excessive volume of paste will separate from the main mass upon melting. The molten solder of the separated mass will not find a surface it can really wet. Therefore, its surface tension will cause the formation of a spherical shape driving the mass, from underneath, to the side of the component body.

23 Parameters (1) The parameters some more significant than others - that could be considered in a model are: 1. Volume of printed paste relative to pad 2. Distance of paste from center of boundary of pad, on the side adjacent to the other pad 3. Placement force 4. Elasticity of printed solder paste 5. Gap between component and solder mask 6. Weight of component 7. Width of component

24 Parameters (2) 8. Thermal stability of flux system - melt viscosity of resins - reaction of activator system 9. Volume of displaced solder mass 10. Distance between boundaries of displaced solder and boundaries of solder joint area 11. Surface tension of molten solder 12. Surface tension of wetable surfaces in solder joint

25 Parameters (3) 13. Surface tension of non-wetable surfaces in gap between the component body and solder mask 14. Wetable surface in solder joint area underneath component 15. Cohesive forces in molten solder Upon melting, the delicate equilibrium between these parameters is disturbed.

26 Reshaping the Paste Deposit Several proposals have been made to change the aperture design in the stencil for passive components in order to eliminate beading Straightforward reduction of the paste deposit on the side of the pad adjacent to the other pad ( J. Keller) Spherical pattern have proven to be incorrect Home-Plate Deposit. Also used in rounded shape (Schutt & Martini Patent AT&T) V-Cut Deposit. (M. Park) U-shape. Placement Accuracy required

27 V-Cut Paste Deposit This design seems to be most effective to reduce the amount of paste in the most critical area of the joint: the center of the area facing the adjacent pad. V-Cut aperture calculation Aperture design seems to be the most influential parameter to eliminate beading.

28 Ultra-Fine Pitch Issues The most common Ultra-Fine Pitch defect besides tombstoning and beading, is bridging. Considering the possible causes for bridging, two different categories should be defined: 1. Low bridges 2. High bridges

29 Low Bridges (1) The so-called low bridges are caused by: - Too much paste on the pad relative to the wetable surface area and the potential volume of solder in the joint - Stencil too thick - Paste on the topside of the stencil after printing - Aperture design

30 Low Bridges (2) - Quenching of the paste in the direction of the neighboring pads by the placement force - Slumping of the paste elastic properties temperature vibrations melt viscosity Aperture design for UFP QFP pads requires a different approach. Example: a 0.4 mm QFP pad: Pad 0.22 mm Aperture 0.20 mm N 2 blanketing helps to prevent low bridging as it will support the wetting of the solder, thus it will finds its way more easy into the joint.

31 High Bridges (1) The so-called High Bridges are caused by: Too much paste on the pad relative to the wetable surface area and the potential volume of solder in the joint. The excellent wetting of the lower end of the component will cause the paste to wick up to the bend in the terminal. At this point the shape of the liquid will no longer follow the outline of the terminal, but it will accumulate into bulky globule finally resulting in a bridge with the mass of solder covering the neighboring terminal.

32 Too much paste is caused by: - Stencil too thick High Bridges (2) - Paste-traces on the print-area on the stencil after printing - Aperture design Difference in thermal mass of the component lead relative to the thermal mass of the pad will sometimes cause the solder to wick up the lead too fast. The fast wicking up the component lead is caused by -T s between lead and pad. Recommendation: Increase the dwell time in the soak-zone to compensate for the surplus in activity of the paste relative to the excellent wetability of the component terminal. Longer soak times may be a problem to the shorter reflow equipment.

33 Reflow Profile Sn62/Sn63/S6M Temperature Recommended minimum reflow profile Cobar No Clean Sn62/Sn63 solder pastes C 1 C/sec. 120 sec C 0.5 C/sec. 40 sec C 2 C/sec sec. Coolest spot min. 210 C for 5 sec. Dwell time min. 30 sec., max. 60 sec Time (sec.)

34 Reflow Profile Lead-Free Temperature Recommended minimum reflow profile Cobar No Clean CuAg solder pastes C 1 C/sec C 0.5 C/sec C 2 C/sec. 235 Coolest spot min. 210 C for 5 sec. Dwell time min. 30 sec., max. 60 sec sec. 40 sec sec Time (sec.)

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