Lead-frame structure, lead-frame, surface mount electronic device and methods of producing same

Abstract

A method of producing a lead-frame structure having two faces and exposing a treated silver surface on at least one of the two faces, the treated silver surface(s) serving the wire bonding, which yields a surface which, after applying resin to it, has excellent adhesion even under severe testing conditions, such as the IPC/JEDEC J-STD-20 MSL standard, and a method of producing a surface mount electronic device including a lead-frame or lead-frame entity and at least one semiconductor device mounted thereon, wherein the lead-frame or lead-frame entity exposes a treated silver surface on at least one of the two faces, wherein the treated silver surface(s) serve(s) the wire bonding, and wherein a resin is applied to the lead-frame or lead-frame entity, which method yields excellent adhesion of the surface of the lead-frame or lead-frame entity even under severe testing conditions.

Claims

1. A method of producing a lead-frame structure having two faces and exposing a treated silver surface on at least one of said two faces, said method comprising the following method steps carried out in this order: (a) providing a lead-frame body structure having two main sides, said lead-frame body structure exclusively exposing a copper surface on each one of said main sides; (b) depositing a silver coating on at least one of said main sides, so that said at least one of said main sides at least partially exposes an untreated silver surface; and (c) electrolytically treating said untreated silver surface on said at least one of said main sides generated in method step (b) with a treatment solution containing at least one hydroxide compound selected from alkali metal hydroxides, alkaline earth metal hydroxides, ammonium hydroxides and mixtures thereof, wherein the lead-frame body structure is a cathode, thereby producing said lead-frame structure which comprises at least two lead-frame entities each one having two faces and at least partially exposing said treated silver surface, characterized in that said at least one of said two faces of at least one of said at least two lead-frame entities either exclusively exposes said treated silver surface or that said at least one of said two faces of said at least one of said at least two lead-frame entities partially exposes said treated silver surface and partially exposes said copper surface, wherein, on each one of said two faces of each one of said at least two lead-frame entities which partially exposes said treated silver surface, the area of said copper surface is smaller than the area of said treated silver surface.

2. A method of producing a surface mount electronic device, said method comprising the following method steps carried out in this order: (a) providing a lead-frame body structure having two main sides, said lead-frame body structure exclusively exposing a copper surface on each one of said main sides; (b) depositing a silver coating on at least one of said main sides, so that said at least one of said main sides at least partially exposes an untreated silver surface; (c) electrolytically treating said untreated silver surface on said at least one of said main sides generated in method step (b) with a treatment solution containing at least one hydroxide compound selected from alkali metal hydroxides, alkaline earth metal hydroxides, ammonium hydroxides and mixtures thereof, wherein the lead-frame body structure is a cathode, thereby producing a lead-frame structure which comprises at least two lead-frame entities each one having two faces and at least partially exposing said treated silver surface; (d) mounting at least one semiconductor device on at least one of said at least two lead-frame entities, and bonding said at least one semiconductor device to said treated silver surface, wherein said bond is capable of creating an electrical connection between said at least one of said at least two lead-frame entities and said at least one semiconductor device; (e) encapsulating said at least one of said at least two lead-frame entities together with said at least one semiconductor device using a resin material, thereby forming at least one surface mount electronic device structure; and (f) singularizing said surface mount electronic device from said surface mount electronic device structure, characterized in that said at least one of said two faces of at least one of said at least two lead-frame entities either exclusively exposes said treated silver surface or that said at least one of said two faces of said at least one of said at least two lead-frame entities partially exposes said treated silver surface and partially exposes said copper surface, wherein, on each one of said two faces of each one of said at least two lead-frame entities which partially exposes said treated silver surface, the area of said copper surface is smaller than the area of said treated silver surface.

3. The method according to claim 1, wherein said at least one of said two faces of at least one of said at least two lead-frame entities exclusively exposes said treated silver surface and no copper surface.

4. The method according to claim 1, wherein said silver coating is at most 2 m thick.

5. The method according to claim 1, wherein said treatment solution additionally contains at least one silicate salt.

6. A method of producing a lead-frame having two faces and exposing a treated silver surface on at least one of said two faces, said method comprising the following method steps carried out in this order: (a) providing a lead-frame body having two main sides, said lead-frame body exclusively exposing a copper surface on each one of said main sides; (b) depositing a silver coating on at least one of said main sides, so that said at least one of said main sides at least partially exposes an untreated silver surface; and (c) electrolytically treating said untreated silver surface on said at least one of said main sides generated in method step (b) with a treatment solution containing at least one hydroxide compound selected from alkali metal hydroxides, alkaline earth metal hydroxides, ammonium hydroxides and mixtures thereof, wherein the lead-frame body is a cathode, thereby producing said lead-frame, characterized in that said at least one of said two faces of said lead-frame either exclusively exposes said treated silver surface or that said at least one of said two faces of said lead-frame partially exposes said treated silver surface and partially exposes said copper surface, wherein, on each one of said two faces of said lead-frame, the area of said copper surface is smaller than the area of said treated silver surface.

7. Method for producing a lead-frame structure having two faces and exposing a treated silver surface on at least one of said two faces, wherein said lead-frame structure comprises at least two lead-frame entities each one having two faces, said method comprising: providing a lead-frame body structure which has two main sides with an untreated silver surface exposed on at least one of said two main sides; and electrolytically treating a lead-frame body structure which has two main sides and which is provided with an untreated silver surface on at least one of said main sides with a treatment solution, wherein said lead-frame body structure is a cathode, wherein the treatment solution contains at least one hydroxide compound selected from alkali metal hydroxides, alkaline earth metal hydroxides, ammonium hydroxides and mixtures thereof, thereby producing said lead-frame structure, characterized in that said at least one of said two faces of at least one of said at least two lead-frame entities exclusively exposes said treated silver surface or that said at least one of said two faces of said at least one of said at least two lead-frame entities partially exposes said treated silver surface and partially exposes a copper surface, wherein, on each one of said two faces of each one of said at least two lead-frame entities which partially exposes said treated silver surface, the area of said copper surface is smaller than the area of said treated silver surface.

8. The method according to claim 2, wherein said at least one of said two faces of at least one of said at least two lead-frame entities exclusively exposes said treated silver surface and no copper surface.

9. The method according to claim 2, wherein said silver coating is at most 2 m thick.

10. The method according to claim 2, wherein said treatment solution additionally contains at least one silicate salt.

Description

(1) The figures shown hereinafter and illustrating the examples explain the present invention in more detail. This explanation is not to be considered a limitation of the scope of the invention but simply serves as its exemplification.

(2) FIG. 1 shows a first embodiment of a lead-frame structure (multiple panel of lead-frame entities) in a top view;

(3) FIG. 2 shows a multiple panel of surface mount electronic device entities (surface mount electronic device structure) comprising a lead-frame structure (multiple-panel; second embodiment) in a top view;

(4) FIG. 3 shows a graph displaying the adhesion of the mold to a lead-frame material in terms of the force F.sub.shear used to shear off buttons applied to lead-frame material; samples examined as they are (Test Flow 1);

(5) FIG. 4 shows a graph displaying the adhesion of the mold to a lead-frame material in terms of the force F.sub.shear used to shear off buttons applied to the lead-frame material; samples examined after extreme heat loading (Test Flow 2);

(6) FIG. 5 shows a graph displaying the adhesion of the mold to a lead-frame material in terms of the force F.sub.shear used to shear off buttons applied to the lead-frame material; samples examined after moisture stress (Test Flow 3);

(7) FIG. 6 shows a graph displaying the adhesion of the mold to a lead-frame material in terms of the force F.sub.shear used to shear off buttons applied to the lead-frame material; mean values obtained with Test Flows 1, 2, 3.

(8) FIGS. 1 and 2 show embodiments of lead-frame structures 100 comprising a plurality of lead-frame entities 110 (to be singularized to yield a plurality of lead-frames or surface mount electronic devices 200, respectively). The figures show the upper faces 170 of the lead-frame structures.

(9) FIG. 1 shows a multiple panel 100 of lead-frame entities 110 in a first embodiment. This multiple panel (lead-frame structure) comprises 14 lead-frame entities as well as portions 300 of this multiple panel which to not pertain to these entities and which due to the singularizing step are removed. Each lead-frame entity comprises a core with die pad 120 where a semiconductor device (for example IC chip, not shown) may be mounted and a plurality of legs 130 which serve the external connection to a circuit board or the like. Wire bonding is performed from the semiconductor device mounted to the core to the individual legs. Singularization of a lead-frame (or of a surface mount electronic device once the semiconductor device is mounted and bonded to the lead-frame entity and the lead-frame entity and semiconductor device are encapsulated) is performed along the dashed line 140.

(10) FIG. 2 shows a multiple panel 100 of lead-frames entities 110, which have semiconductor devices (not shown) being mounted thereon and bonded to the lead-frame entities and which are together with the semiconductor devices encapsulated with a mold 160. Thus, this multiple panel contains a plurality of entities of surface mount electronic devices 200 which may be singularized from this panel as is shown along the dashed line 140. Residual portions 300 of the panel are scrap.

(11) Both embodiments shown in FIGS. 1, 2 are coated with silver: The multiple panel 100 shown in FIG. 1 is completely coated with silver, thus exclusively exposing a treated silver surface on that face which is shown (100% silver surface coverage), whereas the panel 100 shown in FIG. 2 is almost completely coated with silver. In this latter case, an upper edge region 400 of the panel is left free from silver, thus exposing the underlying copper surface. This area not being coated with silver is due to the fact, that the panel (lead-frame body structure) has been immersed into a silver plating bath to coat silver thereon, with the exception of this upper edge region, so that no silver was plated in this region. As this region is not part of the regions of the lead-frame entities 110 however, this copper surface is not considered as a copper surface area in the condition of partial exposure of treated silver surface according to the present invention: In this example, the lead-frame entities are completely coated with silver (as is indicated by the area surrounded by the dashed line 140, for example), so that silver surface coverage in this case is 100% and copper surface coverage is 0%.

EXAMPLES

(12) Sample Preparation:

(13) Strips of C194 copper (copper alloy having 97 wt.-% copper, 3 wt.-% iron, phosphorus and zinc) of 30 cm length and 5 cm width were treated with the method shown in Table 1. The strips were rinsed between successive process steps. A silver coating was deposited on the copper strips and finally treated with the treatment solution containing at least one hydroxide compound using a cathodic treatment. The silver coating completely covered the copper strips and had the thickness as specified.

(14) Three control samples were prepared, namely a first sample which was only silver plated (3 m thickness silver coating), but not further treated at all (Control #1: AgUntreated), a second sample which was silver plated (3 m thickness silver coating) and then treated with the treatment solution containing at least one hydroxide compound (Control #2: AgAgPrep) and a third sample which was neither plated with silver nor treated with any other treatment agent (Control #3: Cu C194Untreated). Silver plating was carried out at the entire surfaces of both sides of the strips.

(15) Further, six samples (Sample #0, Sample #1, Sample #2, Sample #3, Sample #4,

(16) Sample #5) were prepared using the method of the invention including silver plating and treatment with the treatment solution (AgPrep) containing at least one hydroxide compound. These samples differed in thickness of the silver coatings. The samples and relevant parameters are listed in Table 2.

(17) After carrying out the treatments on the strips, eight plastics buttons (cylindrically-shaped buttons having 3 mm diameter and 3 mm height of resin material (epoxy resin) were molded onto one main side of the strip. After the application of the buttons, all the strips were post-mold-cured (Post Mold Cure: 2 hours at 175 C.). Adhesion of the buttons to the strip was then tested by shearing them off the strip and the shear force F.sub.shear [MPa] was measured.

(18) In a first test flow (Test Flow 1), the strips were then examined as prepared, i.e., without any further treatment, by measuring the shear-off force F.sub.shear.

(19) In a second test flow (Test Flow 2), the strips were heat-loaded (Heat Loading A, Extreme Heat Loading) prior to molding the buttons on the silver coating. Heat Loading A consisted of one heat treatment cycle with a peak temperature T.sub.2=300 C. (initially rising the temperature linearly up to T.sub.1=280 C., then holding the temperature at T.sub.1, then linearly rising the temperature to T.sub.2, then linearly decreasing the temperature to T.sub.3=280 C., then holding the temperature at T.sub.3, and finally linearly decreasing the temperature to ambient temperature). Total duration of Heat Loading A was about 6 min. Thereafter the shear-off force F.sub.shear was measured.

(20) In a third test flow (Test Flow 3), after the post-mold cure, the strips were steam aged and thereafter heat loaded (Heat Loading B, Moisture Stress). No heat treatment took place prior to the molding. Heat Loading B consisted of a preconditioning at 93 C. and 93% relative humidity (93% RH) for 18 hours. Heat Loading B consisted of three heat treatment cycles with a peak temperature of 260 C. each (initially rising the temperature linearly up to T.sub.1=150 C., then holding the temperature at T.sub.1, then linearly rising the temperature to T.sub.2=200 C., then holding the temperature at T.sub.2, then further linearly rising the temperature to T.sub.3=260 C., and finally linearly decreasing the temperature to ambient temperature). Total duration of Heat Loading B was about 6 min. After Heat Loading B the shear-off force F.sub.shear was measured.

(21) Test Method:

(22) Shear force F.sub.shear was evaluated by measuring the force required to shear off the buttons from the strips. A measuring device manufactured by DAGE (4000 Plus) was used for this purpose. Shear force was applied at an angle of 90 and at a rate of 50 mm/min over a distance of 100 mm. The force which was required to shear off a respective button was established as the shear force F.sub.shear (in MPa).

(23) Results:

(24) Results of the shear-off force obtained with Test Flow 1 (samples as prepared) are shown in Table 3 and graphically displayed in FIG. 3 (box plots showing mean values and standard deviations of measured shear force values). Each sample yields eight measurements as obtained from the eight buttons.

(25) Results of the shear-off force obtained with Test Flow 2 (Extreme Heat Loading) are shown in Table 4 and graphically displayed in FIG. 4 (box plots showing mean values and standard deviations of measured shear force values). Each sample yields eight measurements as obtained from the eight buttons.

(26) Results of the shear-off force obtained with Test Flow 3 (Moisture Stress) are shown in Table 5 and graphically displayed in FIG. 5 (box plots showing mean values and standard deviations of measured shear force values). Each sample yields eight measurements as obtained from the eight buttons.

(27) A graph summarizing the results of all tests done on all samples is given in FIG. 6. The results shown in this figure are the mean values obtained from Tables 3 to 5. This figure confirms the following: Excellent adhesion (high shear force) is achieved in all cases where the method of the invention was carried out, i.e., with the silver coating being deposited over the entire surface of the sample and therefore covering not only part of the interface between the mold button and the strip but the whole interface (Samples #0 to #5): better adhesion would not be achieved if only part of this interface would be covered by the silver coating as will be apparent from the comparison result obtained with Control #3 where the entire interface consisted of copper, as compared to Control #1 (entire surface coated with 3 m silver). The shear force attains a high level (almost 20 MPa) at very thin silver coatings, such thickness being as thin as 0.08 m (Samples #2 to #5); even with a sample being coated with silver at 0.02 m thick, a high shear force was achieved (Sample #0), lying between about 15 and almost 20 MPa, which was almost as high as a respective sample having a silver coating of 3 m thick (Control #2). Silver yields a higher shear force than copper (Control #1 vs. Control #3), at least if an extreme heat loading is carried out (Test Flow 2). Treatment with the treatment solution containing at least one hydroxide compound yields a considerably higher shear force than without such treatment (Control #1 vs. Control #2).

(28) From this it is evident that providing the silver coating to the entire surface of the lead-frame body or lead-frame body structure will lead to achieve excellent adhesion between the lead-frame surface and the mold. Furthermore, it emerges that high thickness of silver is not necessary to establish good adhesion to the mold. It has further been ascertained that low thickness of silver is also sufficient to ensure good electrical bonding of the SMDs to the silver surface when TSB is used.

(29) TABLE-US-00001 TABLE 1 Process Flow Process Step Temperature Duration Other 1. Cleaning: alkaline degreasing ambient 15 s N/A 2. Predip (Acid activation) ambient 10 s N/A 3. Silver strike (silver cyanide) ambient 10 s 1 A/dm.sup.2 4. Silver treatment AgPrep ambient 30 s 12 A/dm.sup.2 26L (150 g/l NaOH)

(30) TABLE-US-00002 TABLE 2 Samples Sample Category Process Details Control #1 Control Group Ag-Untreated Control #2 Ag-AgPrep Control #3 Cu C194-Untreated Sample #0 New Process Thin Ag 0.021 m + AgP 26L Sample #1 Flow (Table 1) Thin Ag 0.042 m + AgP 26L Sample #2 Thin Ag 0.083 m + AgP 26L Sample #3 Thin Ag 0.125 m + AgP 26L Sample #4 Thin Ag 0.166 m + AgP 26L Sample #5 Thin Ag 0.208 m + AgP 26L

(31) TABLE-US-00003 TABLE 3 Shear Force - Test Flow 1 Button Cont. #1 Cont. #2 Cont. #3 Samp. #0 Samp. #1 Samp. #2 Samp. #3 Samp. #4 Samp. #5 1 8.012 17.510 16.767 19.811 21.256 21.142 20.217 20.601 20.530 2 8.573 20.417 15.731 20.339 18.618 19.869 20.097 20.994 20.443 3 11.719 21.072 16.526 19.894 20.316 20.592 20.356 19.782 20.598 4 10.087 18.511 12.395 18.336 17.782 18.357 19.651 18.320 5 9.889 21.124 13.787 21.142 20.620 20.688 19.724 20.134 20.866 6 11.103 19.657 15.969 19.600 20.480 20.254 19.843 20.993 20.161 7 11.767 22.294 15.304 19.658 18.921 19.798 18.744 19.917 20.822 8 9.680 19.903 14.504 19.005 19.818 18.798 19.089 18.725 19.724 Ave 10.104 20.061 15.123 19.723 19.726 20.163 19.553 20.100 20.183 Max 11.767 22.294 16.767 21.142 21.256 21.142 20.356 20.994 20.866 Min 8.012 17.510 12.395 18.336 17.782 18.798 18.357 18.725 18.320

(32) TABLE-US-00004 TABLE 4 Shear Force - Test Flow 2 Button Cont. #1 Cont. #2 Cont. #3 Samp. #0 Samp. #1 Samp. #2 Samp. #3 Samp. #4 Samp. #5 1 17.131 21.845 3.360 14.518 18.033 21.245 19.557 20.795 19.857 2 17.392 21.082 2.637 15.403 16.679 20.778 20.775 19.607 17.563 3 17.097 20.908 0.000 16.552 20.574 20.530 19.588 4 13.995 18.785 0.000 12.718 12.292 18.297 18.011 18.215 18.991 5 18.278 21.864 0.000 13.928 12.755 19.075 21.635 20.007 20.839 6 18.291 20.769 0.000 14.220 12.770 20.686 19.541 19.388 20.581 7 16.943 21.638 0.000 14.147 13.183 19.781 20.229 20.375 19.396 8 15.747 17.856 0.000 11.611 15.915 19.214 19.502 19.754 19.806 Ave 16.859 20.593 0.750 13.799 14.772 19.956 19.893 19.834 19.578 Max 18.291 21.864 3.360 15.403 18.033 21.245 21.635 20.795 20.839 Min 13.995 17.856 0.000 11.661 12.292 18.297 18.011 18.215 17.563

(33) TABLE-US-00005 TABLE 5 Shear Force - Test Flow 3 Button Cont. #1 Cont. #2 Cont. #3 Samp. #0 Samp. #1 Samp. #2 Samp. #3 Samp. #4 Samp. #5 1 7.470 17.214 10.412 18.183 19.277 19.021 18.692 18.743 18.279 2 4.512 18.333 10.193 18.702 18.270 19.399 18.740 19.424 17.453 3 19.359 10.275 19.280 19.431 19.018 18.989 16.885 4 4.849 15.500 9.200 17.309 16.949 18.133 17.398 18.252 16.458 5 8.102 18.580 10.060 18.288 16.524 18.397 18.812 19.327 18.085 6 7.727 14.387 9.907 18.619 17.903 19.047 18.525 19.510 18.688 7 7.471 18.930 9.837 17.969 18.737 18.994 17.335 18.820 18.162 8 6.149 17.909 9.856 18.468 19.606 18.092 17.564 17.736 17.178 Ave 6.611 17.527 9.968 18.352 18.181 18.812 18.260 18.850 17.649 Max 8.102 19.359 10.412 19.280 19.606 19.431 19.018 19.510 18.688 Min 4.512 14.387 9.200 17.309 16.524 18.092 17.335 17.736 16.458

LIST OF REFERENCES

(34) 100 Lead-frame structure, multiple panel of lead-frame (entities)

(35) 110 Lead-frame entity

(36) 120 Core

(37) 130 Leg

(38) 140 Singularization path, dashed line

(39) 150 Copper surface

(40) 160 Mold (resin)

(41) 170 Upper face

(42) 200 Surface mount electronic device

(43) 300 Portions which do not pertain to lead-frame entities

(44) 400 Face region not coated with silver