WEE, Beng Tatt Alvin (No. 7, Jln Datuk Palembang 1 Tmn Datuk Palemban, Melaka ., 75150, MY)
TAN, Kian Heong (No. 67 Jalan Gp6, Taman Gadong Perdana Melaka, 75200, MY)
NG, Kock Chai (No. 554, Jln Satria 8a Tmn Serom Utama Ii, Sg. Mat, Muar Johor ., 84410, MY)
GOH, Soon Lock (No. 70, Jln Mj1 Tm, Merdeka ., 75350, MY)
WEE, Beng Tatt Alvin (No. 7, Jln Datuk Palembang 1 Tmn Datuk Palemban, Melaka ., 75150, MY)
TAN, Kian Heong (No. 67 Jalan Gp6, Taman Gadong Perdana Melaka, 75200, MY)
NG, Kock Chai (No. 554, Jln Satria 8a Tmn Serom Utama Ii, Sg. Mat, Muar Johor ., 84410, MY)
Patent claims
1. The method of producing an electronic component (53), the method comprising the steps of: - bending lead fingers (6) of a lead frame (27) to their final shape, covering the lead fingers (6) with a conductive layer (15) , and removing a connective bar (3) that is connected to the lead-finger tips (50) .
2. The method of producing an electronic component (53) according to claim 1, the method comprising the further step of covering a semiconductor chip (22) and at least part of the lead frame (27) with an encapsulating compound before the step of bending the lead fingers (6) to their final shape .
3. The method of producing an electronic component (53) according to claim 1, the method comprising the further step of covering a semiconductor chip (22) and at least part of the lead fingers (6) with an encapsulating compound after the step of bending the lead fingers (6) to their final shape .
4. The method of producing an electronic component (53) according to claim 2, the method comprising the further step of attaching a bottom encapsulated body (46) to the bottom surface of die paddle (23) of the lead frame (27) before the step of bending the lead fingers (6) to their final shape.
5. The method of producing an electronic component (53) according to one of claims 2 to 4, the method further comprising the step of removing dam bar (35) from the lead fingers (6) after the step of coating the lead fingers (6) of the encapsulated body (2) .
6. The method of producing an electronic component (53) according to one of claims 2 to 4, the method further comprising the step of removing dam bar (35) from the lead fingers (6) before the step of coating the lead fingers (6) of the encapsulated body (2) .
7. An electronic component (53) comprising the following features : - an encapsulated body (2) with lead fingers (6) attached to the encapsulated body (2), the lead fingers (6) are bent to their final shape and extend away from the encapsulated body (2), one or more connective bars (3) that is respec- tively connected to the lead-finger tips (50), and no extra layer of conductive material (15) covering material of the lead fingers (6) .
8. An electronic component (53) according to claim 7, characterised in that the connective bar (3) comprises one or more alignment holes (33).
9. An electronic component (53) according to claim 7 or 8, characterised in that at least two adjacent lead fingers (6) are connected by a dam bar (35) that is near the encapsulated body (2) .
10. A set of electronic components according to one of the claims 7 to 9 , wherein adjacent lead fingers (6) of two electronic com- ponents are forming one common intermediate region (3, 6) - |
Description
Lead fingers of a semiconductor chip with an even layer of coating
This invention relates to a method to produce lead fingers of an electronic component.
US 6 583 500 and 6 713 852 suggest methods to improve the material composition of a semiconductor lead frame. However, these methods often deliver unsatisfactory results.
The object of this invention is to provide an improved method for achieving an electronic component with good quality.
According to one basic thought of the invention, the quality problems of semiconductor chip manufacturing can be improved by reducing tool marks, which are left on the lead fingers. The lead fingers are coated against corrosion and abrasion and as well as improvement of appearance. Tool marks thins the coating layer and encourage whisker growth, which may develop into a short circuit between lead fingers .
A method of producing an electronic component includes the step of bending of lead fingers of the lead frame to their final shape. Next, the lead fingers are covered with a coating layer by a method such as electroplating. After this, the connective bar that is connected to the lead-finger tips is removed by a means such as cutting.
This method is unlike prior art where the step of covering the lead fingers with a coating layer is performed before the
step of lead forming. An important feature of the coating is its thickness . The step of lead forming leaves tool marks on the surface of the coating layer and causes portions of the coating layer to be thin and uneven.
The invention may includes the first further step of covering a semiconductor chip and at least part of the lead frame strip with an encapsulating compound before the step of bending lead fingers of the lead frame strip to their final shape. Minor tooling changes are usually required to change to this embodiment of the invention from prior art .
In an embodiment of the invention that includes the first further step, a semiconductor chip is bonded to the bare lead frame and electrical wires are attached between contact pads of the semiconductor chip and the lead fingers. Then, the semiconductor chip and part of the lead frame is covered with an encapsulating compound. After this, the lead fingers are formed to its final shape. Then, the lead fingers are covered with a layer of coating. Following this, the connective bar is removed from the lead frame assembly.
The method may include the second further step of covering a semiconductor chip and at least part of the lead frame strip with an encapsulating compound after the step of bending lead fingers to their final shape. The advantage of this method is that it is easy to form the lead fingers of a bare lead frame strip.
In an embodiment of the invention with the second further step, lead fingers of a bare lead frame strip are formed to its final shape. Then, a semiconductor chip is attached to
die paddle of the lead frame and electrical wires are attached between contact pads of the semiconductor chip and the lead fingers. The die paddle may be lifted because of the formed leads and is supported during the chip attachment and bonding of electrical wires. After this, the semiconductor chip, the electrical wires and part of the lead frame strip are covered with an encapsulating compound. Next, the lead fingers are covered with a layer of coating material. Following this, lead fingers are trimmed to needed length.
The method could include the third further step of attaching a bottom-encapsulated body to the bottom surface of the die paddle of the lead frame before the step of bending the lead fingers to their final shape. The bottom-encapsulated body is intended to form the bottom portion of the final encapsulation package.
In an embodiment of the invention, that includes the third further step, lead fingers of a bare lead frame are formed to its final shape. After this, a bottom-encapsulated body is attached to the bottom of the die paddle. Following this, a semiconductor chip is attached to the die paddle and electrical wires are connected between the lead fingers and contact pads of the semiconductor chip. Then, the semiconductor chip, the electrical wires and part of the lead frame strip are covered with an encapsulating compound. Following this, the lead fingers are covered with a layer of coating material. Then, the lead fingers are trimmed to the needed length.
The encapsulating compound has a melting point lower than the melting point of the bottom encapsulated body and contains
adhesive agent that aids the encapsulating compound in bonding to the bottom encapsulated body.
In another embodiment of the invention that includes the third further step, the bottom-encapsulated body is attached to the bare lead frame before the lead fingers are formed to its final shape. The following steps are similar to the steps in the previous embodiment.
The bottom-encapsulated body provide a support for the lead frame when the semiconductor chip is attached to the die paddle and wires are attached between the semiconductor chip and the lead fingers .
The method preferably includes the fourth further step of removing the dam bar of the lead frame before or after the step of covering the lead fingers of the encapsulated body with a coating layer.
Dam bar are redundant after moulding and should be removed.
The step of removing the dam bar exposes an area of the lead finger. The exposed area may develop corrosion, as most lead frames comprise copper material . The step of covering the lead finger with a coating layer after the step of removing the dam bar has the advantage of covering the exposed area.
An electronic component produced according to the invention contains an encapsulated body and lead fingers attached to the encapsulated body. The lead fingers are bent to their final shape. The tip of the lead finger extends away from the encapsulated body. A connective bar is connected to the tip of the lead finger. In addition, there is no extra layer of
conductive material covering the material of the lead fingers .
The lead-finger tips extend away from the encapsulated body to allow space for the connective bar to remain attached to the lead-finger tips during forming of lead fingers. Also, the lead fingers are kept in position by the connective bar.
There is no extra layer of conductive material covering the material of the lead fingers. The lead fingers are bent to their final shape and are attached to the connective bar. The coating of the lead fingers with a layer of conductive material is performed at a later step.
The connective bar electrically shorts the lead fingers. A popular method of coating the lead fingers is electroplating. The electrical shorting of lead fingers facilitates electroplating. Electroplating coats both the lead fingers and the connective bar. The connective bars the proper electroplating of the lead fingers.
According to the invention, the connective bar includes one or more alignment holes. Alignment holes aid the lead frame strip in positioning during production. Production steps, such as lead forming and lead trimming, need accurate positioning of the lead frame strip.
According to the invention, a metal strip known as dam bar is provided near the encapsulated body and connects the lead fingers. The encapsulated body of the electronic component is formed during moulding. The dam bar keeps any excessive
molten compound from reaching the lead fingers during moulding.
According to the invention, a set of electronic component consists of electronic components that are connected by a common region. In a later step, the common region is divided to form the lead fingers and the connective bar.
Figure 1 shows a plan view of a lead forming station according to the invention,
Figure 2 shows a plan view of a lead trimming station according to the invention, Figure 3 shows a cross-section of the lead finger of the prior art, Figure 4 shows a plan view of an assembly station according to the invention, Figure 5 shows a plan view of a moulding station according to the invention,
Figure 6 shows a top view of a lower mould of figure 5, and Figure 7 shows a plan view of a further assembly station according to the invention.
Figures 1 and 2 relate to an embodiment of the method the invention in which a semiconductor chip and part of a lead frame are covered with a moulding compound. Then, lead fingers of the lead frame are formed as showed in figure 1. After this , the lead fingers are trimmed as showed in figure 2.
Figure 1 shows a plan view of a lead forming station in a closed state according to the invention. The bottom of the figure 1 shows of an anvil 7. On the top surface of the anvil
7 is provided an electronic component strip 51. Above the electronic component strip 51 is placed lead forming arms 4.
The electronic component strip 51 includes electronic components 53. Each electronic component 53 consists of an encapsulated body 2 and a lead frame. The lead frame includes lead fingers 6 and connective bars 3. The lead fingers 6 are attached to the left and right sides of the encapsulated body 2.
The connective bar 3 is attached to the end of the lead finger 6. Above the connective bar 3 is placed a forming arm 4.
Each lead finger 6 has two bends and extends away from the encapsulated body 2. The extended the lead finger 6 allows the connective bar 3 to be attached to the lead finger 6 during forming of lead finger 6.
When the forming station 1 is in an open state, the forming arms 4 are separated from the anvil 7. The lead fingers 6 and the connective bar 3 are straight before forming of lead fingers 6. When the forming station 1 closes, the forming arms 4 descend and bend the lead fingers 6 to their final shape.
Alignment holes on the connective bar 3 and guide pins on the anvil 7 aid in aligning the lead frame to the anvil 7 for forming of lead fingers 6. The alignment holes and guide pins are not shown in figure 1.
The lead fingers 6 are covered with a coating layer after the lead fingers 6 are formed. The coating layer comprises of an electrically conductive material such as tin. The coating layer protects the lead fingers 6 against corrosion and abrasion as well as improving the wetting property of the lead finger surfaces to solder.
Figure 2 shows a plan view of a lead trimming station 10 in a closed state according to the invention. The bottom of the figure 2 shows a cutting table 13. Above the cutting table 13 is placed the electronic component 53 of figure 1. Cutting blades 11 are placed next to the lead fingers 6 of the electronic component 53.
Above the lead fingers 6, are provided guide block 12. The guide block 12 guides the lead fingers 6 into position for trimming of lead fingers . The cutting blades 11 are placed over the end of the lead finger 6, which is known as lead- finger tip 50.
In trimming of lead fingers 6, the cutting blades 11 are raised away from the cutting table 13. Then, the electronic component 53 of figure 1 is placed on the cutting table 13. Following this, the cutting blades 11 are lowered and shear the connective bar 3 away from the lead fingers 6. The same shearing action also trims the lead fingers 6 to the needed . length.
Figure 3 shows a cross-section of a lead finger 6 of the prior art. The lead finger 6 has a bend at the top called shoulder 19 and a bend at the bottom known as heel 17. Between the shoulder 19 and the heel 17, there is placed a
vertical stretch of lead finger 6 called leg 16. The heel 17 of the lead finger 6 is connected to a horizontal stretch of the lead finger 6 known as foot 18. A coating layer 15 covers the top and bottom surfaces of the lead finger 6.
The lead finger 6 of the prior art is covered with a layer of coating 15 before forming of lead fingers . The coating layer 15 has a thickness of greater than 7um (micrometers) before lead forming. The lead finger 6 has uneven surfaces due to tool marks caused during forming of lead fingers.
The coating layer 15 is thin and is of less than 1 um depth at the top 40 of the heel 17. Also, the coating layer 15 is uneven at the top 41 point of the foot 18 and at a point 42 below the shoulder 19 of the lead finger 6. The thin coating layer 15 encourages whisker growth, which may form a short circuit between lead fingers 6.
Figures 4, 5, and 6 correspond to another embodiment of the method of the invention where lead fingers of a bare lead frame are formed. Then, a semiconductor chip is attached to the lead frame as shown in figure 4. After this, electrical wires are attached to the lead fingers and a semiconductor chip as showed in figure 4. Following this, the semiconductor is covered with moulding compound as showed in figures 5 and 6. Then, the lead fingers are trimmed as showed in figure 2.
Figure 4 shows a plan view of an assembly station 20 according to the invention. The bottom of the figure shows an assembly table 25. Support blocks 21 are placed on the top surface of the assembly table 25. Above the support blocks 21 is provided a lead frame strip 26.
The lead frame strip 26 consists of lead frames 27. The lead frame 27 includes die paddle 23, lead fingers 6 and connective bars 3. The die paddle 23 is placed over the support block 21. The lead fingers 6 are provided on the left and right sides of the die paddle 23. The end of the lead finger 6 is connected to a connective bar 3.
A semiconductor chip 22 is placed above the die paddle 23. Electrical wires 24 are provided between the lead fingers 6 and the semiconductor chip 22.
In chip assembly, a bare lead frame strip 26 is provided above the support blocks 21. Then, the semiconductor chip 22 is attached to the die paddle 23 by an adhesive, such as epoxy resin. After this, electrical wires 24 are attached between contact pads of the semiconductor chip 22 and the lead fingers 6. The support block 21 provides a support for the die paddle 23 these steps.
Figure 5 shows a plan view of a moulding station according to the invention. The moulding station 34 is in a closed state. At the bottom of the mould-assembly 34 is placed a lower mould 31. An upper mould 32 is placed over the lower mould 31. In the between the lower and upper moulds 31 and 32 is provided a mould cavity 36. An electronic component strip 51 is placed between the lower and upper moulds 31 and 32. The features of the strip 51 are described in figure 1 description.
In package encapsulation, upper mould 32 is separated from the lower mould 31. Next, the die paddle 23 of the lead frame
27 of figure 4, with the semiconductor chip 22 and the electrical wires 24, is placed within the mould cavity 36. Following this, the upper mould 32 is placed tightly over the lower mould 31. Then, molten encapsulating compound is injected into the mould cavity 36 and fills the mould cavity 36. When the molten compound cools, it forms the encapsulated body 2.
The molten encapsulated compound is injection from outside the mould assembly 34 into the mould cavity 36 via a passageway known as gate. The figure shows three possible placement of the gate, which is referred to as top gate 54, bottom gate 55 and side gate 30.
The side gate 30 is placed horizontally on the top surface of the lower mould 31 and links the outer face of mould assembly 34 to the mould cavity 36. The top gate 54 and bottom gate 55 are vertical passageways. The top gate 54 connects the top surface of the upper mould 32 to the bottom surface of the upper mould 32. The bottom gate 55 links the bottom surface of the lower mould 31 to the top surface of the lower mould 31.
At the start of the moulding cycle, molten encapsulating compound is injected into the mould cavity 36 via the gate 30 or 54 or 55. The encapsulating compound fills the gate 30 or 54 or 55 and need to be removed from the gate 30 or 54 or 55 at the end of the moulding cycle. Clearing the passageway allows the molten compound to be injected into the mould cavity 36 in the next moulding cycle.
The side gate 30 is easily cleared of the encapsulation compound when the mould assembly 34 opens at the end of the moulding cycle. The removal of the encapsulating compound in the top gate 54 or bottom gate 55 needs an another step and is cleared by an insertion of a pin into the gate 54 or 55 at the end of the moulding cycle. The gate is slightly tapered in shape to ease the removal of the moulding compound.
Figure 6 shows a top view of the lower mould 31 of figure 5. Over the lower mould 31 is placed the strip of electronic component 51 of figure 5.
A vertical metal strip is found near the encapsulated body 2 and joins the adjacent lead fingers 6 of the lead frame strip 26. This metal strip is known as dam bar 35. The dam bar 35 inhibits any excessive molten encapsulant from reaching the lead fingers 6. Molten encapsulating compound is injected into the mould cavity (36) during moulding operation.
One end of the lead finger 6 is connected to a connective bar
3. The connective bar 3 keeps the lead fingers 6 from shifting during production. On the connective bar 3 are provided alignment holes 33. Alignment holes 33 and guide pins on the mould assembly 34 aid alignment of the lead frame strip 26 with the mould assembly 34 during the moulding operation. The guide pins are not shown in the figure.
The lead fingers 6 and the connective bar 3 that is attached to the lead fingers 6 form a region 60 or 61. A first region 60 connects the two electronic components 53 of the electronic component strip 51. A second region 61 is attached to the left outer area of the electronic component strip 51.
Another second region 61 is connected to the right outer area of the said strip 51.
The side gate 36 is placed below the encapsulated body 2 on the side of the encapsulated body 2 where there are no lead fingers 6. This position allows the bottom surface of the upper mould 32 of figure 5 to forms a cover for the side gate 36 when the mould assembly 34 is in a close state.
Figure 7 corresponds to another embodiment of the method of the invention where the lead fingers of a bare lead frame are formed. Next, a bottom-encapsulated body is attached to the die paddle of the lead frame. Then, a semiconductor chip is attached to the lead frame as shown in figure 7. Then, electrical wires are attached to the lead fingers and a semiconductor chip as showed in figure 7. Following this, the semiconductor is covered with moulding compound as showed in figures 5 and 6. After this, the lead fingers are trimmed as showed in figure 2.
Figure 7 shows a plan view of a further assembly station according to the invention. The further assembly station 45 is similar to the assembly station 20. The main distinction, between the two stations 20 and 45, is in the support of the die paddle 23. The die paddle 23 of station 45 is placed above a bottom encapsulation body 46 instead of the support block 21 of station 20.
The encapsulated body 46 is attached to the bottom surface of the die paddle 23 in an earlier step. The bottom-encapsulated body 46 is intended to form the bottom portion of the encapsulated body 2 of the electronic component 53. The
encapsulation compound for forming body 2 has a lower melting point than the melting point of the body 46 and contains adhesive agent for bonding the encapsulation compound onto the body 46.
Reference numbers :
1 forming station
2 encapsulated body
3 connective bar
4 forming arm
6 lead finger
7 anvil
10 lead trimming station
11 cutting blade
12 guide block
13 cutting table
15 coating layer
16 leg
17 heel
18 foot
19 shoulder
20 assembly station
21 support block
22 semiconductor chip
23 die paddle
24 wire
25 assembly table
26 lead frame strip
27 lead frame
30 side gate
31 lower mould
32 upper mould
33 alignment hole
34 mould assembly
35 dam bar
36 mould cavity
40 top
41 top
42 point
45 assembly station 46 encapsulated body
50 lead-finger tip
51 electronic component strip
53 electronic component
54 top gate 55 bottom gate
60 first region
61 second region
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