Press-in pin, assembly and method for producing such an assembly

Abstract

The invention relates to a press-in pin (10) comprising a main body (12) and a coating (20) which covers the base body (12) at least in sections and has an original barrier layer (22) and a functional layer (24). The original barrier layer (22) consists of at least 80 percent by mass of nickel and the functional layer (24) consists of 30 to 72 percent by mass of silver and / or copper and / or bismuth and / or indium (alloy metals), the remainder being tin and impurities. In order to achieve high temperature resistance, the functional layer has a thickness (d2) of at least 0.7 micrometres (figure 2).

Description

[0001] Press-in pin, assembly and method for producing such an assembly

[0002] Description

[0003] The invention relates to a press-in pin according to claim 1, an assembly which has a printed circuit board and at least one such press-in pin according to claim 6, and a method for producing such an assembly according to claim 11.

[0004] The so-called press-fit technology has been known for a long time and is used on a large scale in electrical engineering. The principle consists in at least one press-fit pin having a press-fit zone (this is usually part of a press-fit connector that has a plurality of such press-fit pins) being pressed into a hole in a printed circuit board. The hole is surrounded by an electrically conductive sleeve, and a gas-tight growth is intended to occur. The press-fit pin always has a base body, which is usually made of a copper alloy. The sleeve is usually made of essentially pure copper. The base body of the press-fit pin and / or the sleeve are often coated to create lubrication that enables press-in and to generate a layer which, as mentioned above, grows gas-tight after press-in through solid diffusion processes. The corresponding coating often contains tin.

[0005] From the generic EP 2 596 157 B1, a press-in pin with such a base body and a tin-containing coating is known. This coating is frequently used in the automotive sector, particularly because it exhibits no whisker growth, or at least significantly reduced whisker growth compared to a pure tin coating. In addition to tin and possible impurities, the coating contains silver and / or copper and / or bismuth in a range between 30 and 72 percent by mass. EP 2 596 157 B1 proposes a thickness of this coating between 0.25 and 0.6 micrometers.

[0006] It should be noted at this point that in this patent application, percentages always refer to percentages by mass, even if this is not explicitly stated.

[0007] The press-in pin described in the above-mentioned patent works very well for its intended purpose, but it has been found that the temperature resistance of an electrical connection made using such a pin is relatively limited.

[0008] High temperature resistance, especially temperature resistance where the temperature can rise above 300°C for more than 15 minutes, is desirable for numerous reasons. Especially if the circuit board into which such a press-fit pin is pressed also has solder connections, then if the press-fit connection has only low temperature resistance, the circuit board must be designed in such a way that the critical temperature is not exceeded even during soldering. This often means that the entire circuit must be built around this "bottleneck," which means that circuits have to be designed too large and a correspondingly large amount of material has to be used, which is of course disadvantageous overall.

[0009] Even during operation of the circuit, a very high, at least short-term, temperature resistance may be desired in order to expand the range of applications.

[0010] Based on this, the present invention aims to provide a press-in pin that exhibits very good temperature resistance, particularly when pressed in. This object is achieved by a press-in pin having the features of claim 1. An assembly comprising such a press-in pin is specified in claim 6. Furthermore, a method for producing such an assembly is proposed in claim 11.

[0011] The press-in pin, like every press-in pin, has a base body which is usually made of a copper alloy with a copper content of at least 51%. A functional layer is also provided, as is generally known from the generic EP 2 596 157 B1 and consists of 30 to 72% silver and / or copper and / or bismuth and / or indium as alloying metals, the remainder being tin and optionally impurities. It has been found that indium is also generally suitable as an alloying metal. Furthermore, a nickel-containing barrier layer with a nickel content of at least 90% is provided between the functional layer and the base body. In the unpressed state of the press-in pin, this is referred to as the "original barrier layer".The original barrier layer covers at least a portion of the base body, in particular its press-in zone, and the functional layer covers at least a portion of the original barrier layer.

[0012] According to the invention, the functional layer is thicker than has been proposed so far in the prior art, namely with a thickness of at least 0.7 pm (micrometers), preferably with a thickness between 0.8 and 1.5 pm, with a thickness of 0.9 to 1.2 pm being particularly preferred.

[0013] It has been found that there is a clear tipping point with regard to the thickness of the functional layer, beyond which the temperature resistance increases dramatically and, in particular, a temperature resistance of over 300°C results with an exposure time of more than 15 minutes.

[0014] To understand this, one must first consider the layer system that exists after the press-in pin is pressed into a – usually tin-coated – sleeve. It should be noted that the pressing process completely disrupts the original functional layer and the original coating of the sleeve.

[0015] The resulting layer system comprises a barrier layer derived from the original barrier layer, which consists essentially of nickel, a bonding layer derived at least partially from the functional layer, and a transition layer located between the bonding layer and the sleeve. If the sleeve was originally coated with tin, this transition layer contains at least a portion of this tin.

[0016] The bonding layer essentially consists of tin and the alloying metal(s) of the functional layer. For simplicity, some of the following passages assume that silver is the only alloying metal. The explanations now apply analogously to the other possible alloying metals copper, bismuth, and indium, including all combinations thereof, although silver is generally preferred. The tin and silver exist in the bonding layer in at least two states: as a "true" alloy with metallic properties, and as an intermetallic phase (also referred to as an intermetallic compound), which has more ceramic properties but is also part of the alloy of the functional layer or bonding layer.For the sake of clarity, a distinction is made below between the intermetallic phase and the metallic alloy, since intermetallic phases are often inaccurately subsumed under the generic term “alloy”. This means that there is at least one region in the compound layer which is in the form of a metallic alloy made of tin and the at least one alloying metal (for example silver), and regions which are each in the form of an intermetallic phase made of silver and tin. The regions of intermetallic phases (also referred to as “intermetallic regions”) can be embedded in the region of the metallic alloy or, for example, begin at the phase boundary of the alloy. The transition layer, which is located between the compound layer and the sleeve, is essentially in the form of an intermetallic phase made of copper and tin and can contain residues of tin.

[0017] According to current knowledge, the aforementioned tipping point is due to the fact that once a certain temperature is exceeded, another layer forms between the functional layer and the nickel-containing barrier layer, which is referred to here as the intermediate layer. This layer consists entirely or at least partially of an intermetallic phase made of nickel from the barrier layer and tin from the metallic tin-silver alloy of the functional layer. It has also been shown that the thickness of this intermediate layer is "self-limiting," meaning it only grows up to a certain thickness, after which growth stops almost completely, regardless of whether the temperature remains high or whether another high-temperature phase follows at a later point in time. In other words, a "one-time aging" process occurs, after which the system returns to a stable state.

[0018] It was further found that the intermediate layer that forms as such is unproblematic both from the point of view of electrical conductivity and from the point of view of the mechanical integrity of the layer system and the press-fit connection as a whole, and the electrical conductivity can even be slightly improved.

[0019] Nevertheless, the formation of this intermediate layer is problematic when using a press-in pin according to the prior art, as described in EP 2 596 157 B1, since it has been found that the "self-limiting thickness" of the intermediate layer is sufficient to consume the tin present in the connection layer as a metallic alloy to such an extent that it is no longer available for growth with the transition layer, so that a gas-tight connection between the press-in pin and the printed circuit board is no longer provided, which can lead to the penetration of atmospheric oxygen and progressive corrosion and thus to the failure of the electrical connection.The effect just described is counteracted according to the invention by providing enough tin in the form of a metallic alloy (for example, a tin-silver alloy) that, after the intermediate layer has completely formed, there is still enough tin in the form of a metallic alloy in the connecting layer to ensure secure intergrowth with the transition layer. One could therefore also say that the layer is chosen to be so thick (and thus so much tin is present in the form of a metallic alloy) that, even if so much tin has been "sacrificed" to the intermediate layer, there is still enough tin in the form of a metallic alloy to ensure sufficient intergrowth with the transition layer until its growth is complete.On the other hand, the layer thickness is preferably chosen small enough to prevent the formation of harmful Sn whiskers due to excessive Sn content, or the occurrence of scraping off of excessively thick coatings during the joining process, resulting in loose metallic particles. This is achieved or ensured by the layer thickness selection specified above.

[0020] Since the potentially problematic intermediate layer forms between the barrier layer and the connecting layer and thus basically also between the original barrier layer and the functional layer, both the press-in pin according to the invention before pressing in and (and this is usually more important) the assembly consisting of a circuit board and at least one pressed-in pin are temperature-resistant.

[0021] After sufficient exposure to temperature, the assembly thus has a layer system formed between the base body of the press-in pin and the sleeve of the printed circuit board, which layer system has: a barrier layer resulting from the original barrier layer, a connecting layer which has regions which are each formed in the form of an intermetallic phase made of the alloy metal(s)^) and tin, and an intermediate layer formed between the barrier layer and the connecting layer, which has at least one region which is formed as an intermetallic phase made of nickel and tin.

[0022] Due to the sufficient excess of tin, the connecting layer further comprises at least one region which is formed as a metallic alloy of tin and the at least one alloying metal, whereby the necessary intergrowth is ensured.

[0023] According to current knowledge, the preferred alloying metal is silver, so that the metallic alloy is often a SnAgO,5 alloy.

[0024] Typically, the circuit board has at least one group of press-in holes and the press-in pin is part of a press-in connector having a plurality of similar press-in pins, which is pressed into the group of press-in holes.

[0025] The main advantage of the invention is that the formed press-in connection is temperature-resistant. In a preferred embodiment, the assembly therefore further comprises at least one component soldered to the circuit board, which component was soldered to the circuit board, in particular in an oven. Due to the temperature resistance of the press-in connection, the soldered component can be soldered after the at least one press-in pin has been pressed into the at least one press-in hole, thus resulting in a somewhat controlled aging of the press-in connection.

[0026] The invention will now be explained in more detail using an embodiment with reference to the schematic figures.

[0027] Here we show:

[0028] Figure 1 shows a part of a press-in pin, in particular its press-in zone, Figure 2 shows the detail D1 from Figure 1,

[0029] Figure 2a shows the detail D2 from Figure 2,

[0030] Figure 3 shows the area of ​​a press-fit hole in a printed circuit board,

[0031] Figure 4 shows detail D3 from Figure 3,

[0032] Figure 5 shows the press-in pin from Figure 1 and the area of ​​the circuit board from Figure 3 after pressing in the press-in pin,

[0033] Figure 6 shows detail D4 from Figure 5,

[0034] Figure 6a shows detail D5 from Figure 6,

[0035] Figure 7 shows what is shown in Figure 6 after an ageing process, in particular at a temperature of at least 300° C with an exposure time of at least 15 minutes, and

[0036] Figure 7a shows detail D6 from Figure 7.

[0037] Figures 1, 2 and 2a schematically show a press-in pin and in particular its coating according to the invention. In the exemplary embodiment shown, the press-in pin is a needle-eye press-in pin known in the prior art, but it can in principle be a press-in pin of any geometry. The press-in pin naturally has a base body 12. This usually consists of a copper alloy, for example spring-hard CuSn6. The copper content of the copper alloy is preferably at least 51%. In principle, however, copper-free or low-copper alloys would also be possible. A coating 20 is applied to this base body, at least in the press-in zone, which coating has at least two layers, here exactly two layers. The first layer, which is deposited directly onto the base body 12, is a nickel-containing layer, preferably a pure nickel layer.In principle, this nickel layer can also contain alloying metals, with the nickel layer preferably having a nickel content of at least 90%. The nickel layer can, in particular, be electroplated, as is known in the art. The nickel layer preferably has a thickness d1 of between 0.5 μm and 2 μm. This layer is referred to as the original barrier layer 22.

[0038] A functional layer 24 is applied to this original barrier layer 22, which can basically be designed and produced as known from the generic EP 2 596 157 B1. In particular, the functional layer can be deposited electrolytically. In a specific embodiment, the alloy metal is silver, so that the functional layer consists of silver and tin, with the silver content being between 30 and 72% and the remainder of the layer—except for unavoidable impurities—consisting of tin. According to the invention, the thickness d2 of this layer is at least 0.7 μm, preferably 0.8 to 1.5 μm, more preferably 0.9 to 1.2 μm. The lower limit is determined by an effect that has already been described and is described again below; an excessively thick layer is unnecessary and would only lead to unnecessarily high press-in forces or to abrasion during the joining process, which could generate harmful loose metallic particles.

[0039] As schematically indicated in Figure 2a, silver and tin are present in two states in the functional layer 24, namely as a metallic alloy in the Ag-Sn alloy region 56c and as an intermetallic phase (IMP) in the Ag-Sn-INP regions 56a, which are, in a sense, "embedded" in the Ag-Sn alloy region 56c. The term "embedding" is, however, to be understood only figuratively. It is important, however, that only a portion of the tin is present as a "true" metallic alloy, while the remaining tin is in the form of a more ceramic intermetallic phase. This intermetallic phase is usually an Ag3Sn-IMP. The regions 56a of this intermetallic phase can be distributed separately from one another throughout the entire functional layer.For the sake of completeness, it should also be mentioned that a region or layer containing an intermetallic phase of copper and nickel typically forms between the base body 12 and the original barrier layer 22. However, this is not important for the following, which is why it is not shown.

[0040] Figure 3 shows a printed circuit board 30 in the area of ​​a press-in hole 32, which is intended for the press-in pin 10 shown in Figure 1 to be pressed into it. Figure 4 is detail D3 from Figure 3.

[0041] The press-in hole 32 is surrounded by a sleeve 36, which is usually made of copper. This sleeve 36 is usually coated on the inside with a tin-containing coating (Sn coating / tin layer 38) that has been tempered to create a CuSn IMP. A typical layer thickness is 1 μm. This can be an essentially pure tin coating, but suitable alloys are also known, with a tin content of at least 20% being preferred. Rarely, metallically uncoated Cu sleeves ("circuit board holes") are also used, which have been post-treated with organic coatings ("BTA"). However, this has no influence on the effects according to the invention.

[0042] Figure 5 shows the situation after the press-in pin 10 has been pressed into the hole 32. To be more precise, it should actually say that the press-in pin is pressed into the coated sleeve 36; however, pressing into the "hole" is used synonymously.

[0043] After pressing in, a layer system forms, the essential components of which are shown schematically in Figures 6 and 6a. The functional layer 24 and the tin layer 38 are essentially completely disrupted by the pressing-in process, and from this, a connecting layer 56 and a transition layer 58 are essentially formed. The layer system thus comprises - from the base body 12 to the sleeve 36 - the following layer sequence: The original barrier layer 22 now forms the barrier layer 52, whereby the disruption is relatively slight here and the barrier layer 52 can be considered essentially identical to the original barrier layer 22. The above-mentioned intermetallic phase between the base body 12 and the barrier layer 52 is still present, but since it is not important here, it is not shown in the layer system in Figure 6.Following the barrier layer 52 is the connecting layer 56, which is responsible for the gas-tight connection between the press-fit pin and the circuit board. This layer is essentially formed from the functional layer 24 and the coating 38 of the sleeve during the press-fitting process by disruption. This layer is followed by the transition layer 58, which is located between the connecting layer 56 and the sleeve 36 and was also formed by the disruption process just described.

[0044] As can be seen from Figure 6a, the connecting layer 56—like the original functional layer 24—contains a metallic Sn-Ag alloy region 56c, in which a metallic alloy is present, and Ag-Sn-INP regions 56a "embedded" therein. Furthermore, regions with other intermetallic phases, in particular of copper and tin, may also be present (Cu-Sn-INP regions 56b).

[0045] The transition layer 58 is essentially an intermetallic phase of copper and tin (Cu-Sn-INP). The adhesion or intergrowth between the bonding layer 56 and the transition layer 58 is based on the presence of tin in the metallic Sn-Ag alloy region 56c.

[0046] If this layer system is now subjected to a relatively high temperature over a certain period of time, for example, 300 °C for 15 minutes, a further layer forms, namely the intermediate layer 54 between the barrier layer 52 and the connecting layer 56, as shown in Figure 7. This intermediate layer is completely or almost completely an intermetallic phase of nickel and tin and is fed from the barrier layer 52 and from the metallic Sn-Ag alloy region 56c of the connecting layer 56. This means that it extracts tin from the connecting layer 56 from the metallic Sn-Ag alloy region 56c. The Ag-Sn IMP regions 56a do not contribute to this, so that the ratio between the metallic Sn-Ag alloy region 56c and the IMP regions 56a, 56b shifts in favor of the IMP regions 56a, 56b, as schematically indicated in Figure 7a.

[0047] It has now been discovered that this intermediate layer 54 does not grow arbitrarily, but after reaching a certain thickness, its growth virtually ceases, even upon further exposure to temperature, because it forms a strong barrier between the nickel of the barrier layer 52 and the tin of the connecting layer 56. This means that only a certain amount of tin can be removed from the Sn-Ag alloy region 56c. This in turn means that if a sufficient amount of metallic tin-silver alloy is originally present in the connecting layer 56, a sufficient amount of tin or metallic tin-silver alloy will be present even after the formation of the intermediate layer 54 to ensure the connection to the transition layer 58.Since it is not possible to influence the original ratio of metallic Sn-Ag alloy to Ag-Sn-IMP, this is ensured according to the invention by the fact that the originally present functional layer 24 is sufficiently thick, namely has a thickness of at least 0.7 pm.

[0048] Thus, starting with a known coating and a known process for its production, a sudden increase in the tolerable temperature can be achieved using simple means.

[0049]

[0050] 10 press-in pins

[0051] 12 Base body (Cu substrate)

[0052] 20 Coating of the press-in pin

[0053] 22 original barrier layer

[0054] 24 functional layer

[0055] 24a Ag-Sn-IMP region (Ag3Sn-IMP)

[0056] 24b Sn-Ag alloy

[0057] 30 circuit board

[0058] 32 press-in hole

[0059] 36 sleeve (Cu substrate)

[0060] 38 Sn coating of the sleeve

[0061] 50 layer system between base body and sleeve

[0062] 52 barrier layer

[0063] 54 Intermediate layer (Ni-Sn-IMP)

[0064] 56 Connection layer

[0065] 56a Ag-Sn-IMP region (Ag3Sn-IMP)

[0066] 56b Cu-Sn-IMP region

[0067] 56c metallic Sn-Ag alloy range

[0068] 58 Transition layer (Cu-Sn-IMP)

Claims

1. Press-in pin (10) with a base body (12) and a coating (20) which covers the base body (12) at least in sections and has an original barrier layer (22) and a functional layer (24), wherein the original barrier layer (22) consists of at least 80 mass percent nickel, and the functional layer (24) consists of 30 to 72 mass percent silver and / or copper and / or bismuth and / or indium (alloy metals), the remainder being tin and impurities, and has a thickness (d2) of at least 0.7 micrometers.

2. Press-in pin (10) according to claim 1, characterized in that, apart from impurities, only silver is present as alloy metal in the functional layer (24).

3. Press-in pin (10) according to claim 1 or 2, characterized in that the functional layer (24) has a thickness (d2) between 0.7 and 1.5 micrometers, preferably between 0.9 and 1.2 micrometers.

4. Press-in pin (10) according to claim 1 or claim 2, characterized in that the original barrier layer (22) consists of at least 90 mass percent nickel, preferably exclusively nickel and impurities.

5. Press-in pin (10) according to at least one of the preceding claims, characterized in that the original barrier layer (22) has a thickness (d) of between 0.5 pm and 2 pm.

6. Assembly comprising: a circuit board (30) having at least one press-in hole (32) surrounded by an electrically conductive sleeve (36), and at least one press-in pin (10) which, in the unpressed state, has the features of at least one of claims 1 to 5 and which is pressed into this press-in hole (32).

7. The assembly according to claim 6, characterized in that a layer system is formed between the base body (12) of the press-in pin (10) and the sleeve (36) of the printed circuit board, which layer system has: a barrier layer (52) resulting from the original barrier layer (22), a connecting layer (56) which has regions (56a) which are each formed in the form of an intermetallic phase made of the alloy metal(s) and tin, and an intermediate layer (54) formed between the barrier layer (52) and the connecting layer (56), which has at least one region which is formed as an intermetallic phase made of nickel and tin, wherein the connecting layer (56) further has at least one region (56c) which is formed as a metallic alloy made of tin and the at least one alloy metal.

8. An assembly according to claim 7, as far as dependent on claim 2, characterized in that the alloy of tin and the at least one alloying metal is an SnAgO,5 alloy.

9. Assembly according to at least one of claims 6 to 8, characterized in that the printed circuit board (30) has at least one group of press-in holes (32) and the press-in pin (10) is part of a press-in plug having a plurality of similar press-in pins (10).

10. The assembly according to at least one of claims 6 to 9, characterized in that it further comprises at least one component soldered to the circuit board.

11. The method for producing an assembly according to claim 10, characterized in that the soldered component is soldered after the at least one press-in pin has been pressed into the at least one press-in hole.

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