DE102017121726A1 - Printed circuit board with solder stop layer and method for at least partially coating a printed circuit board with a solder stop layer - Google Patents

Abstract

Provided are a printed circuit board having a surface and at least one electrical contact which projects in the direction perpendicular to the surface of the printed circuit board over the surface of the printed circuit board, wherein the electrical contact is designed as a solder pad having a contact surface for making electrical contact against this direction or as a conductor track is configured, wherein the surface of the circuit board is at least partially covered with a Lötstoppschicht so that a Lötstopp barrier for the at least one electrical contact, which projects beyond the surface of the circuit board, is formed when this is designed as a solder pad and / or so in that an electrically insulating insulating layer projecting beyond the surface of the printed circuit board is formed by the solder stop layer when the electrical contact is designed as a printed conductor in which the solder stop layer is formed from a hollow At least in sections, the solder stop barrier has a curved surface that has the shape of a meniscus surface or drop surface and / or that the largest thickness of the electrically insulating insulation layer in the direction perpendicular to the surface facing away from the printed circuit board Conductor less than 125% of the thickness (d) of the electrically insulating insulating layer (9) at the edges of the surface facing away from the surface of the circuit board surface of the conductor in the direction parallel to the surface of the conductor and perpendicular to the respective edge and a method for producing such PCB.

Description

The invention relates to a printed circuit board with a Löststoppschicht with the features of the preamble of claim 1 and a method for at least partially coating a printed circuit board with a Lötstoppschicht with the features of the preamble of claim 8.

Circuit boards are usually coated in accordance with current practice with a Lötstopplack, the electrical contacts of the circuit board are excluded from the solder resist. By this measure, the circuit board can be protected from corrosion and mechanical damage, their dielectric strength can be increased and with reduced solder consumption, the formation of solder bridges can be avoided.

A hitherto conventional approach to this coating is to coat the entire surface of the circuit board with a photosensitive solder resist and then, by selective exposure using a film or mask, make the solder resist locally removable so that the contacts can be exposed, for example Use of suitable chemicals. But this has several disadvantages.

Firstly, even with careful process control, it often happens that an offset occurs during the exposure process, which is why the mask layer must be wider than the structure to be exposed. Thus, due to the production process, a gap arises between the protective lacquer layer and the contact, in which, in addition, deposition of residual chemicals may occur. Furthermore, in this way essentially solder mask layers can be realized with the same thickness at all points of the circuit board surface where the layer is not removed, ie no adaptation of the solder mask layers to locally required dielectric strength or locally desired supporting layer thicknesses in the further processing process.

Another already known possibility of applying solder mask layers to printed circuit boards is screen printing. In this procedure too, masks, which cover the contacts that are not to be coated, play an essential role. Accordingly, an undesirable offset also occurs here, and a flush conclusion to contacts or traces can not be achieved. Furthermore, no adaptation of the solder resist layers to locally required dielectric strength or locally desired supporting layer thicknesses in the further processing process can also be carried out by screen printing.

The object of the invention is therefore to provide an improved printed circuit board coated at least in sections with a solder resist and an improved method for at least partially coating printed circuit boards with solder resist, which avoids these disadvantages.

This object is achieved by a printed circuit board with solder stop layer having the features of patent claim 1 and a method for at least partially coating with the features of claim 9. Advantageous developments of the invention are the subject of the dependent claims.

The printed circuit board according to the invention has a surface and at least one electrical contact which projects in the direction perpendicular to the surface of the printed circuit board over the surface of the printed circuit board.

The electrical contact may be designed as a solder pad with a contact surface for making electrical contact against this direction or designed as a conductor track. Of course, if a plurality of electrical contacts are present, they need not all be designed as a soldering pad or all as a conductor track, but they can of course also be designed partly as a soldering pad and partly as a conductor track.

In this case, the surface of the circuit board is at least partially covered with a Lötstoppschicht so that a Lötstopp barrier for at least one electrical contact, which extends beyond the surface of the circuit board, is formed when this is designed as a solder pad and / or so that one is formed over the surface protruding portions of the conductor electrically insulating insulating layer through the Lötstoppschicht when the electrical contact is designed as a conductor.

It is essential to the invention that the solder-stop barrier consists of a hardened solder-stop ink and that the solder-stop barrier at least partially has a curved surface having the shape of a meniscus surface or droplet surface and / or that the largest thickness of the electrically insulating insulation layer is perpendicular to the surface is remote from the circuit board surface of the conductor less than twice as thick as the thickness of the electrically insulating insulating layer at the edges of the surface facing away from the surface of the circuit board surface of the conductor in the direction parallel to the surface of the conductor and perpendicular to the edges thereof.

Due to the fact that the solder-stop layer is realized by a hardened solder-stop ink, ie by a solder-stop material, which is initially a liquid with significantly lower viscosity compared to solder resists, provides the opportunity to achieve a much more targeted modeling of solder resist layers, for example, by using the physics of liquid interfaces to selectively pattern solder resist layers which are then cured , For example, by irradiation with light of a certain wavelength, in particular with UV light and / or by heat treatment. Such solder resists are, for example, from Taiyo Ink MFG. Co. Ltd or Agfa Materials available.

It is essential for solder resist layers on printed conductors that the greatest thickness of the electrically insulating insulating layer in the direction perpendicular to the surface of the printed conductor facing away from the printed circuit board is less than 125% of the thickness of the electrically insulating insulating layer at the edge of the surface facing away from the surface of the printed circuit board Conductor is designed in the direction parallel to the surface of the conductor track and perpendicular to the edge. In this way, much more material-efficient than before can be ensured that a desired breakdown strength for a given printed circuit board is present in all directions. It is particularly preferred if these thicknesses are approximately equal.

For Lötstoppschichten to solder pads is essential that the predetermined by the liquid interface curved surfaces, ie meniscus surfaces that can be generated, for example, by interaction with an edge of the electrical contact, that is a contact surface with the surface of the circuit board connecting side surface of the electrical contact or drop surfaces, especially if such a side surface is not present, can be obtained.

By appropriate choice of parameters can be produced with this approach convex meniscus surfaces or drop surfaces, which is particularly preferable if it is desired that when contacting the contact surface and solder comes to the edge of the electrical contact. On the other hand, concave meniscus surfaces can also be produced in this way, which are particularly suitable for ensuring a high dielectric strength at the edges of the electrical contact, without, as has hitherto been the case, applying the maximum requirements in the entire area of the surface the breakdown strength sufficient dissolving stop layer must be applied.

Conversely, it is also possible, in particular by using capillary effects, to ensure that the solder-stop barrier touches an edge of the at least one electrical contact and / or even adjoins the contact surface continuously and without overlapping.

Another degree of freedom in the modeling of the solder stop barrier and / or the electrical insulation layer can be obtained by structuring the flanks of the at least one electrical contact, for example undercut or at an angle deviating from a surface normal of the surface of the circuit board. In this way, even in situations where a wetting angle is fixed, the geometry of the generated curved meniscus surface can be specifically influenced.

In particular, by applying the teaching according to the invention, the solder-stop layer or solder-stop barrier can be modeled for the first time in such a way that different breakdown strengths and / or supporting layer thicknesses are realized at different points of the solder-stop layer on the same circuit board. For example, a concave meniscus with a stepless and overlap-free connection to the electrical conductor at the edge of a higher breakdown strength ready than in the side offset from the conductor area where the breakdown strength is not needed.

According to a particularly preferred embodiment, the circuit board has a solder stop barrier with a substantially planar portion and the thickness of the solder stop barrier in this substantially planar portion of the thickness of a substantially planar portion of the meniscus surface, which dictates the shape of the curved portion , deviates. In this way, which is practically in particular by differential printing, e.g. the subsequent modification of the solder-stop layer produced in a first step and at least partially cured, can also in these sections also deliberately influence the thickness of the solder-stop layer and thus, e.g. adapted to a dielectric strength required for a given application.

The method according to the invention for at least partially coating such a printed circuit board having a surface and at least one electrical contact projecting in the direction perpendicular to the surface of the printed circuit board over the surface of the printed circuit board with a solder stop layer is characterized in that the solder stop layer is applied as a solder stop ink and is cured.

It is particularly advantageous, if the solder stop ink is applied with an inkjet printer, because thereby the necessary positioning accuracy is achieved without the use of masks.

An equally advantageous alternative to this is that the solder stop ink is applied in a 3D printing process. One of the differences between these two methods may be that the curing step is performed at different times. Thus, when applied by an inkjet process, a plurality of drops of the solder stop ink can first be applied to the surface of the circuit board and then the amount of liquid thus produced cured so that the surface of a larger area of the resulting solder stop layer or solder stop barrier by the interfacial physics of this Lötstopptintenreservoirs is affected.

By contrast, when using a 3D printing process, it is possible to provide that solder stop ink droplets or groups of solder stop ink droplets are cured and then further solder stop ink droplets are applied to the already applied and cured solder stop ink droplets. This provides greater freedom with regard to the targeted modeling of the solder-stop barrier, but can do away with larger areas with curved meniscus surfaces.

It may be particularly advantageous for the flanks of the at least one electrical contact to be patterned, at least in sections, before application of the soldering stop ink, that is to say, for example. undercut, bevelled or otherwise deformed, since in this way the surface interaction with the solder stop ink can be specifically influenced.

In a particularly preferred variant of the method according to the invention, it is provided that in a separate method step, the thickness of a substantially planar portion of the Lötstoppbarriere is modified so that the thickness of the solder stop barrier in this substantially planar portion of the thickness of a substantially planar portion the meniscus surface, which dictates the shape of the curved portion deviates. This can be done in particular by a differential printing, in which subsequently and in advance in the region of the substantially planar portion locally and defined solder stop ink is applied to us cured.

• 1a: ein schematisches Schliffbild einer ersten Leiterplatte mit Lötpad und erfindungsgemäßer Lötstopp-Barriere;
• 1b: ein schematisches Schliffbild einer Variante der ersten Leiterplatte mit lokal angepasster Durchschlagfestigtkeit;
• 2: ein schematisches Schliffbild einer zweiten Leiterplatte mit Leiterbahn und erfindungsgemäßer elektrischer Isolationsschicht;
• 3: ein schematisches Schliffbild einer dritten Leiterplatte mit Lötpad und erfindungsgemäßer Lötstopp-Barriere;
• 4: ein schematisches Schliffbild einer vierten Leiterplatte mit Lötpad und erfindungsgemäßer Lötstopp-Barriere;
• 5: ein schematisches Schliffbild einer fünften Leiterplatte mit Lötpad und erfindungsgemäßer Lötstopp-Barriere; und
• 6: ein schematisches Schliffbild einer sechsten Leiterplatte mit Lötpad und erfindungsgemäßer Lötstopp-Barriere;

The invention will be explained in more detail with reference to figures that illustrate embodiments. It shows:

• 1a a schematic cross section of a first circuit board with solder pad and inventive solder-stop barrier;
• 1b a schematic section of a variant of the first circuit board with locally adapted Durchschlagfestigtkeit;
• 2 : a schematic section of a second printed circuit board with conductor track and inventive electrical insulation layer;
• 3 a schematic cross section of a third circuit board with solder pad and inventive solder-stop barrier;
• 4 a schematic cross-section of a fourth circuit board with solder pad and inventive solder-stop barrier;
• 5 : a schematic section of a fifth circuit board with solder pad and inventive solder stop barrier; and
• 6 a schematic cross-section of a sixth circuit board with solder pad and inventive solder-stop barrier;

1a shows a schematic sectional view of a printed circuit board 1 with a surface 2 on which an electrical contact in the form of the solder pad 3 with contact surface 3a is arranged. The cross section of the soldering pad 3 is substantially rectangular in this embodiment; the flanks 3b , 3c of the solder pad, which is the contact surface 3a with the circuit board 1 connect substantially perpendicular to the surface 2 the circuit board 1 ,

Immediately to the flank 3b . 3c of the soldering pad 3 and stepless at its contact surface 3a then there is a respective designed as a solder-stop barrier solder stop layer 4 . 5 whose curved surfaces 4a , 5a each have the shape of a concave meniscus, the shape of which by the surface interaction of the liquid applied solder stop ink with the solder pad 3 and the surrounding atmosphere is given. Accordingly, in a solder-stop barrier produced in this way, it is also possible, for example, to use capillary effects of the dissolving stop ink to achieve a desired connection to a soldering pad 3 to reach.

1b shows a variant of the first circuit board with locally adapted Durchschlagfestigtkeit, wherein identical reference numerals are used for identically designed components. The difference to 1a is that here by a differential printing process after the design of the solder stop layers with curved surfaces 4a . 5a , in the 1a According to the interfacial physics of this system in a substantially flat, section with the layer thickness zero pass over in a second printing process, a substantially planar portion 4b , 5b was added, in which the Löstoppschicht the thickness t, that is one of the physically predetermined meniscus surface in the plane running section 4b , 5b has different thickness.

2 shows a schematic sectional view of a printed circuit board 6 with a surface 7 on which an electrical contact in the form of a conductor track 8th is arranged, whose cross-section is substantially rectangular in this embodiment. The flanks 8b , 8c of the conductor track, that of the circuit board 6 remote surface 8a the conductor track 8th the circuit board 6 connect, are substantially perpendicular to the surface 7 the circuit board 6 ,

Furthermore, a solder stop layer is present, which is one of the over the surface protruding portions of the conductor track 8th electrically insulating insulation layer 9 forms. The biggest thickness D the electrically insulating insulation layer 9 in the direction perpendicular to that of the circuit board 6 remote surface 8a the conductor track 8th is about as thick as the thickness d the electrically insulating insulation layer at the edges K1 , K2 of the surface of the circuit board 6 remote surface 8a the conductor track 8th in the direction parallel to the surface 8a the conductor track and perpendicular to their edges K1 , K2. Therefore, it is possible to achieve the desired dielectric strength with a minimum cost of materials.

3 shows a schematic sectional view of a printed circuit board 10 with a surface 11 , on the two electrical contacts 12 , which are designed as solder pads, arranged in the direction perpendicular to the surface of the circuit board over the surface 11 the circuit board 10 protrude and a contact surface 12a have for electrical contact against this direction. The cross section of the contacts 12 is substantially rectangular in this embodiment; the flanks 12b . 12c that the contact surface 12a with the circuit board 10 connect substantially perpendicular to the surface 11 the circuit board 10 ,

Between the electrical contacts 12 extends a solder-stop barrier 13 whose surface 13a has the shape of a concave meniscus because it was created by applying a solder stop ink and then curing. The shape is thus determined by the surface interaction of the liquid-applied solder stop ink with the electrical contacts 12 and the surrounding atmosphere. Accordingly, in a solder stop barrier produced in this way 13 Also, for example, capillary effects of the solder stop ink can be used to provide a desired connection to the electrical contacts 12 to reach.

The in the 4 and 5 Shown embodiments also show each schematic sectional images of printed circuit boards 20 . 30 with surfaces 21 . 31 on each of which two electrical contacts 22 . 32 which are designed as a solder pad, are arranged in the direction perpendicular to the surface 21 . 31 the circuit board 20 . 30 over the surface 21 . 31 the circuit board 20 . 30 protrude and a contact surface 22a . 32a have for electrical contact against this direction.

Between the electrical contacts 22 . 32 each extends a solder-stop barrier 23 . 33 whose surface 23a , 33a has the shape of a concave meniscus, as it was created by the application of a solder stop ink and subsequent curing. The shape is thus determined by the surface interaction of the liquid-applied solder stop ink with the electrical contacts 22 . 32 and the surrounding atmosphere.

If the shape of the solder-stop barriers 23 . 33 in the 4 and 5 nevertheless from each other and from the form of in 1 illustrated solder-stop barrier 13 differs, for example, what the minimum thickness in the area between the electrical contacts 22 . 23 or the thickness in the edge region of the electrical contacts 22 . 23 As far as in this embodiment, this is due to the different shape of the flanks 12b . 12c . 22b . 22c . 32b . 32c , In 4 are they conically shaped, in 5 undercut.

Except by such structuring of the flanks 12b . 12c . 22b . 22c . 32b . 32c the electrical contacts 22 . 23 There are also other ways to influence the shape of the meniscus, for example, by changes in viscosity or weight of the solder stop ink, choice of material for or a coating of the electrical contacts or influencing the atmospheric conditions when applying the solder stop ink.

The 6 shows a further embodiment. The schematic sectional view shown in this figure shows a circuit board 40 with surface 41 on the two electrical contacts 42 , which are designed as solder pads, arranged in the direction perpendicular to the surface 41 the circuit board 40 over the surface 41 the circuit board 40 protrude and a contact surface 42a have for electrical contact against this direction.

In this embodiment, it is desirable that also a flank 42b , 42c of the electrical contacts 42 wetting with solder when making a connection via its contact surface. Accordingly, between the electrical contacts 42 a solder-stop barrier 43 modeled from solder-stop ink whose surface 43a has the shape of a convex meniscus or droplet-shaped. This is also possible by applying a solder stop ink and subsequent curing, which clearly underlines the great freedom and possibilities of adaptation in the modeling of the solder stop barriers with the method according to the invention.

LIST OF REFERENCE NUMBERS

1,6,10,20,30,40

circuit board

2,7,11,21,31,41

surface

3

solder pad

3a

contact area

3b, 3c

flank

4.5

solder resist layer

4a, 5a

curved surface

4b, 5b

essentially flat section

8th

conductor path

8a

surface

8b, 8c

flank

9

insulation layer

12,22,32,42

electric contact

12a, 22a, 32a, 42a

contact area

12b, 12c, 22b, 22c, 32b, 32c, 42b, 42c

flank

13,23,33,43

The solder resist barrier

13a, 23a, 33a, 43a

surface

K1, K2

edge

D, d

thickness

Claims (13)

Printed circuit board (1, 6, 10, 20, 30, 40) having a surface (2, 7, 11, 21, 31, 41) and at least one electrical contact (12, 22, 32, 42) extending in the direction perpendicular to the surface (2, 7, 11, 21, 31, 41) of the printed circuit board (1, 6, 10, 20, 30, 40) over the surface (2, 7, 11, 21, 31, 41) of the printed circuit board (1 , 6, 10, 20, 30, 40), the electrical contact (12, 22, 32, 42) acting as a soldering pad (3) having a contact surface (3a, 12a, 22a, 32a, 42a) for making electrical contact therewith Direction is configured or configured as a conductor (8), wherein the surface (2, 7, 11, 21, 31, 41) of the circuit board (1, 6, 10, 20, 30, 40) at least partially with a solder stop layer ( 4, 5), such that a solder-stop barrier (13, 23, 33, 43) for the at least one electrical contact (12, 22, 32, 42) extending over the surface (2, 7, 11, 21 , 31, 41) of the printed circuit board (1, 6, 10, 20, 30, 40) protrudes, when this is used as a solder pad (3) is configured and / or so that an over the surface (2, 7, 11, 21, 31, 41) of the printed circuit board (1, 6, 10, 20, 30, 40) protruding portions of the conductor electrically insulating insulating layer (9) is formed by the solder stop layer, when the electrical contact is designed as a conductor track (8), characterized in that the solder stop layer (4,5) consists of a hardened solder stop ink and is designed so that the solder stop barrier (13,23,33 , 43) at least in sections has a curved surface (4a, 5a, 13a, 23a, 33a, 43a) which has the shape of a meniscus surface or drop surface and / or that the solder stop layer (4, 5) consists of a hardened solder stop ink and so configured in that the maximum thickness (D) of the electrically insulating insulating layer (9) in the direction perpendicular to the surface (8a) of the conductor track (8) facing away from the printed circuit board (6) is less than 125% of the thickness (d) of the electrically insulating Iso Lation layer (9) at the edges (K1, K2) facing away from the surface (7) of the printed circuit board (6) surface (8a) of the conductor track (8) in the direction parallel to the surface (8a) of the conductor track (8) and perpendicular to respective edge (K1, K2) is. PCB (1,6,10,20,30,40) after Claim 1 characterized in that the curved surface (4a, 5a, 13a, 23a, 33a, 43a) of the solder stop barrier (13, 23, 33, 43) is a convex meniscus surface or drop surface. PCB (1,6,10,20,30,40) after Claim 1 , characterized in that the curved surface (4a, 5a, 13a, 23a, 33a, 43a) of the solder-stop barrier (13, 23, 33, 43) is a concave meniscus surface. Printed circuit board (1, 6, 10, 20, 30, 40) according to one of the preceding claims, characterized in that the solder-stop barrier (13, 23, 33, 43) has an edge (12b, 12c, 22b, 22c, 32b, 32c, 42b, 42c) of the at least one electrical contact (12, 22, 32, 42). Printed circuit board (1,6,10,20,30,40) according to any one of the preceding claims, characterized in that the solder-stop barrier (13,23,33,43) steplessly and without overlapping to the contact surface (12a, 22a, 32a, 42a) connects. Printed circuit board (1, 6, 10, 20, 30, 40) according to one of the preceding claims, characterized in that the flanks (12b, 12c, 22b, 22c, 32b, 32c, 42b, 42c) of the at least one electrical contact (12 , 22, 32, 42). Printed circuit board (1,6,10,20,30,40) according to one of the preceding claims, characterized in that the solder stop layer (4,5) is modeled such that at different points of the solder stop layer on the same circuit board (10,20,30 , 40) different breakdown strengths and / or supporting layer thicknesses are realized. Printed circuit board (1,6,10,20,30,40) according to any one of the preceding claims, characterized in that the solder-stop barrier has a substantially planar portion (4b, 5b) and that the thickness of the solder-stop barrier in this Substantially planar portion of the thickness of a substantially planar portion of the meniscus surface, which dictates the shape of the curved surface (4a, 5a) deviates. Method for at least partially coating a printed circuit board (1, 6, 10, 20, 30, 40) according to one of Claims 1 to 8th , characterized in that the solder stop layer (4,5) is applied and cured as Lötstopptinte. Method according to Claim 9 , characterized in that the solder stop ink is applied with an inkjet printer. Method according to Claim 10 , characterized in that the solder stop ink is applied in a 3D printing process. Method according to one of Claims 9 to 11 , characterized in that before the application of the solder stop ink, the flanks (12b, 12c, 22b, 22c, 32b, 32c, 42b, 42c) of the at least one electrical contact (12,22,32,42) are structured at least in sections. Method according to one of Claims 9 to 12 , characterized in that in a separate process step, the thickness (t) of a substantially planar portion (4b, 5b) of the Lötstoppbarriere is modified so that the thickness (t) of the solder stop barrier in this substantially planar portion (4b, 5b ) differs from the thickness of a substantially planar portion of the meniscus surface which dictates the shape of the curved surface (4a, 5a).

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