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The adhesive copper foil tape is usually attached to the inner wall of the shielding cover, the back of the FPC, or the inside of the housing during the assembly of the whole machine. The incoming inspection confirms that the copper foil surface is conductive, the adhesive surface is bonded normally, and the total thickness meets the requirements. The material is released. Installed at room temperature, the appearance is smooth and there is no overflow.
After high-temperature aging testing or actual use for a period of time, a semi transparent to light yellow adhesive mark appears on the edge of the tape. The adhesive marks extend outward along the edge of the copper foil, with widths ranging from a few millimeters to over one millimeter. In densely wired areas, adhesive marks may cover adjacent test points or solder pads, affecting the solderability of subsequent processes; On products with high appearance requirements, adhesive marks directly affect the appearance of the finished product; In extreme cases, the overflowing adhesive layer may bridge two conductive areas that should have been insulated. This issue is not related to the copper foil, nor is it about "adhesive non stick", but rather the flow boundary of the adhesive layer under temperature and pressure.
The physical mechanism of adhesive overflow: Pressure sensitive adhesive is not a solid, but a viscoelastic fluid
Pressure sensitive adhesive behaves as a "semi-solid" at room temperature - it has a certain shape retention ability, but undergoes viscoelastic flow under pressure and time. This fluidity is the physical basis for pressure-sensitive adhesives to wet the surface to be attached and form adhesion. Without liquidity, there is no initial adhesion.
Liquidity is a double-edged sword
When the adhesive layer is pressed between the copper foil and the substrate, it is constrained in two directions simultaneously: in the thickness direction, it is constrained by the copper foil and the substrate, but in the plane direction, it is only constrained by the edges of the copper foil. If the adhesive layer softens to a certain extent at the service temperature or is subjected to continuous pressure, it will flow outward from the direction with the weakest constraint - the edge of the copper foil. This is overflow glue. The occurrence condition of overflow can be summarized as "the flow resistance of the adhesive layer at the edge is less than the driving force". The driving force comes from temperature (reducing the modulus of the adhesive) and pressure (providing the driving force for flow); The resistance comes from the crosslinking density of the adhesive (entanglement and chemical bonds between molecular chains) and the die-cutting quality of the edges.
Temperature is the primary driving variable: the intersection of softening point and service temperature
The modulus of pressure-sensitive adhesive varies with temperature. Below the glass transition temperature Tg, the adhesive layer is in a glassy state, hard and brittle, with almost no fluidity; Above Tg, the adhesive layer enters a high elastic state, the modulus decreases, and begins to exhibit significant viscoelastic flow; As the temperature continues to rise near the softening point, the modulus further decreases and the flow resistance sharply decreases.
The Tg of conventional acrylic pressure-sensitive adhesives is usually between -20 ° C and -40 ° C, which means that the adhesive layer is already in a highly elastic state at room temperature and has a certain degree of fluidity. This fluidity is limited by crosslinking density and adhesive interface constraints at room temperature, and there will be no significant overflow of adhesive. But when the service temperature rises to 60 ° C-85 ° C or even higher, the modulus of the adhesive layer further decreases, the flow resistance at the edge decreases, and overflow begins to occur.
This explains a common phenomenon: after being placed at room temperature for several days, there is no glue overflow at the edges; After 85 ° C/85% RH aging test, obvious adhesive marks appeared on the edges. Adhesive overflow does not occur immediately upon application, but rather gradually occurs under the influence of temperature.
The thickness of the adhesive layer is the second variable: the thicker it is, the greater the "adhesive storage capacity" at the edge
The severity of adhesive overflow is directly related to the thickness of the adhesive layer. The thicker the adhesive layer, the greater the amount of adhesive that can flow per unit edge length, and the wider the overflow width. The width of the 0.025mm thin adhesive layer overflowing at the same temperature is usually less than that of the 0.050mm thick adhesive layer.
This also explains why the "total thickness" label for adhesive backed copper foil tape needs to distinguish between copper foil thickness and adhesive layer thickness. If engineers only focus on the total thickness, they may overlook the impact of adhesive layer thickness on overflow. Two adhesive tapes with the same total thickness, one with a 0.036mm copper foil and a 0.025mm adhesive layer, and the other with a 0.020mm copper foil and a 0.041mm adhesive layer. The latter has a significantly higher risk of adhesive overflow.
Pressure is the third variable: creep under continuous compression
In scenarios such as shield pressing, shell fastening, or long-term stacking, the adhesive layer is subjected to sustained compressive stress. Pressure sensitive adhesives undergo creep under sustained stress, and even at low temperatures, prolonged pressure may cause slow edge overflow. This kind of "stress driven" overflow can also occur at room temperature, but at a slower speed. For applications that require long-term compression, such as shield liner and filling between structural components, it is necessary to confirm the creep characteristics of the adhesive layer under the target compression stress, not just the initial adhesive strength, when selecting.
Die cutting method: Edge quality determines the "starting point" of glue overflow
The impact of die-cutting quality on glue overflow is often underestimated. If the die-cutting tool is not sharp enough or the die-cutting parameters are improper, burrs, adhesive layer wire drawing, or local peeling may occur at the edges of the copper foil and adhesive layer. These edge defects will become the starting point for adhesive overflow - the adhesive layer will flow outward from these weak points, and the overflow width will be greater than that of the tape with flat edges.
For applications that are sensitive to overflow, die-cutting quality should be included as a part of incoming material inspection, checking whether the edges are flat, whether there is adhesive layer drawing and burrs, rather than just checking the size and appearance.
The impact of glue overflow: from appearance to electrical, from immediate to cumulative
The impact of glue overflow is layered. The most direct issue is appearance - adhesive marks can be seen near light colored shells or transparent windows, affecting the overall appearance of the finished product. The next layer is the process - the overflowing adhesive layer covers adjacent solder pads or test points, affecting the wettability of subsequent welding and testing, requiring additional cleaning procedures. The deepest layer is electrical - in densely wired areas, if the overflow adhesive layer bridges two conductive areas, it may introduce the risk of leakage or short circuit. Although pressure-sensitive adhesive itself is insulating, if the adhesive layer contains conductive fillers (double conductive tape), the risk of short circuit caused by overflow adhesive will significantly increase.
For double conductive adhesive copper foil tape, the impact of adhesive overflow needs special attention: conductive particles in the adhesive layer overflow together with the adhesive, forming conductive pathways at the edges. If these pathways come into contact with adjacent conductive structures, the risk of short circuit is higher than that of single conductive tape.
Selection and judgment of adhesive backed copper foil tape based on "service temperature x adhesive layer thickness x die-cutting design"
Firstly, confirm the thickness of the adhesive layer with the supplier, rather than just confirming the total thickness
The thickness of the adhesive layer is a direct variable for adhesive overflow. When selecting, the supplier should be required to provide the thickness of the copper foil and the adhesive layer separately, and evaluate the applicability of the adhesive layer thickness based on the service temperature. For high temperature or long-term compression scenarios, a thinner adhesive layer is a more robust starting point.
Secondly, select the softening point of the adhesive layer based on the service temperature, rather than the bonding strength at room temperature
Request the supplier to provide Tg or softening point data of the adhesive layer to confirm that it is above the upper limit of the service temperature. If the service temperature approaches or exceeds the softening point of the adhesive layer, the risk of adhesive overflow significantly increases, and a higher softening point or higher cross-linking density adhesive layer system needs to be selected.
Thirdly, in applications that are sensitive to adhesive overflow, die-cutting shrinkage design is adopted
If the edge of the tape is close to adjacent pads, test points, or conductive structures, the adhesive layer can be slightly inwardly shrunk relative to the copper foil during die-cutting (such as by 0.1-0.2 mm) to reserve buffer space for overflow. This design is particularly important in dual conductive tape and high-density wiring scenarios.
Fourth, include the appearance of the edges after high-temperature aging in the verification of incoming materials or samples
The appearance inspection at room temperature cannot expose the tendency of glue overflow. For high-temperature service scenarios, edge appearance inspection should be completed during the sample stage after high-temperature storage or temperature cycling to confirm whether the overflow width is within an acceptable range and whether the overflow adhesive layer affects adjacent structures.
Advanced Institute (Shenzhen) Technology Co., LtdThe adhesive backed copper foil tape product line covers single and double conductor structures, bare copper/tin plated/nickel plated substrates, and can provide separate labeling of copper foil thickness and adhesive layer thickness, adhesive layer Tg and softening point data, as well as edge overflow evaluation data after high-temperature aging.
The product supports die-cutting shrinkage design and high-temperature adhesive layer system matching, suitable for high-density wiring and high-temperature service scenarios that are sensitive to overflow adhesive. For more technical information, please visithttp://www.avanzado.cn/.