UltraPad TM Silicone Press Pad with Excellent Mar Resistance (Press Pads for Flex and Rigid Flex PCB Lamination)

Transcription

1 UltraPad TM Silicone Press Pad with Excellent Mar Resistance (Press Pads for Flex and Rigid Flex PCB Lamination) Haibing Zhang, Andy Cloud Arlon Silicone Technologies Governor Lea Road Bear, DE 97 United States Phone: Abstract Silicone based press pads provide uniform pressure during flex and flex-rigid printed circuit board (PCB) lamination at high temperature, 5ºC-25ºC. Use of Arlon UltraPad TM silicone press pad has increased in the PCB lamination industry due to its outstanding thermal stability, predictable end-of-life, long service life, low silicone oil loss, and the low cost per lamination cycle. However, standard Arlon UltraPad TM may not be the perfect choice for some lamination applications, specifically traditional, secondary PCB lamination applications. In these applications, sharp edges from components like connectors can create very high local pressure (up to 6, psi) on a silicone press pad surface resulting in permanent marring or marking. This observation is defined as mar resistance in this technical paper. Laminators may be wary of using a severely marred press pad because of the potential to cause lamination induced defects in the PCB. A test method is developed to quantify mar resistance in this paper. The factors for mar resistance such as temperature, pressure, time, press pad thickness, and crosslink density of silicone rubber are analyzed. This paper also introduces UltraPad TM MR with mar resistance qualities required for applications such as secondary traditional PCB lamination. UltraPad TM MR also has excellent high temperature thermal stability and pressure uniformity retention putting it at the head of the market class for low cost per lamination cycle. Content Background 2 Key Requirements for Silicone Press Pads 3 The Mar Resistance Test Method 4 Factors of Mar Resistance Lamination Temperature Lamination Pressure Lamination Time Press Pad Thickness Crosslink Density of Silicone Rubber 5 Arlon UltraPad TM MR with Excellent Mar Resistance 6 Summary 7 References

2 . Background Printed circuit board (PCB) lamination is a process to achieve the curing or sintering of polymer-based composites which are made of epoxy, polyimide, fluoropolymer, polyester, and other polymers. The lamination process also bonds the polymer composite to metal traces and connectors. A hydraulic press is used to provide pressure and temperature during lamination. Ideally, press pressure is uniform and press plate surfaces are level and smooth, as shown in Figure (a). However, this scenario is unlikely in an actual application. It is very difficult to design a press to provide % pressure uniformity to a laminate. As shown in Figure (b), even if the press plate surfaces are smooth, the PCB surface topography may not be smooth due to required design. So press pads are necessary to provide uniform pressure during PCB lamination, as shown in Figure (c). Press pad elasticity balances uneven pressure in the press by conforming to the uneven surface topography of the PCB []. PCB laminate PCB laminate Press pad PCB laminate Press pad (a) Ideal situation: Pressure is uniform Topography is flat (b) Real situation: Pressure is not uniform Laminate has topography (c) Real world application with conformal press pads Figure Pressure uniformity for PCB lamination in a hydraulic press Although paper based press pads also provide reasonable pressure uniformity for PCB lamination, they are not reusable and unlikely competitive in term of cost per lamination cycle. Elastomeric composites are ideal press pads that provide superior pressure uniformity at the lowest cost per cycle. However, organic elastomers quickly degrade at temperatures greater than 5 C, so they are not able to function across typical printed circuit board press lamination temperatures, 5 C-25 C. Silicone remains stable up to 3 C and consequently is an ideal polymer for PCB lamination press pads. 2

3 2. Key Requirements for Silicone Press Pads Although a number of silicone press pads from different manufactures are available on the market, they do perform differently. Key requirements for a silicone press pad include: Pressure uniformity: the most important function of press pad. Application pressures range from.3 MPa (5psi) to 3.8 MPa (2 psi). Basically, pressure uniformity depends on the durometer and thickness of a press pad and this is quantified in another Arlon technical paper []. Thermal stability: press pads must resist high temperature, typically 5ºC-25ºC, in anaerobic conditions during PCB lamination. Press pad degradation is driven by high temperature and high pressure. Signs of degradation include thickness reduction, silicone oil leakage, and durometer change. Not all silicone press pads are the same though and different brands have very different thermal stability, as detailed in another Arlon technical paper []. Low cost per lamination cycle: low cost with high quality lamination is a key requirement in a cost conscious manufacturing environment. A low press pad price does not always mean low overall cost because this is a function of price and service life cost savings. Smooth surface topography: the surface of a silicone press pad should be relatively smooth so it can provide the best pressure uniformity on the PCB laminate surface. Low tack: crosslinked silicone rubber surfaces generally have high tack causing a PCB or release liner to mildly adhere to a silicone rubber surface. Talc surface treatments and utilization of release films are very popular methods for reducing surface tack. Popular polymer based release films include polyester, polymethylpentene, and polytetrafluoroethylene. Low silicone oil leakage: low molecular weight siloxane (silicone oil) is the byproduct of silicone rubber degradation at high temperature under anaerobic conditions. Silicone oil can leak from the edges and surface of a press pad under pressure. All silicone press pads have some level of silicone oil leakage during service life and the level depends on lamination temperature, pressure, and press pad type. Mar resistance: silicone press pads deform under lamination pressure as shown in Figure (C). After the lamination cycle is complete, this deformation must recover to the original topographically smooth surface. The deformation or surface mar may affect subsequent lamination quality. 3

4 Silicone rubber with surface mar Supporting substrate Figure 2 Silicone press pad with poor mar resistance Silicone rubber with a topographically smooth surface Squeeze-out resistance: Press pads without fabric reinforcement can squeeze-out or laterally deform within the press lamination stack, and the deformation is not recoverable. That s why most silicone press pads are reinforced with a substrate that is well adhered to the silicone polymer. 3. The Mar Resistance Test Method Silicone rubber is an elastic polymer, which flows or moves under pressure. This deformation causes the silicone rubber to change shape although its total volume does not change. The degree of viscoelastic recovery, or mar resistance in this study, is defined as the thickness difference ( t) between the surface under pressure and the surface without pressure, after intermittent deflection pressure is removed and a steady state condition is achieved. See Figure 3. A lower t value can be equated to better mar resistance or the smoother press pad surface after a lamination cycle. Pressure applied by tool to the press pad t Tool and pressure removed from the press pad Figure 3 Thickness difference ( t) after removing pressure 4. Mar Resistance Factors According to our study result, press pad mar resistance is affected by the following factors. 4

5 Lamination temperature Silicone rubber both degrades and continues to crosslink at high lamination temperatures between 5ºC to 25ºC. These changes to the crosslink density, chain length, and molecular weight distribution prevent the polymer chains from recovering to their original locations after pressure is applied and then removed. Consequently, thermal degradation makes mar resistance worse, as shown in Figure 4. Lamination pressure Lamination pressure affects how much silicone rubber deforms and the level of press pad deformation is linearly related to pressure. A high lamination pressure causes more deformation and less recovery after pressure is removed, as shown in Figure 4. Also, a PCB usually has uneven surfaces and attachments protruding. This topography causes local press pad pressure up to times greater than target lamination process. It is not easy to monitor and control these high local pressures so they are usually paramount to mar resistance issues The effect of temperature on mar resistance (UltraPad D75, 45 mils) Lamination temperature (ºC) The effect of pressure on mar resistance (UltraPad D75, 45 mils) Lamination pressure (psi) Figure 4 The effect of lamination temperature and pressure on mar resistance 5

6 Lamination time Under a load, the deformation of polymeric chains is dependent on time because of viscoelastic behavior. After deflection, the elastic component of deformation of a polymer chain can recover to the original shape, but the viscous component of deformation of a polymer chain cannot recover. Longer lamination time results in greater viscous deformation consequently greater permanent deformation. The experimental result is shown in Figure The effect of time on mar resistance (UltraPad D75, 45 mils) Lamination time (min) The effect of thickness on mar resistance (UltraPad D75) Thickness (mil) Figure 5 The effect of lamination time and press pad thickness on mar resistance Thickness A thick press pad material will always have greater deformation in term of thickness deflection than a comparable thinner material. The experimental result shown in Figure 5 also depicts a linear relationship between press pad thickness and mar resistance. Crosslink density of silicone rubber A tightly crosslinked silicone rubber is essential for excellent permanent set resistance [2]. Mar resistance in this study is related to permanent set. Xylene absorption after 24 hours is used to represent polymer crosslink density, and greater levels of xylene absorption indicate lower crosslink density. The test result is shown in Figure 6, which reveals that as crosslink density increases then press pad mar resistance significantly improves. 6

7 The effect of crosslink density (xylene absorption) on mar resistance (UltraPad D75, 45 mils) % % 2% 3% 4% Xylene absorption (%) Figure 6 The effect of crosslink density (xylene absorption) on mar resistance 5. Arlon UltraPad TM MR with Excellent Mar Resistance Arlon has developed a new silicone press pad, UltraPad TM MR, which has much better mar resistance than UltraPad TM D75, (5573R62 series). The comparative test results for.62 thick press pad products can be seen in Figure 7. UltraPad TM MR has similar mar resistance to a leading competitive press pad yet still has the exceptional thermal stability of all UltraPad products. One of the most popular criteria for end of press pad service life is the beginning of silicone oil leakage. Figure 8 shows that time to oil leakage for UltraPad TM MR is nearly twice as long as UltraPad TM D75, and nearly time longer than a leading competitor. Another metric for end of press pad service life is relative pressure uniformity or the percent retention of pressure uniformity []. This metric is less popular in the real world of industrial manufacturing due to testing and calculation complexity, but it is much more accurate and reliable than the criteria of silicone oil leakage. In Reference, the empirical criteria of service life is 5% retention of pressure uniformity []. After combining input from PCB lamination customers and Arlon s experience, 9% retention of pressure uniformity is likely a more realistic criterion for end of service life. Figure 8 shows that the end of service life of UltraPad TM MR is two times greater than UltraPad TM D75, and three times greater than a leading competitive product. 7

8 4.5 Mar resistance (77ºC*psi*2hour) UltraPad D75 Leading competitor UltraPad MR Press Pad Products Figure 7 Mar resistance of UltraPad TM MR, UltraPad TM D75, and a leading competitor Arlon UltraPad TM MR also shows more consistent pressure uniformity as a function of service life time than a leading competitive product. See Figure 8 Accumulated oil leakage (gram) Silicone oil leakage (225ºC*psi) Lamination time (hour) Up/Up (%) % 5% % 95% 9% 85% 8% Pressure uniformity (225ºC*psi) Lamination time (hour) UltraPad D75 Leading competitor UltraPad MR UltraPad D75 Leading competitor UltraPad MR Figure 8 End of Service Life criteria for UltraPad TM MR, UltraPad TM D75, and a leading competitive product 8

9 6. Summary Silicone press pad mar resistance has been extensively investigated. A test method was developed to quantify mar resistance. It was found that the factors of mar resistance include lamination pressure, lamination time, lamination temperature, press pad thickness, and crosslink density of silicone rubber. Low lamination pressure, short lamination time, low lamination temperature, thin press pad thickness, and high crosslink density of silicone rubber yield better product mar resistance. Arlon has developed a new silicone press pad, UltraPad TM MR, with similar mar resistance to a leading competitive product yet with superior service longevity measured by several end of service life metrics. 7. References. Haibing Zhang, Andy Cloud, et al. Low Cost Per Cycle Silicone Press Pad with Predictable End-oflife. Arlon technical paper available at (27). 2. S. S. Rogers. The Vanderbilt Rubber Handbook. Published by R. T. Vanderbilt Co., New York (948) 9